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 */ 28 int 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 */ 36 static 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 */ 53 int 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(d, sizeof(*d) + hash_alg->digest_size, 95 signature, sig_size, fsverity_keyring, 96 VERIFYING_UNSPECIFIED_SIGNATURE, 97 NULL, NULL); 98 kfree(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 LSM_INT_FSVERITY_BUILTINSIG_VALID, 116 signature, 117 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 } 127 128 void __init fsverity_init_signature(void) 129 { 130 fsverity_keyring = 131 keyring_alloc(".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