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/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 }