fs/verity/signature.c

   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 }