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}