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}
123