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 | */ |
27 | int 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 | */ |
35 | static 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 | */ |
48 | int 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(size: 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(data: d, len: sizeof(*d) + hash_alg->digest_size, |
90 | raw_pkcs7: signature, pkcs7_len: sig_size, trusted_keys: fsverity_keyring, |
91 | usage: VERIFYING_UNSPECIFIED_SIGNATURE, |
92 | NULL, NULL); |
93 | kfree(objp: 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 | |
112 | void __init fsverity_init_signature(void) |
113 | { |
114 | fsverity_keyring = |
115 | keyring_alloc(description: ".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 | } |
123 | |