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 static 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         d = kzalloc(sizeof(*d) + hash_alg->digest_size, GFP_KERNEL);
  66         if (!d)
  67                 return -ENOMEM;
  68         memcpy(d->magic, "FSVerity", 8);
  69         d->digest_algorithm = cpu_to_le16(hash_alg - fsverity_hash_algs);
  70         d->digest_size = cpu_to_le16(hash_alg->digest_size);
  71         memcpy(d->digest, vi->file_digest, hash_alg->digest_size);
  72
  73         err = verify_pkcs7_signature(d, sizeof(*d) + hash_alg->digest_size,
  74                                      signature, sig_size, fsverity_keyring,
  75                                      VERIFYING_UNSPECIFIED_SIGNATURE,
  76                                      NULL, NULL);
  77         kfree(d);
  78
  79         if (err) {
  80                 if (err == -ENOKEY)
  81                         fsverity_err(inode,
  82                                      "File's signing cert isn't in the fs-verity keyring");
  83                 else if (err == -EKEYREJECTED)
  84                         fsverity_err(inode, "Incorrect file signature");
  85                 else if (err == -EBADMSG)
  86                         fsverity_err(inode, "Malformed file signature");
  87                 else
  88                         fsverity_err(inode, "Error %d verifying file signature",
  89                                      err);
  90                 return err;
  91         }
  92
  93         return 0;
  94 }
  95
  96 #ifdef CONFIG_SYSCTL
  97 static struct ctl_table_header *fsverity_sysctl_header;
  98
  99 static struct ctl_table fsverity_sysctl_table[] = {
 100         {
 101                 .procname       = "require_signatures",
 102                 .data           = &fsverity_require_signatures,
 103                 .maxlen         = sizeof(int),
 104                 .mode           = 0644,
 105                 .proc_handler   = proc_dointvec_minmax,
 106                 .extra1         = SYSCTL_ZERO,
 107                 .extra2         = SYSCTL_ONE,
 108         },
 109         { }
 110 };
 111
 112 static int __init fsverity_sysctl_init(void)
 113 {
 114         fsverity_sysctl_header = register_sysctl("fs/verity", fsverity_sysctl_table);
 115         if (!fsverity_sysctl_header) {
 116                 pr_err("sysctl registration failed!\n");
 117                 return -ENOMEM;
 118         }
 119         return 0;
 120 }
 121 #else /* !CONFIG_SYSCTL */
 122 static inline int __init fsverity_sysctl_init(void)
 123 {
 124         return 0;
 125 }
 126 #endif /* !CONFIG_SYSCTL */
 127
 128 int __init fsverity_init_signature(void)
 129 {
 130         struct key *ring;
 131         int err;
 132
 133         ring = keyring_alloc(".fs-verity", KUIDT_INIT(0), KGIDT_INIT(0),
 134                              current_cred(), KEY_POS_SEARCH |
 135                                 KEY_USR_VIEW | KEY_USR_READ | KEY_USR_WRITE |
 136                                 KEY_USR_SEARCH | KEY_USR_SETATTR,
 137                              KEY_ALLOC_NOT_IN_QUOTA, NULL, NULL);
 138         if (IS_ERR(ring))
 139                 return PTR_ERR(ring);
 140
 141         err = fsverity_sysctl_init();
 142         if (err)
 143                 goto err_put_ring;
 144
 145         fsverity_keyring = ring;
 146         return 0;
 147
 148 err_put_ring:
 149         key_put(ring);
 150         return err;
 151 }