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(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 } 111 112 void __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