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/security.h>
21#include <linux/slab.h>
22#include <linux/verification.h>
23
24/*
25 * /proc/sys/fs/verity/require_signatures
26 * If 1, all verity files must have a valid builtin signature.
27 */
28int fsverity_require_signatures;
29
30/*
31 * Keyring that contains the trusted X.509 certificates.
32 *
33 * Only root (kuid=0) can modify this. Also, root may use
34 * keyctl_restrict_keyring() to prevent any more additions.
35 */
36static struct key *fsverity_keyring;
37
38/**
39 * fsverity_verify_signature() - check a verity file's signature
40 * @vi: the file's fsverity_info
41 * @signature: the file's built-in signature
42 * @sig_size: size of signature in bytes, or 0 if no signature
43 *
44 * If the file includes a signature of its fs-verity file digest, verify it
45 * against the certificates in the fs-verity keyring. Note that signatures
46 * are verified regardless of the state of the 'fsverity_require_signatures'
47 * variable and the LSM subsystem relies on this behavior to help enforce
48 * file integrity policies. Please discuss changes with the LSM list
49 * (thank you!).
50 *
51 * Return: 0 on success (signature valid or not required); -errno on failure
52 */
53int fsverity_verify_signature(const struct fsverity_info *vi,
54 const u8 *signature, size_t sig_size)
55{
56 const struct inode *inode = vi->inode;
57 const struct fsverity_hash_alg *hash_alg = vi->tree_params.hash_alg;
58 struct fsverity_formatted_digest *d;
59 int err;
60
61 if (sig_size == 0) {
62 if (fsverity_require_signatures) {
63 fsverity_err(inode,
64 "require_signatures=1, rejecting unsigned file!");
65 return -EPERM;
66 }
67 return 0;
68 }
69
70 if (fsverity_keyring->keys.nr_leaves_on_tree == 0) {
71 /*
72 * The ".fs-verity" keyring is empty, due to builtin signatures
73 * being supported by the kernel but not actually being used.
74 * In this case, verify_pkcs7_signature() would always return an
75 * error, usually ENOKEY. It could also be EBADMSG if the
76 * PKCS#7 is malformed, but that isn't very important to
77 * distinguish. So, just skip to ENOKEY to avoid the attack
78 * surface of the PKCS#7 parser, which would otherwise be
79 * reachable by any task able to execute FS_IOC_ENABLE_VERITY.
80 */
81 fsverity_err(inode,
82 "fs-verity keyring is empty, rejecting signed file!");
83 return -ENOKEY;
84 }
85
86 d = kzalloc(sizeof(*d) + hash_alg->digest_size, GFP_KERNEL);
87 if (!d)
88 return -ENOMEM;
89 memcpy(d->magic, "FSVerity", 8);
90 d->digest_algorithm = cpu_to_le16(hash_alg - fsverity_hash_algs);
91 d->digest_size = cpu_to_le16(hash_alg->digest_size);
92 memcpy(d->digest, vi->file_digest, hash_alg->digest_size);
93
94 err = verify_pkcs7_signature(data: d, len: sizeof(*d) + hash_alg->digest_size,
95 raw_pkcs7: signature, pkcs7_len: sig_size, trusted_keys: fsverity_keyring,
96 usage: VERIFYING_UNSPECIFIED_SIGNATURE,
97 NULL, NULL);
98 kfree(objp: d);
99
100 if (err) {
101 if (err == -ENOKEY)
102 fsverity_err(inode,
103 "File's signing cert isn't in the fs-verity keyring");
104 else if (err == -EKEYREJECTED)
105 fsverity_err(inode, "Incorrect file signature");
106 else if (err == -EBADMSG)
107 fsverity_err(inode, "Malformed file signature");
108 else
109 fsverity_err(inode, "Error %d verifying file signature",
110 err);
111 return err;
112 }
113
114 err = security_inode_setintegrity(inode,
115 type: LSM_INT_FSVERITY_BUILTINSIG_VALID,
116 value: signature,
117 size: sig_size);
118
119 if (err) {
120 fsverity_err(inode, "Error %d exposing file signature to LSMs",
121 err);
122 return err;
123 }
124
125 return 0;
126}
127
128void __init fsverity_init_signature(void)
129{
130 fsverity_keyring =
131 keyring_alloc(description: ".fs-verity", KUIDT_INIT(0), KGIDT_INIT(0),
132 current_cred(), KEY_POS_SEARCH |
133 KEY_USR_VIEW | KEY_USR_READ | KEY_USR_WRITE |
134 KEY_USR_SEARCH | KEY_USR_SETATTR,
135 KEY_ALLOC_NOT_IN_QUOTA, NULL, NULL);
136 if (IS_ERR(fsverity_keyring))
137 panic("failed to allocate \".fs-verity\" keyring");
138}
139

source code of linux/fs/verity/signature.c