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1// SPDX-License-Identifier: GPL-2.0
2/*
3 * Verification of builtin signatures
4 *
5 * Copyright 2019 Google LLC
6 */
7
8/*
9 * This file implements verification of fs-verity builtin signatures. Please
10 * take great care before using this feature. It is not the only way to do
11 * signatures with fs-verity, and the alternatives (such as userspace signature
12 * verification, and IMA appraisal) can be much better. For details about the
13 * limitations of this feature, see Documentation/filesystems/fsverity.rst.
14 */
15
16#include "fsverity_private.h"
17
18#include <linux/cred.h>
19#include <linux/key.h>
20#include <linux/slab.h>
21#include <linux/verification.h>
22
23/*
24 * /proc/sys/fs/verity/require_signatures
25 * If 1, all verity files must have a valid builtin signature.
26 */
27int fsverity_require_signatures;
28
29/*
30 * Keyring that contains the trusted X.509 certificates.
31 *
32 * Only root (kuid=0) can modify this. Also, root may use
33 * keyctl_restrict_keyring() to prevent any more additions.
34 */
35static struct key *fsverity_keyring;
36
37/**
38 * fsverity_verify_signature() - check a verity file's signature
39 * @vi: the file's fsverity_info
40 * @signature: the file's built-in signature
41 * @sig_size: size of signature in bytes, or 0 if no signature
42 *
43 * If the file includes a signature of its fs-verity file digest, verify it
44 * against the certificates in the fs-verity keyring.
45 *
46 * Return: 0 on success (signature valid or not required); -errno on failure
47 */
48int fsverity_verify_signature(const struct fsverity_info *vi,
49 const u8 *signature, size_t sig_size)
50{
51 const struct inode *inode = vi->inode;
52 const struct fsverity_hash_alg *hash_alg = vi->tree_params.hash_alg;
53 struct fsverity_formatted_digest *d;
54 int err;
55
56 if (sig_size == 0) {
57 if (fsverity_require_signatures) {
58 fsverity_err(inode,
59 "require_signatures=1, rejecting unsigned file!");
60 return -EPERM;
61 }
62 return 0;
63 }
64
65 if (fsverity_keyring->keys.nr_leaves_on_tree == 0) {
66 /*
67 * The ".fs-verity" keyring is empty, due to builtin signatures
68 * being supported by the kernel but not actually being used.
69 * In this case, verify_pkcs7_signature() would always return an
70 * error, usually ENOKEY. It could also be EBADMSG if the
71 * PKCS#7 is malformed, but that isn't very important to
72 * distinguish. So, just skip to ENOKEY to avoid the attack
73 * surface of the PKCS#7 parser, which would otherwise be
74 * reachable by any task able to execute FS_IOC_ENABLE_VERITY.
75 */
76 fsverity_err(inode,
77 "fs-verity keyring is empty, rejecting signed file!");
78 return -ENOKEY;
79 }
80
81 d = kzalloc(sizeof(*d) + hash_alg->digest_size, GFP_KERNEL);
82 if (!d)
83 return -ENOMEM;
84 memcpy(d->magic, "FSVerity", 8);
85 d->digest_algorithm = cpu_to_le16(hash_alg - fsverity_hash_algs);
86 d->digest_size = cpu_to_le16(hash_alg->digest_size);
87 memcpy(d->digest, vi->file_digest, hash_alg->digest_size);
88
89 err = verify_pkcs7_signature(d, sizeof(*d) + hash_alg->digest_size,
90 signature, sig_size, fsverity_keyring,
91 VERIFYING_UNSPECIFIED_SIGNATURE,
92 NULL, NULL);
93 kfree(d);
94
95 if (err) {
96 if (err == -ENOKEY)
97 fsverity_err(inode,
98 "File's signing cert isn't in the fs-verity keyring");
99 else if (err == -EKEYREJECTED)
100 fsverity_err(inode, "Incorrect file signature");
101 else if (err == -EBADMSG)
102 fsverity_err(inode, "Malformed file signature");
103 else
104 fsverity_err(inode, "Error %d verifying file signature",
105 err);
106 return err;
107 }
108
109 return 0;
110}
111
112void __init fsverity_init_signature(void)
113{
114 fsverity_keyring =
115 keyring_alloc(".fs-verity", KUIDT_INIT(0), KGIDT_INIT(0),
116 current_cred(), KEY_POS_SEARCH |
117 KEY_USR_VIEW | KEY_USR_READ | KEY_USR_WRITE |
118 KEY_USR_SEARCH | KEY_USR_SETATTR,
119 KEY_ALLOC_NOT_IN_QUOTA, NULL, NULL);
120 if (IS_ERR(fsverity_keyring))
121 panic("failed to allocate \".fs-verity\" keyring");
122}
1// SPDX-License-Identifier: GPL-2.0
2/*
3 * Verification of builtin signatures
4 *
5 * Copyright 2019 Google LLC
6 */
7
8#include "fsverity_private.h"
9
10#include <linux/cred.h>
11#include <linux/key.h>
12#include <linux/slab.h>
13#include <linux/verification.h>
14
15/*
16 * /proc/sys/fs/verity/require_signatures
17 * If 1, all verity files must have a valid builtin signature.
18 */
19static int fsverity_require_signatures;
20
21/*
22 * Keyring that contains the trusted X.509 certificates.
23 *
24 * Only root (kuid=0) can modify this. Also, root may use
25 * keyctl_restrict_keyring() to prevent any more additions.
26 */
27static struct key *fsverity_keyring;
28
29/**
30 * fsverity_verify_signature() - check a verity file's signature
31 * @vi: the file's fsverity_info
32 * @signature: the file's built-in signature
33 * @sig_size: size of signature in bytes, or 0 if no signature
34 *
35 * If the file includes a signature of its fs-verity file digest, verify it
36 * against the certificates in the fs-verity keyring.
37 *
38 * Return: 0 on success (signature valid or not required); -errno on failure
39 */
40int fsverity_verify_signature(const struct fsverity_info *vi,
41 const u8 *signature, size_t sig_size)
42{
43 const struct inode *inode = vi->inode;
44 const struct fsverity_hash_alg *hash_alg = vi->tree_params.hash_alg;
45 struct fsverity_formatted_digest *d;
46 int err;
47
48 if (sig_size == 0) {
49 if (fsverity_require_signatures) {
50 fsverity_err(inode,
51 "require_signatures=1, rejecting unsigned file!");
52 return -EPERM;
53 }
54 return 0;
55 }
56
57 d = kzalloc(sizeof(*d) + hash_alg->digest_size, GFP_KERNEL);
58 if (!d)
59 return -ENOMEM;
60 memcpy(d->magic, "FSVerity", 8);
61 d->digest_algorithm = cpu_to_le16(hash_alg - fsverity_hash_algs);
62 d->digest_size = cpu_to_le16(hash_alg->digest_size);
63 memcpy(d->digest, vi->file_digest, hash_alg->digest_size);
64
65 err = verify_pkcs7_signature(d, sizeof(*d) + hash_alg->digest_size,
66 signature, sig_size, fsverity_keyring,
67 VERIFYING_UNSPECIFIED_SIGNATURE,
68 NULL, NULL);
69 kfree(d);
70
71 if (err) {
72 if (err == -ENOKEY)
73 fsverity_err(inode,
74 "File's signing cert isn't in the fs-verity keyring");
75 else if (err == -EKEYREJECTED)
76 fsverity_err(inode, "Incorrect file signature");
77 else if (err == -EBADMSG)
78 fsverity_err(inode, "Malformed file signature");
79 else
80 fsverity_err(inode, "Error %d verifying file signature",
81 err);
82 return err;
83 }
84
85 pr_debug("Valid signature for file digest %s:%*phN\n",
86 hash_alg->name, hash_alg->digest_size, vi->file_digest);
87 return 0;
88}
89
90#ifdef CONFIG_SYSCTL
91static struct ctl_table_header *fsverity_sysctl_header;
92
93static const struct ctl_path fsverity_sysctl_path[] = {
94 { .procname = "fs", },
95 { .procname = "verity", },
96 { }
97};
98
99static struct ctl_table fsverity_sysctl_table[] = {
100 {
101 .procname = "require_signatures",
102 .data = &fsverity_require_signatures,
103 .maxlen = sizeof(int),
104 .mode = 0644,
105 .proc_handler = proc_dointvec_minmax,
106 .extra1 = SYSCTL_ZERO,
107 .extra2 = SYSCTL_ONE,
108 },
109 { }
110};
111
112static int __init fsverity_sysctl_init(void)
113{
114 fsverity_sysctl_header = register_sysctl_paths(fsverity_sysctl_path,
115 fsverity_sysctl_table);
116 if (!fsverity_sysctl_header) {
117 pr_err("sysctl registration failed!\n");
118 return -ENOMEM;
119 }
120 return 0;
121}
122#else /* !CONFIG_SYSCTL */
123static inline int __init fsverity_sysctl_init(void)
124{
125 return 0;
126}
127#endif /* !CONFIG_SYSCTL */
128
129int __init fsverity_init_signature(void)
130{
131 struct key *ring;
132 int err;
133
134 ring = keyring_alloc(".fs-verity", KUIDT_INIT(0), KGIDT_INIT(0),
135 current_cred(), KEY_POS_SEARCH |
136 KEY_USR_VIEW | KEY_USR_READ | KEY_USR_WRITE |
137 KEY_USR_SEARCH | KEY_USR_SETATTR,
138 KEY_ALLOC_NOT_IN_QUOTA, NULL, NULL);
139 if (IS_ERR(ring))
140 return PTR_ERR(ring);
141
142 err = fsverity_sysctl_init();
143 if (err)
144 goto err_put_ring;
145
146 fsverity_keyring = ring;
147 return 0;
148
149err_put_ring:
150 key_put(ring);
151 return err;
152}