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1 | // SPDX-License-Identifier: GPL-2.0 |
2 | /* | |
7bf765dd | 3 | * Verification of builtin signatures |
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4 | * |
5 | * Copyright 2019 Google LLC | |
6 | */ | |
7 | ||
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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 | ||
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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 | */ | |
456ae5fe | 27 | int fsverity_require_signatures; |
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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 | */ | |
35 | static struct key *fsverity_keyring; | |
36 | ||
37 | /** | |
38 | * fsverity_verify_signature() - check a verity file's signature | |
6377a38b | 39 | * @vi: the file's fsverity_info |
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40 | * @signature: the file's built-in signature |
41 | * @sig_size: size of signature in bytes, or 0 if no signature | |
432434c9 | 42 | * |
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43 | * If the file includes a signature of its fs-verity file digest, verify it |
44 | * against the certificates in the fs-verity keyring. | |
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45 | * |
46 | * Return: 0 on success (signature valid or not required); -errno on failure | |
47 | */ | |
48 | int fsverity_verify_signature(const struct fsverity_info *vi, | |
fab634c4 | 49 | const u8 *signature, size_t sig_size) |
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50 | { |
51 | const struct inode *inode = vi->inode; | |
52 | const struct fsverity_hash_alg *hash_alg = vi->tree_params.hash_alg; | |
9e90f30e | 53 | struct fsverity_formatted_digest *d; |
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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 | ||
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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 | ||
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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); | |
ed45e201 | 87 | memcpy(d->digest, vi->file_digest, hash_alg->digest_size); |
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88 | |
89 | err = verify_pkcs7_signature(d, sizeof(*d) + hash_alg->digest_size, | |
fab634c4 | 90 | signature, sig_size, fsverity_keyring, |
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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 | ||
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109 | return 0; |
110 | } | |
111 | ||
e77000cc | 112 | void __init fsverity_init_signature(void) |
432434c9 | 113 | { |
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114 | fsverity_keyring = |
115 | keyring_alloc(".fs-verity", KUIDT_INIT(0), KGIDT_INIT(0), | |
116 | current_cred(), KEY_POS_SEARCH | | |
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117 | KEY_USR_VIEW | KEY_USR_READ | KEY_USR_WRITE | |
118 | KEY_USR_SEARCH | KEY_USR_SETATTR, | |
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119 | KEY_ALLOC_NOT_IN_QUOTA, NULL, NULL); |
120 | if (IS_ERR(fsverity_keyring)) | |
121 | panic("failed to allocate \".fs-verity\" keyring"); | |
432434c9 | 122 | } |