fs/verity/signature.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * fs/verity/signature.c: verification of builtin signatures
   4  *
   5  * Copyright 2019 Google LLC
   6  */
   7
   8 #include "fsverity_private.h"
   9
  10 #include <linux/cred.h>
  11 #include <linux/key.h>
  12 #include <linux/slab.h>
  13 #include <linux/verification.h>
  14
  15 /*
  16  * /proc/sys/fs/verity/require_signatures
  17  * If 1, all verity files must have a valid builtin signature.
  18  */
  19 static int fsverity_require_signatures;
  20
  21 /*
  22  * Keyring that contains the trusted X.509 certificates.
  23  *
  24  * Only root (kuid=0) can modify this.  Also, root may use
  25  * keyctl_restrict_keyring() to prevent any more additions.
  26  */
  27 static struct key *fsverity_keyring;
  28
  29 /**
  30  * fsverity_verify_signature() - check a verity file's signature
  31  * @vi: the file's fsverity_info
  32  * @desc: the file's fsverity_descriptor
  33  * @desc_size: size of @desc
  34  *
  35  * If the file's fs-verity descriptor includes a signature of the file
  36  * measurement, verify it against the certificates in the fs-verity keyring.
  37  *
  38  * Return: 0 on success (signature valid or not required); -errno on failure
  39  */
  40 int fsverity_verify_signature(const struct fsverity_info *vi,
  41                               const struct fsverity_descriptor *desc,
  42                               size_t desc_size)
  43 {
  44         const struct inode *inode = vi->inode;
  45         const struct fsverity_hash_alg *hash_alg = vi->tree_params.hash_alg;
  46         const u32 sig_size = le32_to_cpu(desc->sig_size);
  47         struct fsverity_signed_digest *d;
  48         int err;
  49
  50         if (sig_size == 0) {
  51                 if (fsverity_require_signatures) {
  52                         fsverity_err(inode,
  53                                      "require_signatures=1, rejecting unsigned file!");
  54                         return -EPERM;
  55                 }
  56                 return 0;
  57         }
  58
  59         if (sig_size > desc_size - sizeof(*desc)) {
  60                 fsverity_err(inode, "Signature overflows verity descriptor");
  61                 return -EBADMSG;
  62         }
  63
  64         d = kzalloc(sizeof(*d) + hash_alg->digest_size, GFP_KERNEL);
  65         if (!d)
  66                 return -ENOMEM;
  67         memcpy(d->magic, "FSVerity", 8);
  68         d->digest_algorithm = cpu_to_le16(hash_alg - fsverity_hash_algs);
  69         d->digest_size = cpu_to_le16(hash_alg->digest_size);
  70         memcpy(d->digest, vi->measurement, hash_alg->digest_size);
  71
  72         err = verify_pkcs7_signature(d, sizeof(*d) + hash_alg->digest_size,
  73                                      desc->signature, sig_size,
  74                                      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         pr_debug("Valid signature for file measurement %s:%*phN\n",
  94                  hash_alg->name, hash_alg->digest_size, vi->measurement);
  95         return 0;
  96 }
  97
  98 #ifdef CONFIG_SYSCTL
  99 static struct ctl_table_header *fsverity_sysctl_header;
 100
 101 static const struct ctl_path fsverity_sysctl_path[] = {
 102         { .procname = "fs", },
 103         { .procname = "verity", },
 104         { }
 105 };
 106
 107 static struct ctl_table fsverity_sysctl_table[] = {
 108         {
 109                 .procname       = "require_signatures",
 110                 .data           = &fsverity_require_signatures,
 111                 .maxlen         = sizeof(int),
 112                 .mode           = 0644,
 113                 .proc_handler   = proc_dointvec_minmax,
 114                 .extra1         = SYSCTL_ZERO,
 115                 .extra2         = SYSCTL_ONE,
 116         },
 117         { }
 118 };
 119
 120 static int __init fsverity_sysctl_init(void)
 121 {
 122         fsverity_sysctl_header = register_sysctl_paths(fsverity_sysctl_path,
 123                                                        fsverity_sysctl_table);
 124         if (!fsverity_sysctl_header) {
 125                 pr_err("sysctl registration failed!\n");
 126                 return -ENOMEM;
 127         }
 128         return 0;
 129 }
 130 #else /* !CONFIG_SYSCTL */
 131 static inline int __init fsverity_sysctl_init(void)
 132 {
 133         return 0;
 134 }
 135 #endif /* !CONFIG_SYSCTL */
 136
 137 int __init fsverity_init_signature(void)
 138 {
 139         struct key *ring;
 140         int err;
 141
 142         ring = keyring_alloc(".fs-verity", KUIDT_INIT(0), KGIDT_INIT(0),
 143                              current_cred(), KEY_POS_SEARCH |
 144                                 KEY_USR_VIEW | KEY_USR_READ | KEY_USR_WRITE |
 145                                 KEY_USR_SEARCH | KEY_USR_SETATTR,
 146                              KEY_ALLOC_NOT_IN_QUOTA, NULL, NULL);
 147         if (IS_ERR(ring))
 148                 return PTR_ERR(ring);
 149
 150         err = fsverity_sysctl_init();
 151         if (err)
 152                 goto err_put_ring;
 153
 154         fsverity_keyring = ring;
 155         return 0;
 156
 157 err_put_ring:
 158         key_put(ring);
 159         return err;
 160 }