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1.. SPDX-License-Identifier: GPL-2.0
2
3.. _fsverity:
4
5=======================================================
6fs-verity: read-only file-based authenticity protection
7=======================================================
8
9Introduction
10============
11
12fs-verity (``fs/verity/``) is a support layer that filesystems can
13hook into to support transparent integrity and authenticity protection
14of read-only files. Currently, it is supported by the ext4, f2fs, and
15btrfs filesystems. Like fscrypt, not too much filesystem-specific
16code is needed to support fs-verity.
17
18fs-verity is similar to `dm-verity
19<https://www.kernel.org/doc/Documentation/admin-guide/device-mapper/verity.rst>`_
20but works on files rather than block devices. On regular files on
21filesystems supporting fs-verity, userspace can execute an ioctl that
22causes the filesystem to build a Merkle tree for the file and persist
23it to a filesystem-specific location associated with the file.
24
25After this, the file is made readonly, and all reads from the file are
26automatically verified against the file's Merkle tree. Reads of any
27corrupted data, including mmap reads, will fail.
28
29Userspace can use another ioctl to retrieve the root hash (actually
30the "fs-verity file digest", which is a hash that includes the Merkle
31tree root hash) that fs-verity is enforcing for the file. This ioctl
32executes in constant time, regardless of the file size.
33
34fs-verity is essentially a way to hash a file in constant time,
35subject to the caveat that reads which would violate the hash will
36fail at runtime.
37
38Use cases
39=========
40
41By itself, fs-verity only provides integrity protection, i.e.
42detection of accidental (non-malicious) corruption.
43
44However, because fs-verity makes retrieving the file hash extremely
45efficient, it's primarily meant to be used as a tool to support
46authentication (detection of malicious modifications) or auditing
47(logging file hashes before use).
48
49A standard file hash could be used instead of fs-verity. However,
50this is inefficient if the file is large and only a small portion may
51be accessed. This is often the case for Android application package
52(APK) files, for example. These typically contain many translations,
53classes, and other resources that are infrequently or even never
54accessed on a particular device. It would be slow and wasteful to
55read and hash the entire file before starting the application.
56
57Unlike an ahead-of-time hash, fs-verity also re-verifies data each
58time it's paged in. This ensures that malicious disk firmware can't
59undetectably change the contents of the file at runtime.
60
61fs-verity does not replace or obsolete dm-verity. dm-verity should
62still be used on read-only filesystems. fs-verity is for files that
63must live on a read-write filesystem because they are independently
64updated and potentially user-installed, so dm-verity cannot be used.
65
66fs-verity does not mandate a particular scheme for authenticating its
67file hashes. (Similarly, dm-verity does not mandate a particular
68scheme for authenticating its block device root hashes.) Options for
69authenticating fs-verity file hashes include:
70
71- Trusted userspace code. Often, the userspace code that accesses
72 files can be trusted to authenticate them. Consider e.g. an
73 application that wants to authenticate data files before using them,
74 or an application loader that is part of the operating system (which
75 is already authenticated in a different way, such as by being loaded
76 from a read-only partition that uses dm-verity) and that wants to
77 authenticate applications before loading them. In these cases, this
78 trusted userspace code can authenticate a file's contents by
79 retrieving its fs-verity digest using `FS_IOC_MEASURE_VERITY`_, then
80 verifying a signature of it using any userspace cryptographic
81 library that supports digital signatures.
82
83- Integrity Measurement Architecture (IMA). IMA supports fs-verity
84 file digests as an alternative to its traditional full file digests.
85 "IMA appraisal" enforces that files contain a valid, matching
86 signature in their "security.ima" extended attribute, as controlled
87 by the IMA policy. For more information, see the IMA documentation.
88
89- Integrity Policy Enforcement (IPE). IPE supports enforcing access
90 control decisions based on immutable security properties of files,
91 including those protected by fs-verity's built-in signatures.
92 "IPE policy" specifically allows for the authorization of fs-verity
93 files using properties ``fsverity_digest`` for identifying
94 files by their verity digest, and ``fsverity_signature`` to authorize
95 files with a verified fs-verity's built-in signature. For
96 details on configuring IPE policies and understanding its operational
97 modes, please refer to :doc:`IPE admin guide </admin-guide/LSM/ipe>`.
98
99- Trusted userspace code in combination with `Built-in signature
100 verification`_. This approach should be used only with great care.
101
102User API
103========
104
105FS_IOC_ENABLE_VERITY
106--------------------
107
108The FS_IOC_ENABLE_VERITY ioctl enables fs-verity on a file. It takes
109in a pointer to a struct fsverity_enable_arg, defined as
110follows::
111
112 struct fsverity_enable_arg {
113 __u32 version;
114 __u32 hash_algorithm;
115 __u32 block_size;
116 __u32 salt_size;
117 __u64 salt_ptr;
118 __u32 sig_size;
119 __u32 __reserved1;
120 __u64 sig_ptr;
121 __u64 __reserved2[11];
122 };
123
124This structure contains the parameters of the Merkle tree to build for
125the file. It must be initialized as follows:
126
127- ``version`` must be 1.
128- ``hash_algorithm`` must be the identifier for the hash algorithm to
129 use for the Merkle tree, such as FS_VERITY_HASH_ALG_SHA256. See
130 ``include/uapi/linux/fsverity.h`` for the list of possible values.
131- ``block_size`` is the Merkle tree block size, in bytes. In Linux
132 v6.3 and later, this can be any power of 2 between (inclusively)
133 1024 and the minimum of the system page size and the filesystem
134 block size. In earlier versions, the page size was the only allowed
135 value.
136- ``salt_size`` is the size of the salt in bytes, or 0 if no salt is
137 provided. The salt is a value that is prepended to every hashed
138 block; it can be used to personalize the hashing for a particular
139 file or device. Currently the maximum salt size is 32 bytes.
140- ``salt_ptr`` is the pointer to the salt, or NULL if no salt is
141 provided.
142- ``sig_size`` is the size of the builtin signature in bytes, or 0 if no
143 builtin signature is provided. Currently the builtin signature is
144 (somewhat arbitrarily) limited to 16128 bytes.
145- ``sig_ptr`` is the pointer to the builtin signature, or NULL if no
146 builtin signature is provided. A builtin signature is only needed
147 if the `Built-in signature verification`_ feature is being used. It
148 is not needed for IMA appraisal, and it is not needed if the file
149 signature is being handled entirely in userspace.
150- All reserved fields must be zeroed.
151
152FS_IOC_ENABLE_VERITY causes the filesystem to build a Merkle tree for
153the file and persist it to a filesystem-specific location associated
154with the file, then mark the file as a verity file. This ioctl may
155take a long time to execute on large files, and it is interruptible by
156fatal signals.
157
158FS_IOC_ENABLE_VERITY checks for write access to the inode. However,
159it must be executed on an O_RDONLY file descriptor and no processes
160can have the file open for writing. Attempts to open the file for
161writing while this ioctl is executing will fail with ETXTBSY. (This
162is necessary to guarantee that no writable file descriptors will exist
163after verity is enabled, and to guarantee that the file's contents are
164stable while the Merkle tree is being built over it.)
165
166On success, FS_IOC_ENABLE_VERITY returns 0, and the file becomes a
167verity file. On failure (including the case of interruption by a
168fatal signal), no changes are made to the file.
169
170FS_IOC_ENABLE_VERITY can fail with the following errors:
171
172- ``EACCES``: the process does not have write access to the file
173- ``EBADMSG``: the builtin signature is malformed
174- ``EBUSY``: this ioctl is already running on the file
175- ``EEXIST``: the file already has verity enabled
176- ``EFAULT``: the caller provided inaccessible memory
177- ``EFBIG``: the file is too large to enable verity on
178- ``EINTR``: the operation was interrupted by a fatal signal
179- ``EINVAL``: unsupported version, hash algorithm, or block size; or
180 reserved bits are set; or the file descriptor refers to neither a
181 regular file nor a directory.
182- ``EISDIR``: the file descriptor refers to a directory
183- ``EKEYREJECTED``: the builtin signature doesn't match the file
184- ``EMSGSIZE``: the salt or builtin signature is too long
185- ``ENOKEY``: the ".fs-verity" keyring doesn't contain the certificate
186 needed to verify the builtin signature
187- ``ENOPKG``: fs-verity recognizes the hash algorithm, but it's not
188 available in the kernel's crypto API as currently configured (e.g.
189 for SHA-512, missing CONFIG_CRYPTO_SHA512).
190- ``ENOTTY``: this type of filesystem does not implement fs-verity
191- ``EOPNOTSUPP``: the kernel was not configured with fs-verity
192 support; or the filesystem superblock has not had the 'verity'
193 feature enabled on it; or the filesystem does not support fs-verity
194 on this file. (See `Filesystem support`_.)
195- ``EPERM``: the file is append-only; or, a builtin signature is
196 required and one was not provided.
197- ``EROFS``: the filesystem is read-only
198- ``ETXTBSY``: someone has the file open for writing. This can be the
199 caller's file descriptor, another open file descriptor, or the file
200 reference held by a writable memory map.
201
202FS_IOC_MEASURE_VERITY
203---------------------
204
205The FS_IOC_MEASURE_VERITY ioctl retrieves the digest of a verity file.
206The fs-verity file digest is a cryptographic digest that identifies
207the file contents that are being enforced on reads; it is computed via
208a Merkle tree and is different from a traditional full-file digest.
209
210This ioctl takes in a pointer to a variable-length structure::
211
212 struct fsverity_digest {
213 __u16 digest_algorithm;
214 __u16 digest_size; /* input/output */
215 __u8 digest[];
216 };
217
218``digest_size`` is an input/output field. On input, it must be
219initialized to the number of bytes allocated for the variable-length
220``digest`` field.
221
222On success, 0 is returned and the kernel fills in the structure as
223follows:
224
225- ``digest_algorithm`` will be the hash algorithm used for the file
226 digest. It will match ``fsverity_enable_arg::hash_algorithm``.
227- ``digest_size`` will be the size of the digest in bytes, e.g. 32
228 for SHA-256. (This can be redundant with ``digest_algorithm``.)
229- ``digest`` will be the actual bytes of the digest.
230
231FS_IOC_MEASURE_VERITY is guaranteed to execute in constant time,
232regardless of the size of the file.
233
234FS_IOC_MEASURE_VERITY can fail with the following errors:
235
236- ``EFAULT``: the caller provided inaccessible memory
237- ``ENODATA``: the file is not a verity file
238- ``ENOTTY``: this type of filesystem does not implement fs-verity
239- ``EOPNOTSUPP``: the kernel was not configured with fs-verity
240 support, or the filesystem superblock has not had the 'verity'
241 feature enabled on it. (See `Filesystem support`_.)
242- ``EOVERFLOW``: the digest is longer than the specified
243 ``digest_size`` bytes. Try providing a larger buffer.
244
245FS_IOC_READ_VERITY_METADATA
246---------------------------
247
248The FS_IOC_READ_VERITY_METADATA ioctl reads verity metadata from a
249verity file. This ioctl is available since Linux v5.12.
250
251This ioctl allows writing a server program that takes a verity file
252and serves it to a client program, such that the client can do its own
253fs-verity compatible verification of the file. This only makes sense
254if the client doesn't trust the server and if the server needs to
255provide the storage for the client.
256
257This is a fairly specialized use case, and most fs-verity users won't
258need this ioctl.
259
260This ioctl takes in a pointer to the following structure::
261
262 #define FS_VERITY_METADATA_TYPE_MERKLE_TREE 1
263 #define FS_VERITY_METADATA_TYPE_DESCRIPTOR 2
264 #define FS_VERITY_METADATA_TYPE_SIGNATURE 3
265
266 struct fsverity_read_metadata_arg {
267 __u64 metadata_type;
268 __u64 offset;
269 __u64 length;
270 __u64 buf_ptr;
271 __u64 __reserved;
272 };
273
274``metadata_type`` specifies the type of metadata to read:
275
276- ``FS_VERITY_METADATA_TYPE_MERKLE_TREE`` reads the blocks of the
277 Merkle tree. The blocks are returned in order from the root level
278 to the leaf level. Within each level, the blocks are returned in
279 the same order that their hashes are themselves hashed.
280 See `Merkle tree`_ for more information.
281
282- ``FS_VERITY_METADATA_TYPE_DESCRIPTOR`` reads the fs-verity
283 descriptor. See `fs-verity descriptor`_.
284
285- ``FS_VERITY_METADATA_TYPE_SIGNATURE`` reads the builtin signature
286 which was passed to FS_IOC_ENABLE_VERITY, if any. See `Built-in
287 signature verification`_.
288
289The semantics are similar to those of ``pread()``. ``offset``
290specifies the offset in bytes into the metadata item to read from, and
291``length`` specifies the maximum number of bytes to read from the
292metadata item. ``buf_ptr`` is the pointer to the buffer to read into,
293cast to a 64-bit integer. ``__reserved`` must be 0. On success, the
294number of bytes read is returned. 0 is returned at the end of the
295metadata item. The returned length may be less than ``length``, for
296example if the ioctl is interrupted.
297
298The metadata returned by FS_IOC_READ_VERITY_METADATA isn't guaranteed
299to be authenticated against the file digest that would be returned by
300`FS_IOC_MEASURE_VERITY`_, as the metadata is expected to be used to
301implement fs-verity compatible verification anyway (though absent a
302malicious disk, the metadata will indeed match). E.g. to implement
303this ioctl, the filesystem is allowed to just read the Merkle tree
304blocks from disk without actually verifying the path to the root node.
305
306FS_IOC_READ_VERITY_METADATA can fail with the following errors:
307
308- ``EFAULT``: the caller provided inaccessible memory
309- ``EINTR``: the ioctl was interrupted before any data was read
310- ``EINVAL``: reserved fields were set, or ``offset + length``
311 overflowed
312- ``ENODATA``: the file is not a verity file, or
313 FS_VERITY_METADATA_TYPE_SIGNATURE was requested but the file doesn't
314 have a builtin signature
315- ``ENOTTY``: this type of filesystem does not implement fs-verity, or
316 this ioctl is not yet implemented on it
317- ``EOPNOTSUPP``: the kernel was not configured with fs-verity
318 support, or the filesystem superblock has not had the 'verity'
319 feature enabled on it. (See `Filesystem support`_.)
320
321FS_IOC_GETFLAGS
322---------------
323
324The existing ioctl FS_IOC_GETFLAGS (which isn't specific to fs-verity)
325can also be used to check whether a file has fs-verity enabled or not.
326To do so, check for FS_VERITY_FL (0x00100000) in the returned flags.
327
328The verity flag is not settable via FS_IOC_SETFLAGS. You must use
329FS_IOC_ENABLE_VERITY instead, since parameters must be provided.
330
331statx
332-----
333
334Since Linux v5.5, the statx() system call sets STATX_ATTR_VERITY if
335the file has fs-verity enabled. This can perform better than
336FS_IOC_GETFLAGS and FS_IOC_MEASURE_VERITY because it doesn't require
337opening the file, and opening verity files can be expensive.
338
339.. _accessing_verity_files:
340
341Accessing verity files
342======================
343
344Applications can transparently access a verity file just like a
345non-verity one, with the following exceptions:
346
347- Verity files are readonly. They cannot be opened for writing or
348 truncate()d, even if the file mode bits allow it. Attempts to do
349 one of these things will fail with EPERM. However, changes to
350 metadata such as owner, mode, timestamps, and xattrs are still
351 allowed, since these are not measured by fs-verity. Verity files
352 can also still be renamed, deleted, and linked to.
353
354- Direct I/O is not supported on verity files. Attempts to use direct
355 I/O on such files will fall back to buffered I/O.
356
357- DAX (Direct Access) is not supported on verity files, because this
358 would circumvent the data verification.
359
360- Reads of data that doesn't match the verity Merkle tree will fail
361 with EIO (for read()) or SIGBUS (for mmap() reads).
362
363- If the sysctl "fs.verity.require_signatures" is set to 1 and the
364 file is not signed by a key in the ".fs-verity" keyring, then
365 opening the file will fail. See `Built-in signature verification`_.
366
367Direct access to the Merkle tree is not supported. Therefore, if a
368verity file is copied, or is backed up and restored, then it will lose
369its "verity"-ness. fs-verity is primarily meant for files like
370executables that are managed by a package manager.
371
372File digest computation
373=======================
374
375This section describes how fs-verity hashes the file contents using a
376Merkle tree to produce the digest which cryptographically identifies
377the file contents. This algorithm is the same for all filesystems
378that support fs-verity.
379
380Userspace only needs to be aware of this algorithm if it needs to
381compute fs-verity file digests itself, e.g. in order to sign files.
382
383.. _fsverity_merkle_tree:
384
385Merkle tree
386-----------
387
388The file contents is divided into blocks, where the block size is
389configurable but is usually 4096 bytes. The end of the last block is
390zero-padded if needed. Each block is then hashed, producing the first
391level of hashes. Then, the hashes in this first level are grouped
392into 'blocksize'-byte blocks (zero-padding the ends as needed) and
393these blocks are hashed, producing the second level of hashes. This
394proceeds up the tree until only a single block remains. The hash of
395this block is the "Merkle tree root hash".
396
397If the file fits in one block and is nonempty, then the "Merkle tree
398root hash" is simply the hash of the single data block. If the file
399is empty, then the "Merkle tree root hash" is all zeroes.
400
401The "blocks" here are not necessarily the same as "filesystem blocks".
402
403If a salt was specified, then it's zero-padded to the closest multiple
404of the input size of the hash algorithm's compression function, e.g.
40564 bytes for SHA-256 or 128 bytes for SHA-512. The padded salt is
406prepended to every data or Merkle tree block that is hashed.
407
408The purpose of the block padding is to cause every hash to be taken
409over the same amount of data, which simplifies the implementation and
410keeps open more possibilities for hardware acceleration. The purpose
411of the salt padding is to make the salting "free" when the salted hash
412state is precomputed, then imported for each hash.
413
414Example: in the recommended configuration of SHA-256 and 4K blocks,
415128 hash values fit in each block. Thus, each level of the Merkle
416tree is approximately 128 times smaller than the previous, and for
417large files the Merkle tree's size converges to approximately 1/127 of
418the original file size. However, for small files, the padding is
419significant, making the space overhead proportionally more.
420
421.. _fsverity_descriptor:
422
423fs-verity descriptor
424--------------------
425
426By itself, the Merkle tree root hash is ambiguous. For example, it
427can't a distinguish a large file from a small second file whose data
428is exactly the top-level hash block of the first file. Ambiguities
429also arise from the convention of padding to the next block boundary.
430
431To solve this problem, the fs-verity file digest is actually computed
432as a hash of the following structure, which contains the Merkle tree
433root hash as well as other fields such as the file size::
434
435 struct fsverity_descriptor {
436 __u8 version; /* must be 1 */
437 __u8 hash_algorithm; /* Merkle tree hash algorithm */
438 __u8 log_blocksize; /* log2 of size of data and tree blocks */
439 __u8 salt_size; /* size of salt in bytes; 0 if none */
440 __le32 __reserved_0x04; /* must be 0 */
441 __le64 data_size; /* size of file the Merkle tree is built over */
442 __u8 root_hash[64]; /* Merkle tree root hash */
443 __u8 salt[32]; /* salt prepended to each hashed block */
444 __u8 __reserved[144]; /* must be 0's */
445 };
446
447Built-in signature verification
448===============================
449
450CONFIG_FS_VERITY_BUILTIN_SIGNATURES=y adds supports for in-kernel
451verification of fs-verity builtin signatures.
452
453**IMPORTANT**! Please take great care before using this feature.
454It is not the only way to do signatures with fs-verity, and the
455alternatives (such as userspace signature verification, and IMA
456appraisal) can be much better. It's also easy to fall into a trap
457of thinking this feature solves more problems than it actually does.
458
459Enabling this option adds the following:
460
4611. At boot time, the kernel creates a keyring named ".fs-verity". The
462 root user can add trusted X.509 certificates to this keyring using
463 the add_key() system call.
464
4652. `FS_IOC_ENABLE_VERITY`_ accepts a pointer to a PKCS#7 formatted
466 detached signature in DER format of the file's fs-verity digest.
467 On success, the ioctl persists the signature alongside the Merkle
468 tree. Then, any time the file is opened, the kernel verifies the
469 file's actual digest against this signature, using the certificates
470 in the ".fs-verity" keyring. This verification happens as long as the
471 file's signature exists, regardless of the state of the sysctl variable
472 "fs.verity.require_signatures" described in the next item. The IPE LSM
473 relies on this behavior to recognize and label fsverity files
474 that contain a verified built-in fsverity signature.
475
4763. A new sysctl "fs.verity.require_signatures" is made available.
477 When set to 1, the kernel requires that all verity files have a
478 correctly signed digest as described in (2).
479
480The data that the signature as described in (2) must be a signature of
481is the fs-verity file digest in the following format::
482
483 struct fsverity_formatted_digest {
484 char magic[8]; /* must be "FSVerity" */
485 __le16 digest_algorithm;
486 __le16 digest_size;
487 __u8 digest[];
488 };
489
490That's it. It should be emphasized again that fs-verity builtin
491signatures are not the only way to do signatures with fs-verity. See
492`Use cases`_ for an overview of ways in which fs-verity can be used.
493fs-verity builtin signatures have some major limitations that should
494be carefully considered before using them:
495
496- Builtin signature verification does *not* make the kernel enforce
497 that any files actually have fs-verity enabled. Thus, it is not a
498 complete authentication policy. Currently, if it is used, one
499 way to complete the authentication policy is for trusted userspace
500 code to explicitly check whether files have fs-verity enabled with a
501 signature before they are accessed. (With
502 fs.verity.require_signatures=1, just checking whether fs-verity is
503 enabled suffices.) But, in this case the trusted userspace code
504 could just store the signature alongside the file and verify it
505 itself using a cryptographic library, instead of using this feature.
506
507- Another approach is to utilize fs-verity builtin signature
508 verification in conjunction with the IPE LSM, which supports defining
509 a kernel-enforced, system-wide authentication policy that allows only
510 files with a verified fs-verity builtin signature to perform certain
511 operations, such as execution. Note that IPE doesn't require
512 fs.verity.require_signatures=1.
513 Please refer to :doc:`IPE admin guide </admin-guide/LSM/ipe>` for
514 more details.
515
516- A file's builtin signature can only be set at the same time that
517 fs-verity is being enabled on the file. Changing or deleting the
518 builtin signature later requires re-creating the file.
519
520- Builtin signature verification uses the same set of public keys for
521 all fs-verity enabled files on the system. Different keys cannot be
522 trusted for different files; each key is all or nothing.
523
524- The sysctl fs.verity.require_signatures applies system-wide.
525 Setting it to 1 only works when all users of fs-verity on the system
526 agree that it should be set to 1. This limitation can prevent
527 fs-verity from being used in cases where it would be helpful.
528
529- Builtin signature verification can only use signature algorithms
530 that are supported by the kernel. For example, the kernel does not
531 yet support Ed25519, even though this is often the signature
532 algorithm that is recommended for new cryptographic designs.
533
534- fs-verity builtin signatures are in PKCS#7 format, and the public
535 keys are in X.509 format. These formats are commonly used,
536 including by some other kernel features (which is why the fs-verity
537 builtin signatures use them), and are very feature rich.
538 Unfortunately, history has shown that code that parses and handles
539 these formats (which are from the 1990s and are based on ASN.1)
540 often has vulnerabilities as a result of their complexity. This
541 complexity is not inherent to the cryptography itself.
542
543 fs-verity users who do not need advanced features of X.509 and
544 PKCS#7 should strongly consider using simpler formats, such as plain
545 Ed25519 keys and signatures, and verifying signatures in userspace.
546
547 fs-verity users who choose to use X.509 and PKCS#7 anyway should
548 still consider that verifying those signatures in userspace is more
549 flexible (for other reasons mentioned earlier in this document) and
550 eliminates the need to enable CONFIG_FS_VERITY_BUILTIN_SIGNATURES
551 and its associated increase in kernel attack surface. In some cases
552 it can even be necessary, since advanced X.509 and PKCS#7 features
553 do not always work as intended with the kernel. For example, the
554 kernel does not check X.509 certificate validity times.
555
556 Note: IMA appraisal, which supports fs-verity, does not use PKCS#7
557 for its signatures, so it partially avoids the issues discussed
558 here. IMA appraisal does use X.509.
559
560Filesystem support
561==================
562
563fs-verity is supported by several filesystems, described below. The
564CONFIG_FS_VERITY kconfig option must be enabled to use fs-verity on
565any of these filesystems.
566
567``include/linux/fsverity.h`` declares the interface between the
568``fs/verity/`` support layer and filesystems. Briefly, filesystems
569must provide an ``fsverity_operations`` structure that provides
570methods to read and write the verity metadata to a filesystem-specific
571location, including the Merkle tree blocks and
572``fsverity_descriptor``. Filesystems must also call functions in
573``fs/verity/`` at certain times, such as when a file is opened or when
574pages have been read into the pagecache. (See `Verifying data`_.)
575
576ext4
577----
578
579ext4 supports fs-verity since Linux v5.4 and e2fsprogs v1.45.2.
580
581To create verity files on an ext4 filesystem, the filesystem must have
582been formatted with ``-O verity`` or had ``tune2fs -O verity`` run on
583it. "verity" is an RO_COMPAT filesystem feature, so once set, old
584kernels will only be able to mount the filesystem readonly, and old
585versions of e2fsck will be unable to check the filesystem.
586
587Originally, an ext4 filesystem with the "verity" feature could only be
588mounted when its block size was equal to the system page size
589(typically 4096 bytes). In Linux v6.3, this limitation was removed.
590
591ext4 sets the EXT4_VERITY_FL on-disk inode flag on verity files. It
592can only be set by `FS_IOC_ENABLE_VERITY`_, and it cannot be cleared.
593
594ext4 also supports encryption, which can be used simultaneously with
595fs-verity. In this case, the plaintext data is verified rather than
596the ciphertext. This is necessary in order to make the fs-verity file
597digest meaningful, since every file is encrypted differently.
598
599ext4 stores the verity metadata (Merkle tree and fsverity_descriptor)
600past the end of the file, starting at the first 64K boundary beyond
601i_size. This approach works because (a) verity files are readonly,
602and (b) pages fully beyond i_size aren't visible to userspace but can
603be read/written internally by ext4 with only some relatively small
604changes to ext4. This approach avoids having to depend on the
605EA_INODE feature and on rearchitecturing ext4's xattr support to
606support paging multi-gigabyte xattrs into memory, and to support
607encrypting xattrs. Note that the verity metadata *must* be encrypted
608when the file is, since it contains hashes of the plaintext data.
609
610ext4 only allows verity on extent-based files.
611
612f2fs
613----
614
615f2fs supports fs-verity since Linux v5.4 and f2fs-tools v1.11.0.
616
617To create verity files on an f2fs filesystem, the filesystem must have
618been formatted with ``-O verity``.
619
620f2fs sets the FADVISE_VERITY_BIT on-disk inode flag on verity files.
621It can only be set by `FS_IOC_ENABLE_VERITY`_, and it cannot be
622cleared.
623
624Like ext4, f2fs stores the verity metadata (Merkle tree and
625fsverity_descriptor) past the end of the file, starting at the first
62664K boundary beyond i_size. See explanation for ext4 above.
627Moreover, f2fs supports at most 4096 bytes of xattr entries per inode
628which usually wouldn't be enough for even a single Merkle tree block.
629
630f2fs doesn't support enabling verity on files that currently have
631atomic or volatile writes pending.
632
633btrfs
634-----
635
636btrfs supports fs-verity since Linux v5.15. Verity-enabled inodes are
637marked with a RO_COMPAT inode flag, and the verity metadata is stored
638in separate btree items.
639
640Implementation details
641======================
642
643Verifying data
644--------------
645
646fs-verity ensures that all reads of a verity file's data are verified,
647regardless of which syscall is used to do the read (e.g. mmap(),
648read(), pread()) and regardless of whether it's the first read or a
649later read (unless the later read can return cached data that was
650already verified). Below, we describe how filesystems implement this.
651
652Pagecache
653~~~~~~~~~
654
655For filesystems using Linux's pagecache, the ``->read_folio()`` and
656``->readahead()`` methods must be modified to verify folios before
657they are marked Uptodate. Merely hooking ``->read_iter()`` would be
658insufficient, since ``->read_iter()`` is not used for memory maps.
659
660Therefore, fs/verity/ provides the function fsverity_verify_blocks()
661which verifies data that has been read into the pagecache of a verity
662inode. The containing folio must still be locked and not Uptodate, so
663it's not yet readable by userspace. As needed to do the verification,
664fsverity_verify_blocks() will call back into the filesystem to read
665hash blocks via fsverity_operations::read_merkle_tree_page().
666
667fsverity_verify_blocks() returns false if verification failed; in this
668case, the filesystem must not set the folio Uptodate. Following this,
669as per the usual Linux pagecache behavior, attempts by userspace to
670read() from the part of the file containing the folio will fail with
671EIO, and accesses to the folio within a memory map will raise SIGBUS.
672
673In principle, verifying a data block requires verifying the entire
674path in the Merkle tree from the data block to the root hash.
675However, for efficiency the filesystem may cache the hash blocks.
676Therefore, fsverity_verify_blocks() only ascends the tree reading hash
677blocks until an already-verified hash block is seen. It then verifies
678the path to that block.
679
680This optimization, which is also used by dm-verity, results in
681excellent sequential read performance. This is because usually (e.g.
682127 in 128 times for 4K blocks and SHA-256) the hash block from the
683bottom level of the tree will already be cached and checked from
684reading a previous data block. However, random reads perform worse.
685
686Block device based filesystems
687~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
688
689Block device based filesystems (e.g. ext4 and f2fs) in Linux also use
690the pagecache, so the above subsection applies too. However, they
691also usually read many data blocks from a file at once, grouped into a
692structure called a "bio". To make it easier for these types of
693filesystems to support fs-verity, fs/verity/ also provides a function
694fsverity_verify_bio() which verifies all data blocks in a bio.
695
696ext4 and f2fs also support encryption. If a verity file is also
697encrypted, the data must be decrypted before being verified. To
698support this, these filesystems allocate a "post-read context" for
699each bio and store it in ``->bi_private``::
700
701 struct bio_post_read_ctx {
702 struct bio *bio;
703 struct work_struct work;
704 unsigned int cur_step;
705 unsigned int enabled_steps;
706 };
707
708``enabled_steps`` is a bitmask that specifies whether decryption,
709verity, or both is enabled. After the bio completes, for each needed
710postprocessing step the filesystem enqueues the bio_post_read_ctx on a
711workqueue, and then the workqueue work does the decryption or
712verification. Finally, folios where no decryption or verity error
713occurred are marked Uptodate, and the folios are unlocked.
714
715On many filesystems, files can contain holes. Normally,
716``->readahead()`` simply zeroes hole blocks and considers the
717corresponding data to be up-to-date; no bios are issued. To prevent
718this case from bypassing fs-verity, filesystems use
719fsverity_verify_blocks() to verify hole blocks.
720
721Filesystems also disable direct I/O on verity files, since otherwise
722direct I/O would bypass fs-verity.
723
724Userspace utility
725=================
726
727This document focuses on the kernel, but a userspace utility for
728fs-verity can be found at:
729
730 https://git.kernel.org/pub/scm/fs/fsverity/fsverity-utils.git
731
732See the README.md file in the fsverity-utils source tree for details,
733including examples of setting up fs-verity protected files.
734
735Tests
736=====
737
738To test fs-verity, use xfstests. For example, using `kvm-xfstests
739<https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-quickstart.md>`_::
740
741 kvm-xfstests -c ext4,f2fs,btrfs -g verity
742
743FAQ
744===
745
746This section answers frequently asked questions about fs-verity that
747weren't already directly answered in other parts of this document.
748
749:Q: Why isn't fs-verity part of IMA?
750:A: fs-verity and IMA (Integrity Measurement Architecture) have
751 different focuses. fs-verity is a filesystem-level mechanism for
752 hashing individual files using a Merkle tree. In contrast, IMA
753 specifies a system-wide policy that specifies which files are
754 hashed and what to do with those hashes, such as log them,
755 authenticate them, or add them to a measurement list.
756
757 IMA supports the fs-verity hashing mechanism as an alternative
758 to full file hashes, for those who want the performance and
759 security benefits of the Merkle tree based hash. However, it
760 doesn't make sense to force all uses of fs-verity to be through
761 IMA. fs-verity already meets many users' needs even as a
762 standalone filesystem feature, and it's testable like other
763 filesystem features e.g. with xfstests.
764
765:Q: Isn't fs-verity useless because the attacker can just modify the
766 hashes in the Merkle tree, which is stored on-disk?
767:A: To verify the authenticity of an fs-verity file you must verify
768 the authenticity of the "fs-verity file digest", which
769 incorporates the root hash of the Merkle tree. See `Use cases`_.
770
771:Q: Isn't fs-verity useless because the attacker can just replace a
772 verity file with a non-verity one?
773:A: See `Use cases`_. In the initial use case, it's really trusted
774 userspace code that authenticates the files; fs-verity is just a
775 tool to do this job efficiently and securely. The trusted
776 userspace code will consider non-verity files to be inauthentic.
777
778:Q: Why does the Merkle tree need to be stored on-disk? Couldn't you
779 store just the root hash?
780:A: If the Merkle tree wasn't stored on-disk, then you'd have to
781 compute the entire tree when the file is first accessed, even if
782 just one byte is being read. This is a fundamental consequence of
783 how Merkle tree hashing works. To verify a leaf node, you need to
784 verify the whole path to the root hash, including the root node
785 (the thing which the root hash is a hash of). But if the root
786 node isn't stored on-disk, you have to compute it by hashing its
787 children, and so on until you've actually hashed the entire file.
788
789 That defeats most of the point of doing a Merkle tree-based hash,
790 since if you have to hash the whole file ahead of time anyway,
791 then you could simply do sha256(file) instead. That would be much
792 simpler, and a bit faster too.
793
794 It's true that an in-memory Merkle tree could still provide the
795 advantage of verification on every read rather than just on the
796 first read. However, it would be inefficient because every time a
797 hash page gets evicted (you can't pin the entire Merkle tree into
798 memory, since it may be very large), in order to restore it you
799 again need to hash everything below it in the tree. This again
800 defeats most of the point of doing a Merkle tree-based hash, since
801 a single block read could trigger re-hashing gigabytes of data.
802
803:Q: But couldn't you store just the leaf nodes and compute the rest?
804:A: See previous answer; this really just moves up one level, since
805 one could alternatively interpret the data blocks as being the
806 leaf nodes of the Merkle tree. It's true that the tree can be
807 computed much faster if the leaf level is stored rather than just
808 the data, but that's only because each level is less than 1% the
809 size of the level below (assuming the recommended settings of
810 SHA-256 and 4K blocks). For the exact same reason, by storing
811 "just the leaf nodes" you'd already be storing over 99% of the
812 tree, so you might as well simply store the whole tree.
813
814:Q: Can the Merkle tree be built ahead of time, e.g. distributed as
815 part of a package that is installed to many computers?
816:A: This isn't currently supported. It was part of the original
817 design, but was removed to simplify the kernel UAPI and because it
818 wasn't a critical use case. Files are usually installed once and
819 used many times, and cryptographic hashing is somewhat fast on
820 most modern processors.
821
822:Q: Why doesn't fs-verity support writes?
823:A: Write support would be very difficult and would require a
824 completely different design, so it's well outside the scope of
825 fs-verity. Write support would require:
826
827 - A way to maintain consistency between the data and hashes,
828 including all levels of hashes, since corruption after a crash
829 (especially of potentially the entire file!) is unacceptable.
830 The main options for solving this are data journalling,
831 copy-on-write, and log-structured volume. But it's very hard to
832 retrofit existing filesystems with new consistency mechanisms.
833 Data journalling is available on ext4, but is very slow.
834
835 - Rebuilding the Merkle tree after every write, which would be
836 extremely inefficient. Alternatively, a different authenticated
837 dictionary structure such as an "authenticated skiplist" could
838 be used. However, this would be far more complex.
839
840 Compare it to dm-verity vs. dm-integrity. dm-verity is very
841 simple: the kernel just verifies read-only data against a
842 read-only Merkle tree. In contrast, dm-integrity supports writes
843 but is slow, is much more complex, and doesn't actually support
844 full-device authentication since it authenticates each sector
845 independently, i.e. there is no "root hash". It doesn't really
846 make sense for the same device-mapper target to support these two
847 very different cases; the same applies to fs-verity.
848
849:Q: Since verity files are immutable, why isn't the immutable bit set?
850:A: The existing "immutable" bit (FS_IMMUTABLE_FL) already has a
851 specific set of semantics which not only make the file contents
852 read-only, but also prevent the file from being deleted, renamed,
853 linked to, or having its owner or mode changed. These extra
854 properties are unwanted for fs-verity, so reusing the immutable
855 bit isn't appropriate.
856
857:Q: Why does the API use ioctls instead of setxattr() and getxattr()?
858:A: Abusing the xattr interface for basically arbitrary syscalls is
859 heavily frowned upon by most of the Linux filesystem developers.
860 An xattr should really just be an xattr on-disk, not an API to
861 e.g. magically trigger construction of a Merkle tree.
862
863:Q: Does fs-verity support remote filesystems?
864:A: So far all filesystems that have implemented fs-verity support are
865 local filesystems, but in principle any filesystem that can store
866 per-file verity metadata can support fs-verity, regardless of
867 whether it's local or remote. Some filesystems may have fewer
868 options of where to store the verity metadata; one possibility is
869 to store it past the end of the file and "hide" it from userspace
870 by manipulating i_size. The data verification functions provided
871 by ``fs/verity/`` also assume that the filesystem uses the Linux
872 pagecache, but both local and remote filesystems normally do so.
873
874:Q: Why is anything filesystem-specific at all? Shouldn't fs-verity
875 be implemented entirely at the VFS level?
876:A: There are many reasons why this is not possible or would be very
877 difficult, including the following:
878
879 - To prevent bypassing verification, folios must not be marked
880 Uptodate until they've been verified. Currently, each
881 filesystem is responsible for marking folios Uptodate via
882 ``->readahead()``. Therefore, currently it's not possible for
883 the VFS to do the verification on its own. Changing this would
884 require significant changes to the VFS and all filesystems.
885
886 - It would require defining a filesystem-independent way to store
887 the verity metadata. Extended attributes don't work for this
888 because (a) the Merkle tree may be gigabytes, but many
889 filesystems assume that all xattrs fit into a single 4K
890 filesystem block, and (b) ext4 and f2fs encryption doesn't
891 encrypt xattrs, yet the Merkle tree *must* be encrypted when the
892 file contents are, because it stores hashes of the plaintext
893 file contents.
894
895 So the verity metadata would have to be stored in an actual
896 file. Using a separate file would be very ugly, since the
897 metadata is fundamentally part of the file to be protected, and
898 it could cause problems where users could delete the real file
899 but not the metadata file or vice versa. On the other hand,
900 having it be in the same file would break applications unless
901 filesystems' notion of i_size were divorced from the VFS's,
902 which would be complex and require changes to all filesystems.
903
904 - It's desirable that FS_IOC_ENABLE_VERITY uses the filesystem's
905 transaction mechanism so that either the file ends up with
906 verity enabled, or no changes were made. Allowing intermediate
907 states to occur after a crash may cause problems.
1.. SPDX-License-Identifier: GPL-2.0
2
3.. _fsverity:
4
5=======================================================
6fs-verity: read-only file-based authenticity protection
7=======================================================
8
9Introduction
10============
11
12fs-verity (``fs/verity/``) is a support layer that filesystems can
13hook into to support transparent integrity and authenticity protection
14of read-only files. Currently, it is supported by the ext4, f2fs, and
15btrfs filesystems. Like fscrypt, not too much filesystem-specific
16code is needed to support fs-verity.
17
18fs-verity is similar to `dm-verity
19<https://www.kernel.org/doc/Documentation/device-mapper/verity.txt>`_
20but works on files rather than block devices. On regular files on
21filesystems supporting fs-verity, userspace can execute an ioctl that
22causes the filesystem to build a Merkle tree for the file and persist
23it to a filesystem-specific location associated with the file.
24
25After this, the file is made readonly, and all reads from the file are
26automatically verified against the file's Merkle tree. Reads of any
27corrupted data, including mmap reads, will fail.
28
29Userspace can use another ioctl to retrieve the root hash (actually
30the "fs-verity file digest", which is a hash that includes the Merkle
31tree root hash) that fs-verity is enforcing for the file. This ioctl
32executes in constant time, regardless of the file size.
33
34fs-verity is essentially a way to hash a file in constant time,
35subject to the caveat that reads which would violate the hash will
36fail at runtime.
37
38Use cases
39=========
40
41By itself, the base fs-verity feature only provides integrity
42protection, i.e. detection of accidental (non-malicious) corruption.
43
44However, because fs-verity makes retrieving the file hash extremely
45efficient, it's primarily meant to be used as a tool to support
46authentication (detection of malicious modifications) or auditing
47(logging file hashes before use).
48
49Trusted userspace code (e.g. operating system code running on a
50read-only partition that is itself authenticated by dm-verity) can
51authenticate the contents of an fs-verity file by using the
52`FS_IOC_MEASURE_VERITY`_ ioctl to retrieve its hash, then verifying a
53digital signature of it.
54
55A standard file hash could be used instead of fs-verity. However,
56this is inefficient if the file is large and only a small portion may
57be accessed. This is often the case for Android application package
58(APK) files, for example. These typically contain many translations,
59classes, and other resources that are infrequently or even never
60accessed on a particular device. It would be slow and wasteful to
61read and hash the entire file before starting the application.
62
63Unlike an ahead-of-time hash, fs-verity also re-verifies data each
64time it's paged in. This ensures that malicious disk firmware can't
65undetectably change the contents of the file at runtime.
66
67fs-verity does not replace or obsolete dm-verity. dm-verity should
68still be used on read-only filesystems. fs-verity is for files that
69must live on a read-write filesystem because they are independently
70updated and potentially user-installed, so dm-verity cannot be used.
71
72The base fs-verity feature is a hashing mechanism only; actually
73authenticating the files may be done by:
74
75* Userspace-only
76
77* Builtin signature verification + userspace policy
78
79 fs-verity optionally supports a simple signature verification
80 mechanism where users can configure the kernel to require that
81 all fs-verity files be signed by a key loaded into a keyring;
82 see `Built-in signature verification`_.
83
84* Integrity Measurement Architecture (IMA)
85
86 IMA supports including fs-verity file digests and signatures in the
87 IMA measurement list and verifying fs-verity based file signatures
88 stored as security.ima xattrs, based on policy.
89
90
91User API
92========
93
94FS_IOC_ENABLE_VERITY
95--------------------
96
97The FS_IOC_ENABLE_VERITY ioctl enables fs-verity on a file. It takes
98in a pointer to a struct fsverity_enable_arg, defined as
99follows::
100
101 struct fsverity_enable_arg {
102 __u32 version;
103 __u32 hash_algorithm;
104 __u32 block_size;
105 __u32 salt_size;
106 __u64 salt_ptr;
107 __u32 sig_size;
108 __u32 __reserved1;
109 __u64 sig_ptr;
110 __u64 __reserved2[11];
111 };
112
113This structure contains the parameters of the Merkle tree to build for
114the file, and optionally contains a signature. It must be initialized
115as follows:
116
117- ``version`` must be 1.
118- ``hash_algorithm`` must be the identifier for the hash algorithm to
119 use for the Merkle tree, such as FS_VERITY_HASH_ALG_SHA256. See
120 ``include/uapi/linux/fsverity.h`` for the list of possible values.
121- ``block_size`` must be the Merkle tree block size. Currently, this
122 must be equal to the system page size, which is usually 4096 bytes.
123 Other sizes may be supported in the future. This value is not
124 necessarily the same as the filesystem block size.
125- ``salt_size`` is the size of the salt in bytes, or 0 if no salt is
126 provided. The salt is a value that is prepended to every hashed
127 block; it can be used to personalize the hashing for a particular
128 file or device. Currently the maximum salt size is 32 bytes.
129- ``salt_ptr`` is the pointer to the salt, or NULL if no salt is
130 provided.
131- ``sig_size`` is the size of the signature in bytes, or 0 if no
132 signature is provided. Currently the signature is (somewhat
133 arbitrarily) limited to 16128 bytes. See `Built-in signature
134 verification`_ for more information.
135- ``sig_ptr`` is the pointer to the signature, or NULL if no
136 signature is provided.
137- All reserved fields must be zeroed.
138
139FS_IOC_ENABLE_VERITY causes the filesystem to build a Merkle tree for
140the file and persist it to a filesystem-specific location associated
141with the file, then mark the file as a verity file. This ioctl may
142take a long time to execute on large files, and it is interruptible by
143fatal signals.
144
145FS_IOC_ENABLE_VERITY checks for write access to the inode. However,
146it must be executed on an O_RDONLY file descriptor and no processes
147can have the file open for writing. Attempts to open the file for
148writing while this ioctl is executing will fail with ETXTBSY. (This
149is necessary to guarantee that no writable file descriptors will exist
150after verity is enabled, and to guarantee that the file's contents are
151stable while the Merkle tree is being built over it.)
152
153On success, FS_IOC_ENABLE_VERITY returns 0, and the file becomes a
154verity file. On failure (including the case of interruption by a
155fatal signal), no changes are made to the file.
156
157FS_IOC_ENABLE_VERITY can fail with the following errors:
158
159- ``EACCES``: the process does not have write access to the file
160- ``EBADMSG``: the signature is malformed
161- ``EBUSY``: this ioctl is already running on the file
162- ``EEXIST``: the file already has verity enabled
163- ``EFAULT``: the caller provided inaccessible memory
164- ``EINTR``: the operation was interrupted by a fatal signal
165- ``EINVAL``: unsupported version, hash algorithm, or block size; or
166 reserved bits are set; or the file descriptor refers to neither a
167 regular file nor a directory.
168- ``EISDIR``: the file descriptor refers to a directory
169- ``EKEYREJECTED``: the signature doesn't match the file
170- ``EMSGSIZE``: the salt or signature is too long
171- ``ENOKEY``: the fs-verity keyring doesn't contain the certificate
172 needed to verify the signature
173- ``ENOPKG``: fs-verity recognizes the hash algorithm, but it's not
174 available in the kernel's crypto API as currently configured (e.g.
175 for SHA-512, missing CONFIG_CRYPTO_SHA512).
176- ``ENOTTY``: this type of filesystem does not implement fs-verity
177- ``EOPNOTSUPP``: the kernel was not configured with fs-verity
178 support; or the filesystem superblock has not had the 'verity'
179 feature enabled on it; or the filesystem does not support fs-verity
180 on this file. (See `Filesystem support`_.)
181- ``EPERM``: the file is append-only; or, a signature is required and
182 one was not provided.
183- ``EROFS``: the filesystem is read-only
184- ``ETXTBSY``: someone has the file open for writing. This can be the
185 caller's file descriptor, another open file descriptor, or the file
186 reference held by a writable memory map.
187
188FS_IOC_MEASURE_VERITY
189---------------------
190
191The FS_IOC_MEASURE_VERITY ioctl retrieves the digest of a verity file.
192The fs-verity file digest is a cryptographic digest that identifies
193the file contents that are being enforced on reads; it is computed via
194a Merkle tree and is different from a traditional full-file digest.
195
196This ioctl takes in a pointer to a variable-length structure::
197
198 struct fsverity_digest {
199 __u16 digest_algorithm;
200 __u16 digest_size; /* input/output */
201 __u8 digest[];
202 };
203
204``digest_size`` is an input/output field. On input, it must be
205initialized to the number of bytes allocated for the variable-length
206``digest`` field.
207
208On success, 0 is returned and the kernel fills in the structure as
209follows:
210
211- ``digest_algorithm`` will be the hash algorithm used for the file
212 digest. It will match ``fsverity_enable_arg::hash_algorithm``.
213- ``digest_size`` will be the size of the digest in bytes, e.g. 32
214 for SHA-256. (This can be redundant with ``digest_algorithm``.)
215- ``digest`` will be the actual bytes of the digest.
216
217FS_IOC_MEASURE_VERITY is guaranteed to execute in constant time,
218regardless of the size of the file.
219
220FS_IOC_MEASURE_VERITY can fail with the following errors:
221
222- ``EFAULT``: the caller provided inaccessible memory
223- ``ENODATA``: the file is not a verity file
224- ``ENOTTY``: this type of filesystem does not implement fs-verity
225- ``EOPNOTSUPP``: the kernel was not configured with fs-verity
226 support, or the filesystem superblock has not had the 'verity'
227 feature enabled on it. (See `Filesystem support`_.)
228- ``EOVERFLOW``: the digest is longer than the specified
229 ``digest_size`` bytes. Try providing a larger buffer.
230
231FS_IOC_READ_VERITY_METADATA
232---------------------------
233
234The FS_IOC_READ_VERITY_METADATA ioctl reads verity metadata from a
235verity file. This ioctl is available since Linux v5.12.
236
237This ioctl allows writing a server program that takes a verity file
238and serves it to a client program, such that the client can do its own
239fs-verity compatible verification of the file. This only makes sense
240if the client doesn't trust the server and if the server needs to
241provide the storage for the client.
242
243This is a fairly specialized use case, and most fs-verity users won't
244need this ioctl.
245
246This ioctl takes in a pointer to the following structure::
247
248 #define FS_VERITY_METADATA_TYPE_MERKLE_TREE 1
249 #define FS_VERITY_METADATA_TYPE_DESCRIPTOR 2
250 #define FS_VERITY_METADATA_TYPE_SIGNATURE 3
251
252 struct fsverity_read_metadata_arg {
253 __u64 metadata_type;
254 __u64 offset;
255 __u64 length;
256 __u64 buf_ptr;
257 __u64 __reserved;
258 };
259
260``metadata_type`` specifies the type of metadata to read:
261
262- ``FS_VERITY_METADATA_TYPE_MERKLE_TREE`` reads the blocks of the
263 Merkle tree. The blocks are returned in order from the root level
264 to the leaf level. Within each level, the blocks are returned in
265 the same order that their hashes are themselves hashed.
266 See `Merkle tree`_ for more information.
267
268- ``FS_VERITY_METADATA_TYPE_DESCRIPTOR`` reads the fs-verity
269 descriptor. See `fs-verity descriptor`_.
270
271- ``FS_VERITY_METADATA_TYPE_SIGNATURE`` reads the signature which was
272 passed to FS_IOC_ENABLE_VERITY, if any. See `Built-in signature
273 verification`_.
274
275The semantics are similar to those of ``pread()``. ``offset``
276specifies the offset in bytes into the metadata item to read from, and
277``length`` specifies the maximum number of bytes to read from the
278metadata item. ``buf_ptr`` is the pointer to the buffer to read into,
279cast to a 64-bit integer. ``__reserved`` must be 0. On success, the
280number of bytes read is returned. 0 is returned at the end of the
281metadata item. The returned length may be less than ``length``, for
282example if the ioctl is interrupted.
283
284The metadata returned by FS_IOC_READ_VERITY_METADATA isn't guaranteed
285to be authenticated against the file digest that would be returned by
286`FS_IOC_MEASURE_VERITY`_, as the metadata is expected to be used to
287implement fs-verity compatible verification anyway (though absent a
288malicious disk, the metadata will indeed match). E.g. to implement
289this ioctl, the filesystem is allowed to just read the Merkle tree
290blocks from disk without actually verifying the path to the root node.
291
292FS_IOC_READ_VERITY_METADATA can fail with the following errors:
293
294- ``EFAULT``: the caller provided inaccessible memory
295- ``EINTR``: the ioctl was interrupted before any data was read
296- ``EINVAL``: reserved fields were set, or ``offset + length``
297 overflowed
298- ``ENODATA``: the file is not a verity file, or
299 FS_VERITY_METADATA_TYPE_SIGNATURE was requested but the file doesn't
300 have a built-in signature
301- ``ENOTTY``: this type of filesystem does not implement fs-verity, or
302 this ioctl is not yet implemented on it
303- ``EOPNOTSUPP``: the kernel was not configured with fs-verity
304 support, or the filesystem superblock has not had the 'verity'
305 feature enabled on it. (See `Filesystem support`_.)
306
307FS_IOC_GETFLAGS
308---------------
309
310The existing ioctl FS_IOC_GETFLAGS (which isn't specific to fs-verity)
311can also be used to check whether a file has fs-verity enabled or not.
312To do so, check for FS_VERITY_FL (0x00100000) in the returned flags.
313
314The verity flag is not settable via FS_IOC_SETFLAGS. You must use
315FS_IOC_ENABLE_VERITY instead, since parameters must be provided.
316
317statx
318-----
319
320Since Linux v5.5, the statx() system call sets STATX_ATTR_VERITY if
321the file has fs-verity enabled. This can perform better than
322FS_IOC_GETFLAGS and FS_IOC_MEASURE_VERITY because it doesn't require
323opening the file, and opening verity files can be expensive.
324
325Accessing verity files
326======================
327
328Applications can transparently access a verity file just like a
329non-verity one, with the following exceptions:
330
331- Verity files are readonly. They cannot be opened for writing or
332 truncate()d, even if the file mode bits allow it. Attempts to do
333 one of these things will fail with EPERM. However, changes to
334 metadata such as owner, mode, timestamps, and xattrs are still
335 allowed, since these are not measured by fs-verity. Verity files
336 can also still be renamed, deleted, and linked to.
337
338- Direct I/O is not supported on verity files. Attempts to use direct
339 I/O on such files will fall back to buffered I/O.
340
341- DAX (Direct Access) is not supported on verity files, because this
342 would circumvent the data verification.
343
344- Reads of data that doesn't match the verity Merkle tree will fail
345 with EIO (for read()) or SIGBUS (for mmap() reads).
346
347- If the sysctl "fs.verity.require_signatures" is set to 1 and the
348 file is not signed by a key in the fs-verity keyring, then opening
349 the file will fail. See `Built-in signature verification`_.
350
351Direct access to the Merkle tree is not supported. Therefore, if a
352verity file is copied, or is backed up and restored, then it will lose
353its "verity"-ness. fs-verity is primarily meant for files like
354executables that are managed by a package manager.
355
356File digest computation
357=======================
358
359This section describes how fs-verity hashes the file contents using a
360Merkle tree to produce the digest which cryptographically identifies
361the file contents. This algorithm is the same for all filesystems
362that support fs-verity.
363
364Userspace only needs to be aware of this algorithm if it needs to
365compute fs-verity file digests itself, e.g. in order to sign files.
366
367.. _fsverity_merkle_tree:
368
369Merkle tree
370-----------
371
372The file contents is divided into blocks, where the block size is
373configurable but is usually 4096 bytes. The end of the last block is
374zero-padded if needed. Each block is then hashed, producing the first
375level of hashes. Then, the hashes in this first level are grouped
376into 'blocksize'-byte blocks (zero-padding the ends as needed) and
377these blocks are hashed, producing the second level of hashes. This
378proceeds up the tree until only a single block remains. The hash of
379this block is the "Merkle tree root hash".
380
381If the file fits in one block and is nonempty, then the "Merkle tree
382root hash" is simply the hash of the single data block. If the file
383is empty, then the "Merkle tree root hash" is all zeroes.
384
385The "blocks" here are not necessarily the same as "filesystem blocks".
386
387If a salt was specified, then it's zero-padded to the closest multiple
388of the input size of the hash algorithm's compression function, e.g.
38964 bytes for SHA-256 or 128 bytes for SHA-512. The padded salt is
390prepended to every data or Merkle tree block that is hashed.
391
392The purpose of the block padding is to cause every hash to be taken
393over the same amount of data, which simplifies the implementation and
394keeps open more possibilities for hardware acceleration. The purpose
395of the salt padding is to make the salting "free" when the salted hash
396state is precomputed, then imported for each hash.
397
398Example: in the recommended configuration of SHA-256 and 4K blocks,
399128 hash values fit in each block. Thus, each level of the Merkle
400tree is approximately 128 times smaller than the previous, and for
401large files the Merkle tree's size converges to approximately 1/127 of
402the original file size. However, for small files, the padding is
403significant, making the space overhead proportionally more.
404
405.. _fsverity_descriptor:
406
407fs-verity descriptor
408--------------------
409
410By itself, the Merkle tree root hash is ambiguous. For example, it
411can't a distinguish a large file from a small second file whose data
412is exactly the top-level hash block of the first file. Ambiguities
413also arise from the convention of padding to the next block boundary.
414
415To solve this problem, the fs-verity file digest is actually computed
416as a hash of the following structure, which contains the Merkle tree
417root hash as well as other fields such as the file size::
418
419 struct fsverity_descriptor {
420 __u8 version; /* must be 1 */
421 __u8 hash_algorithm; /* Merkle tree hash algorithm */
422 __u8 log_blocksize; /* log2 of size of data and tree blocks */
423 __u8 salt_size; /* size of salt in bytes; 0 if none */
424 __le32 __reserved_0x04; /* must be 0 */
425 __le64 data_size; /* size of file the Merkle tree is built over */
426 __u8 root_hash[64]; /* Merkle tree root hash */
427 __u8 salt[32]; /* salt prepended to each hashed block */
428 __u8 __reserved[144]; /* must be 0's */
429 };
430
431Built-in signature verification
432===============================
433
434With CONFIG_FS_VERITY_BUILTIN_SIGNATURES=y, fs-verity supports putting
435a portion of an authentication policy (see `Use cases`_) in the
436kernel. Specifically, it adds support for:
437
4381. At fs-verity module initialization time, a keyring ".fs-verity" is
439 created. The root user can add trusted X.509 certificates to this
440 keyring using the add_key() system call, then (when done)
441 optionally use keyctl_restrict_keyring() to prevent additional
442 certificates from being added.
443
4442. `FS_IOC_ENABLE_VERITY`_ accepts a pointer to a PKCS#7 formatted
445 detached signature in DER format of the file's fs-verity digest.
446 On success, this signature is persisted alongside the Merkle tree.
447 Then, any time the file is opened, the kernel will verify the
448 file's actual digest against this signature, using the certificates
449 in the ".fs-verity" keyring.
450
4513. A new sysctl "fs.verity.require_signatures" is made available.
452 When set to 1, the kernel requires that all verity files have a
453 correctly signed digest as described in (2).
454
455fs-verity file digests must be signed in the following format, which
456is similar to the structure used by `FS_IOC_MEASURE_VERITY`_::
457
458 struct fsverity_formatted_digest {
459 char magic[8]; /* must be "FSVerity" */
460 __le16 digest_algorithm;
461 __le16 digest_size;
462 __u8 digest[];
463 };
464
465fs-verity's built-in signature verification support is meant as a
466relatively simple mechanism that can be used to provide some level of
467authenticity protection for verity files, as an alternative to doing
468the signature verification in userspace or using IMA-appraisal.
469However, with this mechanism, userspace programs still need to check
470that the verity bit is set, and there is no protection against verity
471files being swapped around.
472
473Filesystem support
474==================
475
476fs-verity is supported by several filesystems, described below. The
477CONFIG_FS_VERITY kconfig option must be enabled to use fs-verity on
478any of these filesystems.
479
480``include/linux/fsverity.h`` declares the interface between the
481``fs/verity/`` support layer and filesystems. Briefly, filesystems
482must provide an ``fsverity_operations`` structure that provides
483methods to read and write the verity metadata to a filesystem-specific
484location, including the Merkle tree blocks and
485``fsverity_descriptor``. Filesystems must also call functions in
486``fs/verity/`` at certain times, such as when a file is opened or when
487pages have been read into the pagecache. (See `Verifying data`_.)
488
489ext4
490----
491
492ext4 supports fs-verity since Linux v5.4 and e2fsprogs v1.45.2.
493
494To create verity files on an ext4 filesystem, the filesystem must have
495been formatted with ``-O verity`` or had ``tune2fs -O verity`` run on
496it. "verity" is an RO_COMPAT filesystem feature, so once set, old
497kernels will only be able to mount the filesystem readonly, and old
498versions of e2fsck will be unable to check the filesystem. Moreover,
499currently ext4 only supports mounting a filesystem with the "verity"
500feature when its block size is equal to PAGE_SIZE (often 4096 bytes).
501
502ext4 sets the EXT4_VERITY_FL on-disk inode flag on verity files. It
503can only be set by `FS_IOC_ENABLE_VERITY`_, and it cannot be cleared.
504
505ext4 also supports encryption, which can be used simultaneously with
506fs-verity. In this case, the plaintext data is verified rather than
507the ciphertext. This is necessary in order to make the fs-verity file
508digest meaningful, since every file is encrypted differently.
509
510ext4 stores the verity metadata (Merkle tree and fsverity_descriptor)
511past the end of the file, starting at the first 64K boundary beyond
512i_size. This approach works because (a) verity files are readonly,
513and (b) pages fully beyond i_size aren't visible to userspace but can
514be read/written internally by ext4 with only some relatively small
515changes to ext4. This approach avoids having to depend on the
516EA_INODE feature and on rearchitecturing ext4's xattr support to
517support paging multi-gigabyte xattrs into memory, and to support
518encrypting xattrs. Note that the verity metadata *must* be encrypted
519when the file is, since it contains hashes of the plaintext data.
520
521Currently, ext4 verity only supports the case where the Merkle tree
522block size, filesystem block size, and page size are all the same. It
523also only supports extent-based files.
524
525f2fs
526----
527
528f2fs supports fs-verity since Linux v5.4 and f2fs-tools v1.11.0.
529
530To create verity files on an f2fs filesystem, the filesystem must have
531been formatted with ``-O verity``.
532
533f2fs sets the FADVISE_VERITY_BIT on-disk inode flag on verity files.
534It can only be set by `FS_IOC_ENABLE_VERITY`_, and it cannot be
535cleared.
536
537Like ext4, f2fs stores the verity metadata (Merkle tree and
538fsverity_descriptor) past the end of the file, starting at the first
53964K boundary beyond i_size. See explanation for ext4 above.
540Moreover, f2fs supports at most 4096 bytes of xattr entries per inode
541which wouldn't be enough for even a single Merkle tree block.
542
543Currently, f2fs verity only supports a Merkle tree block size of 4096.
544Also, f2fs doesn't support enabling verity on files that currently
545have atomic or volatile writes pending.
546
547btrfs
548-----
549
550btrfs supports fs-verity since Linux v5.15. Verity-enabled inodes are
551marked with a RO_COMPAT inode flag, and the verity metadata is stored
552in separate btree items.
553
554Implementation details
555======================
556
557Verifying data
558--------------
559
560fs-verity ensures that all reads of a verity file's data are verified,
561regardless of which syscall is used to do the read (e.g. mmap(),
562read(), pread()) and regardless of whether it's the first read or a
563later read (unless the later read can return cached data that was
564already verified). Below, we describe how filesystems implement this.
565
566Pagecache
567~~~~~~~~~
568
569For filesystems using Linux's pagecache, the ``->read_folio()`` and
570``->readahead()`` methods must be modified to verify pages before they
571are marked Uptodate. Merely hooking ``->read_iter()`` would be
572insufficient, since ``->read_iter()`` is not used for memory maps.
573
574Therefore, fs/verity/ provides a function fsverity_verify_page() which
575verifies a page that has been read into the pagecache of a verity
576inode, but is still locked and not Uptodate, so it's not yet readable
577by userspace. As needed to do the verification,
578fsverity_verify_page() will call back into the filesystem to read
579Merkle tree pages via fsverity_operations::read_merkle_tree_page().
580
581fsverity_verify_page() returns false if verification failed; in this
582case, the filesystem must not set the page Uptodate. Following this,
583as per the usual Linux pagecache behavior, attempts by userspace to
584read() from the part of the file containing the page will fail with
585EIO, and accesses to the page within a memory map will raise SIGBUS.
586
587fsverity_verify_page() currently only supports the case where the
588Merkle tree block size is equal to PAGE_SIZE (often 4096 bytes).
589
590In principle, fsverity_verify_page() verifies the entire path in the
591Merkle tree from the data page to the root hash. However, for
592efficiency the filesystem may cache the hash pages. Therefore,
593fsverity_verify_page() only ascends the tree reading hash pages until
594an already-verified hash page is seen, as indicated by the PageChecked
595bit being set. It then verifies the path to that page.
596
597This optimization, which is also used by dm-verity, results in
598excellent sequential read performance. This is because usually (e.g.
599127 in 128 times for 4K blocks and SHA-256) the hash page from the
600bottom level of the tree will already be cached and checked from
601reading a previous data page. However, random reads perform worse.
602
603Block device based filesystems
604~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
605
606Block device based filesystems (e.g. ext4 and f2fs) in Linux also use
607the pagecache, so the above subsection applies too. However, they
608also usually read many pages from a file at once, grouped into a
609structure called a "bio". To make it easier for these types of
610filesystems to support fs-verity, fs/verity/ also provides a function
611fsverity_verify_bio() which verifies all pages in a bio.
612
613ext4 and f2fs also support encryption. If a verity file is also
614encrypted, the pages must be decrypted before being verified. To
615support this, these filesystems allocate a "post-read context" for
616each bio and store it in ``->bi_private``::
617
618 struct bio_post_read_ctx {
619 struct bio *bio;
620 struct work_struct work;
621 unsigned int cur_step;
622 unsigned int enabled_steps;
623 };
624
625``enabled_steps`` is a bitmask that specifies whether decryption,
626verity, or both is enabled. After the bio completes, for each needed
627postprocessing step the filesystem enqueues the bio_post_read_ctx on a
628workqueue, and then the workqueue work does the decryption or
629verification. Finally, pages where no decryption or verity error
630occurred are marked Uptodate, and the pages are unlocked.
631
632On many filesystems, files can contain holes. Normally,
633``->readahead()`` simply zeroes holes and sets the corresponding pages
634Uptodate; no bios are issued. To prevent this case from bypassing
635fs-verity, these filesystems use fsverity_verify_page() to verify hole
636pages.
637
638Filesystems also disable direct I/O on verity files, since otherwise
639direct I/O would bypass fs-verity.
640
641Userspace utility
642=================
643
644This document focuses on the kernel, but a userspace utility for
645fs-verity can be found at:
646
647 https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/fsverity-utils.git
648
649See the README.md file in the fsverity-utils source tree for details,
650including examples of setting up fs-verity protected files.
651
652Tests
653=====
654
655To test fs-verity, use xfstests. For example, using `kvm-xfstests
656<https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-quickstart.md>`_::
657
658 kvm-xfstests -c ext4,f2fs,btrfs -g verity
659
660FAQ
661===
662
663This section answers frequently asked questions about fs-verity that
664weren't already directly answered in other parts of this document.
665
666:Q: Why isn't fs-verity part of IMA?
667:A: fs-verity and IMA (Integrity Measurement Architecture) have
668 different focuses. fs-verity is a filesystem-level mechanism for
669 hashing individual files using a Merkle tree. In contrast, IMA
670 specifies a system-wide policy that specifies which files are
671 hashed and what to do with those hashes, such as log them,
672 authenticate them, or add them to a measurement list.
673
674 IMA supports the fs-verity hashing mechanism as an alternative
675 to full file hashes, for those who want the performance and
676 security benefits of the Merkle tree based hash. However, it
677 doesn't make sense to force all uses of fs-verity to be through
678 IMA. fs-verity already meets many users' needs even as a
679 standalone filesystem feature, and it's testable like other
680 filesystem features e.g. with xfstests.
681
682:Q: Isn't fs-verity useless because the attacker can just modify the
683 hashes in the Merkle tree, which is stored on-disk?
684:A: To verify the authenticity of an fs-verity file you must verify
685 the authenticity of the "fs-verity file digest", which
686 incorporates the root hash of the Merkle tree. See `Use cases`_.
687
688:Q: Isn't fs-verity useless because the attacker can just replace a
689 verity file with a non-verity one?
690:A: See `Use cases`_. In the initial use case, it's really trusted
691 userspace code that authenticates the files; fs-verity is just a
692 tool to do this job efficiently and securely. The trusted
693 userspace code will consider non-verity files to be inauthentic.
694
695:Q: Why does the Merkle tree need to be stored on-disk? Couldn't you
696 store just the root hash?
697:A: If the Merkle tree wasn't stored on-disk, then you'd have to
698 compute the entire tree when the file is first accessed, even if
699 just one byte is being read. This is a fundamental consequence of
700 how Merkle tree hashing works. To verify a leaf node, you need to
701 verify the whole path to the root hash, including the root node
702 (the thing which the root hash is a hash of). But if the root
703 node isn't stored on-disk, you have to compute it by hashing its
704 children, and so on until you've actually hashed the entire file.
705
706 That defeats most of the point of doing a Merkle tree-based hash,
707 since if you have to hash the whole file ahead of time anyway,
708 then you could simply do sha256(file) instead. That would be much
709 simpler, and a bit faster too.
710
711 It's true that an in-memory Merkle tree could still provide the
712 advantage of verification on every read rather than just on the
713 first read. However, it would be inefficient because every time a
714 hash page gets evicted (you can't pin the entire Merkle tree into
715 memory, since it may be very large), in order to restore it you
716 again need to hash everything below it in the tree. This again
717 defeats most of the point of doing a Merkle tree-based hash, since
718 a single block read could trigger re-hashing gigabytes of data.
719
720:Q: But couldn't you store just the leaf nodes and compute the rest?
721:A: See previous answer; this really just moves up one level, since
722 one could alternatively interpret the data blocks as being the
723 leaf nodes of the Merkle tree. It's true that the tree can be
724 computed much faster if the leaf level is stored rather than just
725 the data, but that's only because each level is less than 1% the
726 size of the level below (assuming the recommended settings of
727 SHA-256 and 4K blocks). For the exact same reason, by storing
728 "just the leaf nodes" you'd already be storing over 99% of the
729 tree, so you might as well simply store the whole tree.
730
731:Q: Can the Merkle tree be built ahead of time, e.g. distributed as
732 part of a package that is installed to many computers?
733:A: This isn't currently supported. It was part of the original
734 design, but was removed to simplify the kernel UAPI and because it
735 wasn't a critical use case. Files are usually installed once and
736 used many times, and cryptographic hashing is somewhat fast on
737 most modern processors.
738
739:Q: Why doesn't fs-verity support writes?
740:A: Write support would be very difficult and would require a
741 completely different design, so it's well outside the scope of
742 fs-verity. Write support would require:
743
744 - A way to maintain consistency between the data and hashes,
745 including all levels of hashes, since corruption after a crash
746 (especially of potentially the entire file!) is unacceptable.
747 The main options for solving this are data journalling,
748 copy-on-write, and log-structured volume. But it's very hard to
749 retrofit existing filesystems with new consistency mechanisms.
750 Data journalling is available on ext4, but is very slow.
751
752 - Rebuilding the Merkle tree after every write, which would be
753 extremely inefficient. Alternatively, a different authenticated
754 dictionary structure such as an "authenticated skiplist" could
755 be used. However, this would be far more complex.
756
757 Compare it to dm-verity vs. dm-integrity. dm-verity is very
758 simple: the kernel just verifies read-only data against a
759 read-only Merkle tree. In contrast, dm-integrity supports writes
760 but is slow, is much more complex, and doesn't actually support
761 full-device authentication since it authenticates each sector
762 independently, i.e. there is no "root hash". It doesn't really
763 make sense for the same device-mapper target to support these two
764 very different cases; the same applies to fs-verity.
765
766:Q: Since verity files are immutable, why isn't the immutable bit set?
767:A: The existing "immutable" bit (FS_IMMUTABLE_FL) already has a
768 specific set of semantics which not only make the file contents
769 read-only, but also prevent the file from being deleted, renamed,
770 linked to, or having its owner or mode changed. These extra
771 properties are unwanted for fs-verity, so reusing the immutable
772 bit isn't appropriate.
773
774:Q: Why does the API use ioctls instead of setxattr() and getxattr()?
775:A: Abusing the xattr interface for basically arbitrary syscalls is
776 heavily frowned upon by most of the Linux filesystem developers.
777 An xattr should really just be an xattr on-disk, not an API to
778 e.g. magically trigger construction of a Merkle tree.
779
780:Q: Does fs-verity support remote filesystems?
781:A: So far all filesystems that have implemented fs-verity support are
782 local filesystems, but in principle any filesystem that can store
783 per-file verity metadata can support fs-verity, regardless of
784 whether it's local or remote. Some filesystems may have fewer
785 options of where to store the verity metadata; one possibility is
786 to store it past the end of the file and "hide" it from userspace
787 by manipulating i_size. The data verification functions provided
788 by ``fs/verity/`` also assume that the filesystem uses the Linux
789 pagecache, but both local and remote filesystems normally do so.
790
791:Q: Why is anything filesystem-specific at all? Shouldn't fs-verity
792 be implemented entirely at the VFS level?
793:A: There are many reasons why this is not possible or would be very
794 difficult, including the following:
795
796 - To prevent bypassing verification, pages must not be marked
797 Uptodate until they've been verified. Currently, each
798 filesystem is responsible for marking pages Uptodate via
799 ``->readahead()``. Therefore, currently it's not possible for
800 the VFS to do the verification on its own. Changing this would
801 require significant changes to the VFS and all filesystems.
802
803 - It would require defining a filesystem-independent way to store
804 the verity metadata. Extended attributes don't work for this
805 because (a) the Merkle tree may be gigabytes, but many
806 filesystems assume that all xattrs fit into a single 4K
807 filesystem block, and (b) ext4 and f2fs encryption doesn't
808 encrypt xattrs, yet the Merkle tree *must* be encrypted when the
809 file contents are, because it stores hashes of the plaintext
810 file contents.
811
812 So the verity metadata would have to be stored in an actual
813 file. Using a separate file would be very ugly, since the
814 metadata is fundamentally part of the file to be protected, and
815 it could cause problems where users could delete the real file
816 but not the metadata file or vice versa. On the other hand,
817 having it be in the same file would break applications unless
818 filesystems' notion of i_size were divorced from the VFS's,
819 which would be complex and require changes to all filesystems.
820
821 - It's desirable that FS_IOC_ENABLE_VERITY uses the filesystem's
822 transaction mechanism so that either the file ends up with
823 verity enabled, or no changes were made. Allowing intermediate
824 states to occur after a crash may cause problems.