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1 ============================
2 KERNEL KEY RETENTION SERVICE
3 ============================
4
5This service allows cryptographic keys, authentication tokens, cross-domain
6user mappings, and similar to be cached in the kernel for the use of
7filesystems and other kernel services.
8
9Keyrings are permitted; these are a special type of key that can hold links to
10other keys. Processes each have three standard keyring subscriptions that a
11kernel service can search for relevant keys.
12
13The key service can be configured on by enabling:
14
15 "Security options"/"Enable access key retention support" (CONFIG_KEYS)
16
17This document has the following sections:
18
19 - Key overview
20 - Key service overview
21 - Key access permissions
22 - SELinux support
23 - New procfs files
24 - Userspace system call interface
25 - Kernel services
26 - Notes on accessing payload contents
27 - Defining a key type
28 - Request-key callback service
29 - Garbage collection
30
31
32============
33KEY OVERVIEW
34============
35
36In this context, keys represent units of cryptographic data, authentication
37tokens, keyrings, etc.. These are represented in the kernel by struct key.
38
39Each key has a number of attributes:
40
41 - A serial number.
42 - A type.
43 - A description (for matching a key in a search).
44 - Access control information.
45 - An expiry time.
46 - A payload.
47 - State.
48
49
50 (*) Each key is issued a serial number of type key_serial_t that is unique for
51 the lifetime of that key. All serial numbers are positive non-zero 32-bit
52 integers.
53
54 Userspace programs can use a key's serial numbers as a way to gain access
55 to it, subject to permission checking.
56
57 (*) Each key is of a defined "type". Types must be registered inside the
58 kernel by a kernel service (such as a filesystem) before keys of that type
59 can be added or used. Userspace programs cannot define new types directly.
60
61 Key types are represented in the kernel by struct key_type. This defines a
62 number of operations that can be performed on a key of that type.
63
64 Should a type be removed from the system, all the keys of that type will
65 be invalidated.
66
67 (*) Each key has a description. This should be a printable string. The key
68 type provides an operation to perform a match between the description on a
69 key and a criterion string.
70
71 (*) Each key has an owner user ID, a group ID and a permissions mask. These
72 are used to control what a process may do to a key from userspace, and
73 whether a kernel service will be able to find the key.
74
75 (*) Each key can be set to expire at a specific time by the key type's
76 instantiation function. Keys can also be immortal.
77
78 (*) Each key can have a payload. This is a quantity of data that represent the
79 actual "key". In the case of a keyring, this is a list of keys to which
80 the keyring links; in the case of a user-defined key, it's an arbitrary
81 blob of data.
82
83 Having a payload is not required; and the payload can, in fact, just be a
84 value stored in the struct key itself.
85
86 When a key is instantiated, the key type's instantiation function is
87 called with a blob of data, and that then creates the key's payload in
88 some way.
89
90 Similarly, when userspace wants to read back the contents of the key, if
91 permitted, another key type operation will be called to convert the key's
92 attached payload back into a blob of data.
93
94 (*) Each key can be in one of a number of basic states:
95
96 (*) Uninstantiated. The key exists, but does not have any data attached.
97 Keys being requested from userspace will be in this state.
98
99 (*) Instantiated. This is the normal state. The key is fully formed, and
100 has data attached.
101
102 (*) Negative. This is a relatively short-lived state. The key acts as a
103 note saying that a previous call out to userspace failed, and acts as
104 a throttle on key lookups. A negative key can be updated to a normal
105 state.
106
107 (*) Expired. Keys can have lifetimes set. If their lifetime is exceeded,
108 they traverse to this state. An expired key can be updated back to a
109 normal state.
110
111 (*) Revoked. A key is put in this state by userspace action. It can't be
112 found or operated upon (apart from by unlinking it).
113
114 (*) Dead. The key's type was unregistered, and so the key is now useless.
115
116Keys in the last three states are subject to garbage collection. See the
117section on "Garbage collection".
118
119
120====================
121KEY SERVICE OVERVIEW
122====================
123
124The key service provides a number of features besides keys:
125
126 (*) The key service defines two special key types:
127
128 (+) "keyring"
129
130 Keyrings are special keys that contain a list of other keys. Keyring
131 lists can be modified using various system calls. Keyrings should not
132 be given a payload when created.
133
134 (+) "user"
135
136 A key of this type has a description and a payload that are arbitrary
137 blobs of data. These can be created, updated and read by userspace,
138 and aren't intended for use by kernel services.
139
140 (*) Each process subscribes to three keyrings: a thread-specific keyring, a
141 process-specific keyring, and a session-specific keyring.
142
143 The thread-specific keyring is discarded from the child when any sort of
144 clone, fork, vfork or execve occurs. A new keyring is created only when
145 required.
146
147 The process-specific keyring is replaced with an empty one in the child on
148 clone, fork, vfork unless CLONE_THREAD is supplied, in which case it is
149 shared. execve also discards the process's process keyring and creates a
150 new one.
151
152 The session-specific keyring is persistent across clone, fork, vfork and
153 execve, even when the latter executes a set-UID or set-GID binary. A
154 process can, however, replace its current session keyring with a new one
155 by using PR_JOIN_SESSION_KEYRING. It is permitted to request an anonymous
156 new one, or to attempt to create or join one of a specific name.
157
158 The ownership of the thread keyring changes when the real UID and GID of
159 the thread changes.
160
161 (*) Each user ID resident in the system holds two special keyrings: a user
162 specific keyring and a default user session keyring. The default session
163 keyring is initialised with a link to the user-specific keyring.
164
165 When a process changes its real UID, if it used to have no session key, it
166 will be subscribed to the default session key for the new UID.
167
168 If a process attempts to access its session key when it doesn't have one,
169 it will be subscribed to the default for its current UID.
170
171 (*) Each user has two quotas against which the keys they own are tracked. One
172 limits the total number of keys and keyrings, the other limits the total
173 amount of description and payload space that can be consumed.
174
175 The user can view information on this and other statistics through procfs
176 files. The root user may also alter the quota limits through sysctl files
177 (see the section "New procfs files").
178
179 Process-specific and thread-specific keyrings are not counted towards a
180 user's quota.
181
182 If a system call that modifies a key or keyring in some way would put the
183 user over quota, the operation is refused and error EDQUOT is returned.
184
185 (*) There's a system call interface by which userspace programs can create and
186 manipulate keys and keyrings.
187
188 (*) There's a kernel interface by which services can register types and search
189 for keys.
190
191 (*) There's a way for the a search done from the kernel to call back to
192 userspace to request a key that can't be found in a process's keyrings.
193
194 (*) An optional filesystem is available through which the key database can be
195 viewed and manipulated.
196
197
198======================
199KEY ACCESS PERMISSIONS
200======================
201
202Keys have an owner user ID, a group access ID, and a permissions mask. The mask
203has up to eight bits each for possessor, user, group and other access. Only
204six of each set of eight bits are defined. These permissions granted are:
205
206 (*) View
207
208 This permits a key or keyring's attributes to be viewed - including key
209 type and description.
210
211 (*) Read
212
213 This permits a key's payload to be viewed or a keyring's list of linked
214 keys.
215
216 (*) Write
217
218 This permits a key's payload to be instantiated or updated, or it allows a
219 link to be added to or removed from a keyring.
220
221 (*) Search
222
223 This permits keyrings to be searched and keys to be found. Searches can
224 only recurse into nested keyrings that have search permission set.
225
226 (*) Link
227
228 This permits a key or keyring to be linked to. To create a link from a
229 keyring to a key, a process must have Write permission on the keyring and
230 Link permission on the key.
231
232 (*) Set Attribute
233
234 This permits a key's UID, GID and permissions mask to be changed.
235
236For changing the ownership, group ID or permissions mask, being the owner of
237the key or having the sysadmin capability is sufficient.
238
239
240===============
241SELINUX SUPPORT
242===============
243
244The security class "key" has been added to SELinux so that mandatory access
245controls can be applied to keys created within various contexts. This support
246is preliminary, and is likely to change quite significantly in the near future.
247Currently, all of the basic permissions explained above are provided in SELinux
248as well; SELinux is simply invoked after all basic permission checks have been
249performed.
250
251The value of the file /proc/self/attr/keycreate influences the labeling of
252newly-created keys. If the contents of that file correspond to an SELinux
253security context, then the key will be assigned that context. Otherwise, the
254key will be assigned the current context of the task that invoked the key
255creation request. Tasks must be granted explicit permission to assign a
256particular context to newly-created keys, using the "create" permission in the
257key security class.
258
259The default keyrings associated with users will be labeled with the default
260context of the user if and only if the login programs have been instrumented to
261properly initialize keycreate during the login process. Otherwise, they will
262be labeled with the context of the login program itself.
263
264Note, however, that the default keyrings associated with the root user are
265labeled with the default kernel context, since they are created early in the
266boot process, before root has a chance to log in.
267
268The keyrings associated with new threads are each labeled with the context of
269their associated thread, and both session and process keyrings are handled
270similarly.
271
272
273================
274NEW PROCFS FILES
275================
276
277Two files have been added to procfs by which an administrator can find out
278about the status of the key service:
279
280 (*) /proc/keys
281
282 This lists the keys that are currently viewable by the task reading the
283 file, giving information about their type, description and permissions.
284 It is not possible to view the payload of the key this way, though some
285 information about it may be given.
286
287 The only keys included in the list are those that grant View permission to
288 the reading process whether or not it possesses them. Note that LSM
289 security checks are still performed, and may further filter out keys that
290 the current process is not authorised to view.
291
292 The contents of the file look like this:
293
294 SERIAL FLAGS USAGE EXPY PERM UID GID TYPE DESCRIPTION: SUMMARY
295 00000001 I----- 39 perm 1f3f0000 0 0 keyring _uid_ses.0: 1/4
296 00000002 I----- 2 perm 1f3f0000 0 0 keyring _uid.0: empty
297 00000007 I----- 1 perm 1f3f0000 0 0 keyring _pid.1: empty
298 0000018d I----- 1 perm 1f3f0000 0 0 keyring _pid.412: empty
299 000004d2 I--Q-- 1 perm 1f3f0000 32 -1 keyring _uid.32: 1/4
300 000004d3 I--Q-- 3 perm 1f3f0000 32 -1 keyring _uid_ses.32: empty
301 00000892 I--QU- 1 perm 1f000000 0 0 user metal:copper: 0
302 00000893 I--Q-N 1 35s 1f3f0000 0 0 user metal:silver: 0
303 00000894 I--Q-- 1 10h 003f0000 0 0 user metal:gold: 0
304
305 The flags are:
306
307 I Instantiated
308 R Revoked
309 D Dead
310 Q Contributes to user's quota
311 U Under construction by callback to userspace
312 N Negative key
313
314 This file must be enabled at kernel configuration time as it allows anyone
315 to list the keys database.
316
317 (*) /proc/key-users
318
319 This file lists the tracking data for each user that has at least one key
320 on the system. Such data includes quota information and statistics:
321
322 [root@andromeda root]# cat /proc/key-users
323 0: 46 45/45 1/100 13/10000
324 29: 2 2/2 2/100 40/10000
325 32: 2 2/2 2/100 40/10000
326 38: 2 2/2 2/100 40/10000
327
328 The format of each line is
329 <UID>: User ID to which this applies
330 <usage> Structure refcount
331 <inst>/<keys> Total number of keys and number instantiated
332 <keys>/<max> Key count quota
333 <bytes>/<max> Key size quota
334
335
336Four new sysctl files have been added also for the purpose of controlling the
337quota limits on keys:
338
339 (*) /proc/sys/kernel/keys/root_maxkeys
340 /proc/sys/kernel/keys/root_maxbytes
341
342 These files hold the maximum number of keys that root may have and the
343 maximum total number of bytes of data that root may have stored in those
344 keys.
345
346 (*) /proc/sys/kernel/keys/maxkeys
347 /proc/sys/kernel/keys/maxbytes
348
349 These files hold the maximum number of keys that each non-root user may
350 have and the maximum total number of bytes of data that each of those
351 users may have stored in their keys.
352
353Root may alter these by writing each new limit as a decimal number string to
354the appropriate file.
355
356
357===============================
358USERSPACE SYSTEM CALL INTERFACE
359===============================
360
361Userspace can manipulate keys directly through three new syscalls: add_key,
362request_key and keyctl. The latter provides a number of functions for
363manipulating keys.
364
365When referring to a key directly, userspace programs should use the key's
366serial number (a positive 32-bit integer). However, there are some special
367values available for referring to special keys and keyrings that relate to the
368process making the call:
369
370 CONSTANT VALUE KEY REFERENCED
371 ============================== ====== ===========================
372 KEY_SPEC_THREAD_KEYRING -1 thread-specific keyring
373 KEY_SPEC_PROCESS_KEYRING -2 process-specific keyring
374 KEY_SPEC_SESSION_KEYRING -3 session-specific keyring
375 KEY_SPEC_USER_KEYRING -4 UID-specific keyring
376 KEY_SPEC_USER_SESSION_KEYRING -5 UID-session keyring
377 KEY_SPEC_GROUP_KEYRING -6 GID-specific keyring
378 KEY_SPEC_REQKEY_AUTH_KEY -7 assumed request_key()
379 authorisation key
380
381
382The main syscalls are:
383
384 (*) Create a new key of given type, description and payload and add it to the
385 nominated keyring:
386
387 key_serial_t add_key(const char *type, const char *desc,
388 const void *payload, size_t plen,
389 key_serial_t keyring);
390
391 If a key of the same type and description as that proposed already exists
392 in the keyring, this will try to update it with the given payload, or it
393 will return error EEXIST if that function is not supported by the key
394 type. The process must also have permission to write to the key to be able
395 to update it. The new key will have all user permissions granted and no
396 group or third party permissions.
397
398 Otherwise, this will attempt to create a new key of the specified type and
399 description, and to instantiate it with the supplied payload and attach it
400 to the keyring. In this case, an error will be generated if the process
401 does not have permission to write to the keyring.
402
403 The payload is optional, and the pointer can be NULL if not required by
404 the type. The payload is plen in size, and plen can be zero for an empty
405 payload.
406
407 A new keyring can be generated by setting type "keyring", the keyring name
408 as the description (or NULL) and setting the payload to NULL.
409
410 User defined keys can be created by specifying type "user". It is
411 recommended that a user defined key's description by prefixed with a type
412 ID and a colon, such as "krb5tgt:" for a Kerberos 5 ticket granting
413 ticket.
414
415 Any other type must have been registered with the kernel in advance by a
416 kernel service such as a filesystem.
417
418 The ID of the new or updated key is returned if successful.
419
420
421 (*) Search the process's keyrings for a key, potentially calling out to
422 userspace to create it.
423
424 key_serial_t request_key(const char *type, const char *description,
425 const char *callout_info,
426 key_serial_t dest_keyring);
427
428 This function searches all the process's keyrings in the order thread,
429 process, session for a matching key. This works very much like
430 KEYCTL_SEARCH, including the optional attachment of the discovered key to
431 a keyring.
432
433 If a key cannot be found, and if callout_info is not NULL, then
434 /sbin/request-key will be invoked in an attempt to obtain a key. The
435 callout_info string will be passed as an argument to the program.
436
437 See also Documentation/security/keys-request-key.txt.
438
439
440The keyctl syscall functions are:
441
442 (*) Map a special key ID to a real key ID for this process:
443
444 key_serial_t keyctl(KEYCTL_GET_KEYRING_ID, key_serial_t id,
445 int create);
446
447 The special key specified by "id" is looked up (with the key being created
448 if necessary) and the ID of the key or keyring thus found is returned if
449 it exists.
450
451 If the key does not yet exist, the key will be created if "create" is
452 non-zero; and the error ENOKEY will be returned if "create" is zero.
453
454
455 (*) Replace the session keyring this process subscribes to with a new one:
456
457 key_serial_t keyctl(KEYCTL_JOIN_SESSION_KEYRING, const char *name);
458
459 If name is NULL, an anonymous keyring is created attached to the process
460 as its session keyring, displacing the old session keyring.
461
462 If name is not NULL, if a keyring of that name exists, the process
463 attempts to attach it as the session keyring, returning an error if that
464 is not permitted; otherwise a new keyring of that name is created and
465 attached as the session keyring.
466
467 To attach to a named keyring, the keyring must have search permission for
468 the process's ownership.
469
470 The ID of the new session keyring is returned if successful.
471
472
473 (*) Update the specified key:
474
475 long keyctl(KEYCTL_UPDATE, key_serial_t key, const void *payload,
476 size_t plen);
477
478 This will try to update the specified key with the given payload, or it
479 will return error EOPNOTSUPP if that function is not supported by the key
480 type. The process must also have permission to write to the key to be able
481 to update it.
482
483 The payload is of length plen, and may be absent or empty as for
484 add_key().
485
486
487 (*) Revoke a key:
488
489 long keyctl(KEYCTL_REVOKE, key_serial_t key);
490
491 This makes a key unavailable for further operations. Further attempts to
492 use the key will be met with error EKEYREVOKED, and the key will no longer
493 be findable.
494
495
496 (*) Change the ownership of a key:
497
498 long keyctl(KEYCTL_CHOWN, key_serial_t key, uid_t uid, gid_t gid);
499
500 This function permits a key's owner and group ID to be changed. Either one
501 of uid or gid can be set to -1 to suppress that change.
502
503 Only the superuser can change a key's owner to something other than the
504 key's current owner. Similarly, only the superuser can change a key's
505 group ID to something other than the calling process's group ID or one of
506 its group list members.
507
508
509 (*) Change the permissions mask on a key:
510
511 long keyctl(KEYCTL_SETPERM, key_serial_t key, key_perm_t perm);
512
513 This function permits the owner of a key or the superuser to change the
514 permissions mask on a key.
515
516 Only bits the available bits are permitted; if any other bits are set,
517 error EINVAL will be returned.
518
519
520 (*) Describe a key:
521
522 long keyctl(KEYCTL_DESCRIBE, key_serial_t key, char *buffer,
523 size_t buflen);
524
525 This function returns a summary of the key's attributes (but not its
526 payload data) as a string in the buffer provided.
527
528 Unless there's an error, it always returns the amount of data it could
529 produce, even if that's too big for the buffer, but it won't copy more
530 than requested to userspace. If the buffer pointer is NULL then no copy
531 will take place.
532
533 A process must have view permission on the key for this function to be
534 successful.
535
536 If successful, a string is placed in the buffer in the following format:
537
538 <type>;<uid>;<gid>;<perm>;<description>
539
540 Where type and description are strings, uid and gid are decimal, and perm
541 is hexadecimal. A NUL character is included at the end of the string if
542 the buffer is sufficiently big.
543
544 This can be parsed with
545
546 sscanf(buffer, "%[^;];%d;%d;%o;%s", type, &uid, &gid, &mode, desc);
547
548
549 (*) Clear out a keyring:
550
551 long keyctl(KEYCTL_CLEAR, key_serial_t keyring);
552
553 This function clears the list of keys attached to a keyring. The calling
554 process must have write permission on the keyring, and it must be a
555 keyring (or else error ENOTDIR will result).
556
557
558 (*) Link a key into a keyring:
559
560 long keyctl(KEYCTL_LINK, key_serial_t keyring, key_serial_t key);
561
562 This function creates a link from the keyring to the key. The process must
563 have write permission on the keyring and must have link permission on the
564 key.
565
566 Should the keyring not be a keyring, error ENOTDIR will result; and if the
567 keyring is full, error ENFILE will result.
568
569 The link procedure checks the nesting of the keyrings, returning ELOOP if
570 it appears too deep or EDEADLK if the link would introduce a cycle.
571
572 Any links within the keyring to keys that match the new key in terms of
573 type and description will be discarded from the keyring as the new one is
574 added.
575
576
577 (*) Unlink a key or keyring from another keyring:
578
579 long keyctl(KEYCTL_UNLINK, key_serial_t keyring, key_serial_t key);
580
581 This function looks through the keyring for the first link to the
582 specified key, and removes it if found. Subsequent links to that key are
583 ignored. The process must have write permission on the keyring.
584
585 If the keyring is not a keyring, error ENOTDIR will result; and if the key
586 is not present, error ENOENT will be the result.
587
588
589 (*) Search a keyring tree for a key:
590
591 key_serial_t keyctl(KEYCTL_SEARCH, key_serial_t keyring,
592 const char *type, const char *description,
593 key_serial_t dest_keyring);
594
595 This searches the keyring tree headed by the specified keyring until a key
596 is found that matches the type and description criteria. Each keyring is
597 checked for keys before recursion into its children occurs.
598
599 The process must have search permission on the top level keyring, or else
600 error EACCES will result. Only keyrings that the process has search
601 permission on will be recursed into, and only keys and keyrings for which
602 a process has search permission can be matched. If the specified keyring
603 is not a keyring, ENOTDIR will result.
604
605 If the search succeeds, the function will attempt to link the found key
606 into the destination keyring if one is supplied (non-zero ID). All the
607 constraints applicable to KEYCTL_LINK apply in this case too.
608
609 Error ENOKEY, EKEYREVOKED or EKEYEXPIRED will be returned if the search
610 fails. On success, the resulting key ID will be returned.
611
612
613 (*) Read the payload data from a key:
614
615 long keyctl(KEYCTL_READ, key_serial_t keyring, char *buffer,
616 size_t buflen);
617
618 This function attempts to read the payload data from the specified key
619 into the buffer. The process must have read permission on the key to
620 succeed.
621
622 The returned data will be processed for presentation by the key type. For
623 instance, a keyring will return an array of key_serial_t entries
624 representing the IDs of all the keys to which it is subscribed. The user
625 defined key type will return its data as is. If a key type does not
626 implement this function, error EOPNOTSUPP will result.
627
628 As much of the data as can be fitted into the buffer will be copied to
629 userspace if the buffer pointer is not NULL.
630
631 On a successful return, the function will always return the amount of data
632 available rather than the amount copied.
633
634
635 (*) Instantiate a partially constructed key.
636
637 long keyctl(KEYCTL_INSTANTIATE, key_serial_t key,
638 const void *payload, size_t plen,
639 key_serial_t keyring);
640 long keyctl(KEYCTL_INSTANTIATE_IOV, key_serial_t key,
641 const struct iovec *payload_iov, unsigned ioc,
642 key_serial_t keyring);
643
644 If the kernel calls back to userspace to complete the instantiation of a
645 key, userspace should use this call to supply data for the key before the
646 invoked process returns, or else the key will be marked negative
647 automatically.
648
649 The process must have write access on the key to be able to instantiate
650 it, and the key must be uninstantiated.
651
652 If a keyring is specified (non-zero), the key will also be linked into
653 that keyring, however all the constraints applying in KEYCTL_LINK apply in
654 this case too.
655
656 The payload and plen arguments describe the payload data as for add_key().
657
658 The payload_iov and ioc arguments describe the payload data in an iovec
659 array instead of a single buffer.
660
661
662 (*) Negatively instantiate a partially constructed key.
663
664 long keyctl(KEYCTL_NEGATE, key_serial_t key,
665 unsigned timeout, key_serial_t keyring);
666 long keyctl(KEYCTL_REJECT, key_serial_t key,
667 unsigned timeout, unsigned error, key_serial_t keyring);
668
669 If the kernel calls back to userspace to complete the instantiation of a
670 key, userspace should use this call mark the key as negative before the
671 invoked process returns if it is unable to fulfil the request.
672
673 The process must have write access on the key to be able to instantiate
674 it, and the key must be uninstantiated.
675
676 If a keyring is specified (non-zero), the key will also be linked into
677 that keyring, however all the constraints applying in KEYCTL_LINK apply in
678 this case too.
679
680 If the key is rejected, future searches for it will return the specified
681 error code until the rejected key expires. Negating the key is the same
682 as rejecting the key with ENOKEY as the error code.
683
684
685 (*) Set the default request-key destination keyring.
686
687 long keyctl(KEYCTL_SET_REQKEY_KEYRING, int reqkey_defl);
688
689 This sets the default keyring to which implicitly requested keys will be
690 attached for this thread. reqkey_defl should be one of these constants:
691
692 CONSTANT VALUE NEW DEFAULT KEYRING
693 ====================================== ====== =======================
694 KEY_REQKEY_DEFL_NO_CHANGE -1 No change
695 KEY_REQKEY_DEFL_DEFAULT 0 Default[1]
696 KEY_REQKEY_DEFL_THREAD_KEYRING 1 Thread keyring
697 KEY_REQKEY_DEFL_PROCESS_KEYRING 2 Process keyring
698 KEY_REQKEY_DEFL_SESSION_KEYRING 3 Session keyring
699 KEY_REQKEY_DEFL_USER_KEYRING 4 User keyring
700 KEY_REQKEY_DEFL_USER_SESSION_KEYRING 5 User session keyring
701 KEY_REQKEY_DEFL_GROUP_KEYRING 6 Group keyring
702
703 The old default will be returned if successful and error EINVAL will be
704 returned if reqkey_defl is not one of the above values.
705
706 The default keyring can be overridden by the keyring indicated to the
707 request_key() system call.
708
709 Note that this setting is inherited across fork/exec.
710
711 [1] The default is: the thread keyring if there is one, otherwise
712 the process keyring if there is one, otherwise the session keyring if
713 there is one, otherwise the user default session keyring.
714
715
716 (*) Set the timeout on a key.
717
718 long keyctl(KEYCTL_SET_TIMEOUT, key_serial_t key, unsigned timeout);
719
720 This sets or clears the timeout on a key. The timeout can be 0 to clear
721 the timeout or a number of seconds to set the expiry time that far into
722 the future.
723
724 The process must have attribute modification access on a key to set its
725 timeout. Timeouts may not be set with this function on negative, revoked
726 or expired keys.
727
728
729 (*) Assume the authority granted to instantiate a key
730
731 long keyctl(KEYCTL_ASSUME_AUTHORITY, key_serial_t key);
732
733 This assumes or divests the authority required to instantiate the
734 specified key. Authority can only be assumed if the thread has the
735 authorisation key associated with the specified key in its keyrings
736 somewhere.
737
738 Once authority is assumed, searches for keys will also search the
739 requester's keyrings using the requester's security label, UID, GID and
740 groups.
741
742 If the requested authority is unavailable, error EPERM will be returned,
743 likewise if the authority has been revoked because the target key is
744 already instantiated.
745
746 If the specified key is 0, then any assumed authority will be divested.
747
748 The assumed authoritative key is inherited across fork and exec.
749
750
751 (*) Get the LSM security context attached to a key.
752
753 long keyctl(KEYCTL_GET_SECURITY, key_serial_t key, char *buffer,
754 size_t buflen)
755
756 This function returns a string that represents the LSM security context
757 attached to a key in the buffer provided.
758
759 Unless there's an error, it always returns the amount of data it could
760 produce, even if that's too big for the buffer, but it won't copy more
761 than requested to userspace. If the buffer pointer is NULL then no copy
762 will take place.
763
764 A NUL character is included at the end of the string if the buffer is
765 sufficiently big. This is included in the returned count. If no LSM is
766 in force then an empty string will be returned.
767
768 A process must have view permission on the key for this function to be
769 successful.
770
771
772 (*) Install the calling process's session keyring on its parent.
773
774 long keyctl(KEYCTL_SESSION_TO_PARENT);
775
776 This functions attempts to install the calling process's session keyring
777 on to the calling process's parent, replacing the parent's current session
778 keyring.
779
780 The calling process must have the same ownership as its parent, the
781 keyring must have the same ownership as the calling process, the calling
782 process must have LINK permission on the keyring and the active LSM module
783 mustn't deny permission, otherwise error EPERM will be returned.
784
785 Error ENOMEM will be returned if there was insufficient memory to complete
786 the operation, otherwise 0 will be returned to indicate success.
787
788 The keyring will be replaced next time the parent process leaves the
789 kernel and resumes executing userspace.
790
791
792===============
793KERNEL SERVICES
794===============
795
796The kernel services for key management are fairly simple to deal with. They can
797be broken down into two areas: keys and key types.
798
799Dealing with keys is fairly straightforward. Firstly, the kernel service
800registers its type, then it searches for a key of that type. It should retain
801the key as long as it has need of it, and then it should release it. For a
802filesystem or device file, a search would probably be performed during the open
803call, and the key released upon close. How to deal with conflicting keys due to
804two different users opening the same file is left to the filesystem author to
805solve.
806
807To access the key manager, the following header must be #included:
808
809 <linux/key.h>
810
811Specific key types should have a header file under include/keys/ that should be
812used to access that type. For keys of type "user", for example, that would be:
813
814 <keys/user-type.h>
815
816Note that there are two different types of pointers to keys that may be
817encountered:
818
819 (*) struct key *
820
821 This simply points to the key structure itself. Key structures will be at
822 least four-byte aligned.
823
824 (*) key_ref_t
825
826 This is equivalent to a struct key *, but the least significant bit is set
827 if the caller "possesses" the key. By "possession" it is meant that the
828 calling processes has a searchable link to the key from one of its
829 keyrings. There are three functions for dealing with these:
830
831 key_ref_t make_key_ref(const struct key *key,
832 unsigned long possession);
833
834 struct key *key_ref_to_ptr(const key_ref_t key_ref);
835
836 unsigned long is_key_possessed(const key_ref_t key_ref);
837
838 The first function constructs a key reference from a key pointer and
839 possession information (which must be 0 or 1 and not any other value).
840
841 The second function retrieves the key pointer from a reference and the
842 third retrieves the possession flag.
843
844When accessing a key's payload contents, certain precautions must be taken to
845prevent access vs modification races. See the section "Notes on accessing
846payload contents" for more information.
847
848(*) To search for a key, call:
849
850 struct key *request_key(const struct key_type *type,
851 const char *description,
852 const char *callout_info);
853
854 This is used to request a key or keyring with a description that matches
855 the description specified according to the key type's match function. This
856 permits approximate matching to occur. If callout_string is not NULL, then
857 /sbin/request-key will be invoked in an attempt to obtain the key from
858 userspace. In that case, callout_string will be passed as an argument to
859 the program.
860
861 Should the function fail error ENOKEY, EKEYEXPIRED or EKEYREVOKED will be
862 returned.
863
864 If successful, the key will have been attached to the default keyring for
865 implicitly obtained request-key keys, as set by KEYCTL_SET_REQKEY_KEYRING.
866
867 See also Documentation/security/keys-request-key.txt.
868
869
870(*) To search for a key, passing auxiliary data to the upcaller, call:
871
872 struct key *request_key_with_auxdata(const struct key_type *type,
873 const char *description,
874 const void *callout_info,
875 size_t callout_len,
876 void *aux);
877
878 This is identical to request_key(), except that the auxiliary data is
879 passed to the key_type->request_key() op if it exists, and the callout_info
880 is a blob of length callout_len, if given (the length may be 0).
881
882
883(*) A key can be requested asynchronously by calling one of:
884
885 struct key *request_key_async(const struct key_type *type,
886 const char *description,
887 const void *callout_info,
888 size_t callout_len);
889
890 or:
891
892 struct key *request_key_async_with_auxdata(const struct key_type *type,
893 const char *description,
894 const char *callout_info,
895 size_t callout_len,
896 void *aux);
897
898 which are asynchronous equivalents of request_key() and
899 request_key_with_auxdata() respectively.
900
901 These two functions return with the key potentially still under
902 construction. To wait for construction completion, the following should be
903 called:
904
905 int wait_for_key_construction(struct key *key, bool intr);
906
907 The function will wait for the key to finish being constructed and then
908 invokes key_validate() to return an appropriate value to indicate the state
909 of the key (0 indicates the key is usable).
910
911 If intr is true, then the wait can be interrupted by a signal, in which
912 case error ERESTARTSYS will be returned.
913
914
915(*) When it is no longer required, the key should be released using:
916
917 void key_put(struct key *key);
918
919 Or:
920
921 void key_ref_put(key_ref_t key_ref);
922
923 These can be called from interrupt context. If CONFIG_KEYS is not set then
924 the argument will not be parsed.
925
926
927(*) Extra references can be made to a key by calling the following function:
928
929 struct key *key_get(struct key *key);
930
931 These need to be disposed of by calling key_put() when they've been
932 finished with. The key pointer passed in will be returned. If the pointer
933 is NULL or CONFIG_KEYS is not set then the key will not be dereferenced and
934 no increment will take place.
935
936
937(*) A key's serial number can be obtained by calling:
938
939 key_serial_t key_serial(struct key *key);
940
941 If key is NULL or if CONFIG_KEYS is not set then 0 will be returned (in the
942 latter case without parsing the argument).
943
944
945(*) If a keyring was found in the search, this can be further searched by:
946
947 key_ref_t keyring_search(key_ref_t keyring_ref,
948 const struct key_type *type,
949 const char *description)
950
951 This searches the keyring tree specified for a matching key. Error ENOKEY
952 is returned upon failure (use IS_ERR/PTR_ERR to determine). If successful,
953 the returned key will need to be released.
954
955 The possession attribute from the keyring reference is used to control
956 access through the permissions mask and is propagated to the returned key
957 reference pointer if successful.
958
959
960(*) To check the validity of a key, this function can be called:
961
962 int validate_key(struct key *key);
963
964 This checks that the key in question hasn't expired or and hasn't been
965 revoked. Should the key be invalid, error EKEYEXPIRED or EKEYREVOKED will
966 be returned. If the key is NULL or if CONFIG_KEYS is not set then 0 will be
967 returned (in the latter case without parsing the argument).
968
969
970(*) To register a key type, the following function should be called:
971
972 int register_key_type(struct key_type *type);
973
974 This will return error EEXIST if a type of the same name is already
975 present.
976
977
978(*) To unregister a key type, call:
979
980 void unregister_key_type(struct key_type *type);
981
982
983Under some circumstances, it may be desirable to deal with a bundle of keys.
984The facility provides access to the keyring type for managing such a bundle:
985
986 struct key_type key_type_keyring;
987
988This can be used with a function such as request_key() to find a specific
989keyring in a process's keyrings. A keyring thus found can then be searched
990with keyring_search(). Note that it is not possible to use request_key() to
991search a specific keyring, so using keyrings in this way is of limited utility.
992
993
994===================================
995NOTES ON ACCESSING PAYLOAD CONTENTS
996===================================
997
998The simplest payload is just a number in key->payload.value. In this case,
999there's no need to indulge in RCU or locking when accessing the payload.
1000
1001More complex payload contents must be allocated and a pointer to them set in
1002key->payload.data. One of the following ways must be selected to access the
1003data:
1004
1005 (1) Unmodifiable key type.
1006
1007 If the key type does not have a modify method, then the key's payload can
1008 be accessed without any form of locking, provided that it's known to be
1009 instantiated (uninstantiated keys cannot be "found").
1010
1011 (2) The key's semaphore.
1012
1013 The semaphore could be used to govern access to the payload and to control
1014 the payload pointer. It must be write-locked for modifications and would
1015 have to be read-locked for general access. The disadvantage of doing this
1016 is that the accessor may be required to sleep.
1017
1018 (3) RCU.
1019
1020 RCU must be used when the semaphore isn't already held; if the semaphore
1021 is held then the contents can't change under you unexpectedly as the
1022 semaphore must still be used to serialise modifications to the key. The
1023 key management code takes care of this for the key type.
1024
1025 However, this means using:
1026
1027 rcu_read_lock() ... rcu_dereference() ... rcu_read_unlock()
1028
1029 to read the pointer, and:
1030
1031 rcu_dereference() ... rcu_assign_pointer() ... call_rcu()
1032
1033 to set the pointer and dispose of the old contents after a grace period.
1034 Note that only the key type should ever modify a key's payload.
1035
1036 Furthermore, an RCU controlled payload must hold a struct rcu_head for the
1037 use of call_rcu() and, if the payload is of variable size, the length of
1038 the payload. key->datalen cannot be relied upon to be consistent with the
1039 payload just dereferenced if the key's semaphore is not held.
1040
1041
1042===================
1043DEFINING A KEY TYPE
1044===================
1045
1046A kernel service may want to define its own key type. For instance, an AFS
1047filesystem might want to define a Kerberos 5 ticket key type. To do this, it
1048author fills in a key_type struct and registers it with the system.
1049
1050Source files that implement key types should include the following header file:
1051
1052 <linux/key-type.h>
1053
1054The structure has a number of fields, some of which are mandatory:
1055
1056 (*) const char *name
1057
1058 The name of the key type. This is used to translate a key type name
1059 supplied by userspace into a pointer to the structure.
1060
1061
1062 (*) size_t def_datalen
1063
1064 This is optional - it supplies the default payload data length as
1065 contributed to the quota. If the key type's payload is always or almost
1066 always the same size, then this is a more efficient way to do things.
1067
1068 The data length (and quota) on a particular key can always be changed
1069 during instantiation or update by calling:
1070
1071 int key_payload_reserve(struct key *key, size_t datalen);
1072
1073 With the revised data length. Error EDQUOT will be returned if this is not
1074 viable.
1075
1076
1077 (*) int (*vet_description)(const char *description);
1078
1079 This optional method is called to vet a key description. If the key type
1080 doesn't approve of the key description, it may return an error, otherwise
1081 it should return 0.
1082
1083
1084 (*) int (*instantiate)(struct key *key, const void *data, size_t datalen);
1085
1086 This method is called to attach a payload to a key during construction.
1087 The payload attached need not bear any relation to the data passed to this
1088 function.
1089
1090 If the amount of data attached to the key differs from the size in
1091 keytype->def_datalen, then key_payload_reserve() should be called.
1092
1093 This method does not have to lock the key in order to attach a payload.
1094 The fact that KEY_FLAG_INSTANTIATED is not set in key->flags prevents
1095 anything else from gaining access to the key.
1096
1097 It is safe to sleep in this method.
1098
1099
1100 (*) int (*update)(struct key *key, const void *data, size_t datalen);
1101
1102 If this type of key can be updated, then this method should be provided.
1103 It is called to update a key's payload from the blob of data provided.
1104
1105 key_payload_reserve() should be called if the data length might change
1106 before any changes are actually made. Note that if this succeeds, the type
1107 is committed to changing the key because it's already been altered, so all
1108 memory allocation must be done first.
1109
1110 The key will have its semaphore write-locked before this method is called,
1111 but this only deters other writers; any changes to the key's payload must
1112 be made under RCU conditions, and call_rcu() must be used to dispose of
1113 the old payload.
1114
1115 key_payload_reserve() should be called before the changes are made, but
1116 after all allocations and other potentially failing function calls are
1117 made.
1118
1119 It is safe to sleep in this method.
1120
1121
1122 (*) int (*match)(const struct key *key, const void *desc);
1123
1124 This method is called to match a key against a description. It should
1125 return non-zero if the two match, zero if they don't.
1126
1127 This method should not need to lock the key in any way. The type and
1128 description can be considered invariant, and the payload should not be
1129 accessed (the key may not yet be instantiated).
1130
1131 It is not safe to sleep in this method; the caller may hold spinlocks.
1132
1133
1134 (*) void (*revoke)(struct key *key);
1135
1136 This method is optional. It is called to discard part of the payload
1137 data upon a key being revoked. The caller will have the key semaphore
1138 write-locked.
1139
1140 It is safe to sleep in this method, though care should be taken to avoid
1141 a deadlock against the key semaphore.
1142
1143
1144 (*) void (*destroy)(struct key *key);
1145
1146 This method is optional. It is called to discard the payload data on a key
1147 when it is being destroyed.
1148
1149 This method does not need to lock the key to access the payload; it can
1150 consider the key as being inaccessible at this time. Note that the key's
1151 type may have been changed before this function is called.
1152
1153 It is not safe to sleep in this method; the caller may hold spinlocks.
1154
1155
1156 (*) void (*describe)(const struct key *key, struct seq_file *p);
1157
1158 This method is optional. It is called during /proc/keys reading to
1159 summarise a key's description and payload in text form.
1160
1161 This method will be called with the RCU read lock held. rcu_dereference()
1162 should be used to read the payload pointer if the payload is to be
1163 accessed. key->datalen cannot be trusted to stay consistent with the
1164 contents of the payload.
1165
1166 The description will not change, though the key's state may.
1167
1168 It is not safe to sleep in this method; the RCU read lock is held by the
1169 caller.
1170
1171
1172 (*) long (*read)(const struct key *key, char __user *buffer, size_t buflen);
1173
1174 This method is optional. It is called by KEYCTL_READ to translate the
1175 key's payload into something a blob of data for userspace to deal with.
1176 Ideally, the blob should be in the same format as that passed in to the
1177 instantiate and update methods.
1178
1179 If successful, the blob size that could be produced should be returned
1180 rather than the size copied.
1181
1182 This method will be called with the key's semaphore read-locked. This will
1183 prevent the key's payload changing. It is not necessary to use RCU locking
1184 when accessing the key's payload. It is safe to sleep in this method, such
1185 as might happen when the userspace buffer is accessed.
1186
1187
1188 (*) int (*request_key)(struct key_construction *cons, const char *op,
1189 void *aux);
1190
1191 This method is optional. If provided, request_key() and friends will
1192 invoke this function rather than upcalling to /sbin/request-key to operate
1193 upon a key of this type.
1194
1195 The aux parameter is as passed to request_key_async_with_auxdata() and
1196 similar or is NULL otherwise. Also passed are the construction record for
1197 the key to be operated upon and the operation type (currently only
1198 "create").
1199
1200 This method is permitted to return before the upcall is complete, but the
1201 following function must be called under all circumstances to complete the
1202 instantiation process, whether or not it succeeds, whether or not there's
1203 an error:
1204
1205 void complete_request_key(struct key_construction *cons, int error);
1206
1207 The error parameter should be 0 on success, -ve on error. The
1208 construction record is destroyed by this action and the authorisation key
1209 will be revoked. If an error is indicated, the key under construction
1210 will be negatively instantiated if it wasn't already instantiated.
1211
1212 If this method returns an error, that error will be returned to the
1213 caller of request_key*(). complete_request_key() must be called prior to
1214 returning.
1215
1216 The key under construction and the authorisation key can be found in the
1217 key_construction struct pointed to by cons:
1218
1219 (*) struct key *key;
1220
1221 The key under construction.
1222
1223 (*) struct key *authkey;
1224
1225 The authorisation key.
1226
1227
1228============================
1229REQUEST-KEY CALLBACK SERVICE
1230============================
1231
1232To create a new key, the kernel will attempt to execute the following command
1233line:
1234
1235 /sbin/request-key create <key> <uid> <gid> \
1236 <threadring> <processring> <sessionring> <callout_info>
1237
1238<key> is the key being constructed, and the three keyrings are the process
1239keyrings from the process that caused the search to be issued. These are
1240included for two reasons:
1241
1242 (1) There may be an authentication token in one of the keyrings that is
1243 required to obtain the key, eg: a Kerberos Ticket-Granting Ticket.
1244
1245 (2) The new key should probably be cached in one of these rings.
1246
1247This program should set it UID and GID to those specified before attempting to
1248access any more keys. It may then look around for a user specific process to
1249hand the request off to (perhaps a path held in placed in another key by, for
1250example, the KDE desktop manager).
1251
1252The program (or whatever it calls) should finish construction of the key by
1253calling KEYCTL_INSTANTIATE or KEYCTL_INSTANTIATE_IOV, which also permits it to
1254cache the key in one of the keyrings (probably the session ring) before
1255returning. Alternatively, the key can be marked as negative with KEYCTL_NEGATE
1256or KEYCTL_REJECT; this also permits the key to be cached in one of the
1257keyrings.
1258
1259If it returns with the key remaining in the unconstructed state, the key will
1260be marked as being negative, it will be added to the session keyring, and an
1261error will be returned to the key requestor.
1262
1263Supplementary information may be provided from whoever or whatever invoked this
1264service. This will be passed as the <callout_info> parameter. If no such
1265information was made available, then "-" will be passed as this parameter
1266instead.
1267
1268
1269Similarly, the kernel may attempt to update an expired or a soon to expire key
1270by executing:
1271
1272 /sbin/request-key update <key> <uid> <gid> \
1273 <threadring> <processring> <sessionring>
1274
1275In this case, the program isn't required to actually attach the key to a ring;
1276the rings are provided for reference.
1277
1278
1279==================
1280GARBAGE COLLECTION
1281==================
1282
1283Dead keys (for which the type has been removed) will be automatically unlinked
1284from those keyrings that point to them and deleted as soon as possible by a
1285background garbage collector.
1286
1287Similarly, revoked and expired keys will be garbage collected, but only after a
1288certain amount of time has passed. This time is set as a number of seconds in:
1289
1290 /proc/sys/kernel/keys/gc_delay