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         if (fsverity_keyring->keys.nr_leaves_on_tree == 0) {
  65                 /*
  66                  * The ".fs-verity" keyring is empty, due to builtin signatures
  67                  * being supported by the kernel but not actually being used.
  68                  * In this case, verify_pkcs7_signature() would always return an
  69                  * error, usually ENOKEY.  It could also be EBADMSG if the
  70                  * PKCS#7 is malformed, but that isn't very important to
  71                  * distinguish.  So, just skip to ENOKEY to avoid the attack
  72                  * surface of the PKCS#7 parser, which would otherwise be
  73                  * reachable by any task able to execute FS_IOC_ENABLE_VERITY.
  74                  */
  75                 fsverity_err(inode,
  76                              "fs-verity keyring is empty, rejecting signed file!");
  77                 return -ENOKEY;
  78         }
  79
  80         d = kzalloc(sizeof(*d) + hash_alg->digest_size, GFP_KERNEL);
  81         if (!d)
  82                 return -ENOMEM;
  83         memcpy(d->magic, "FSVerity", 8);
  84         d->digest_algorithm = cpu_to_le16(hash_alg - fsverity_hash_algs);
  85         d->digest_size = cpu_to_le16(hash_alg->digest_size);
  86         memcpy(d->digest, vi->measurement, hash_alg->digest_size);
  87
  88         err = verify_pkcs7_signature(d, sizeof(*d) + hash_alg->digest_size,
  89                                      desc->signature, sig_size,
  90                                      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         pr_debug("Valid signature for file measurement %s:%*phN\n",
 110                  hash_alg->name, hash_alg->digest_size, vi->measurement);
 111         return 0;
 112 }
 113
 114 #ifdef CONFIG_SYSCTL
 115 static struct ctl_table_header *fsverity_sysctl_header;
 116
 117 static const struct ctl_path fsverity_sysctl_path[] = {
 118         { .procname = "fs", },
 119         { .procname = "verity", },
 120         { }
 121 };
 122
 123 static struct ctl_table fsverity_sysctl_table[] = {
 124         {
 125                 .procname       = "require_signatures",
 126                 .data           = &fsverity_require_signatures,
 127                 .maxlen         = sizeof(int),
 128                 .mode           = 0644,
 129                 .proc_handler   = proc_dointvec_minmax,
 130                 .extra1         = SYSCTL_ZERO,
 131                 .extra2         = SYSCTL_ONE,
 132         },
 133         { }
 134 };
 135
 136 static int __init fsverity_sysctl_init(void)
 137 {
 138         fsverity_sysctl_header = register_sysctl_paths(fsverity_sysctl_path,
 139                                                        fsverity_sysctl_table);
 140         if (!fsverity_sysctl_header) {
 141                 pr_err("sysctl registration failed!\n");
 142                 return -ENOMEM;
 143         }
 144         return 0;
 145 }
 146 #else /* !CONFIG_SYSCTL */
 147 static inline int __init fsverity_sysctl_init(void)
 148 {
 149         return 0;
 150 }
 151 #endif /* !CONFIG_SYSCTL */
 152
 153 int __init fsverity_init_signature(void)
 154 {
 155         struct key *ring;
 156         int err;
 157
 158         ring = keyring_alloc(".fs-verity", KUIDT_INIT(0), KGIDT_INIT(0),
 159                              current_cred(), KEY_POS_SEARCH |
 160                                 KEY_USR_VIEW | KEY_USR_READ | KEY_USR_WRITE |
 161                                 KEY_USR_SEARCH | KEY_USR_SETATTR,
 162                              KEY_ALLOC_NOT_IN_QUOTA, NULL, NULL);
 163         if (IS_ERR(ring))
 164                 return PTR_ERR(ring);
 165
 166         err = fsverity_sysctl_init();
 167         if (err)
 168                 goto err_put_ring;
 169
 170         fsverity_keyring = ring;
 171         return 0;
 172
 173 err_put_ring:
 174         key_put(ring);
 175         return err;
 176 }