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 #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  * @signature: the file's built-in signature
  33  * @sig_size: size of signature in bytes, or 0 if no signature
  34  *
  35  * If the file includes a signature of its fs-verity file digest, verify it
  36  * 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 u8 *signature, size_t sig_size)
  42 {
  43         const struct inode *inode = vi->inode;
  44         const struct fsverity_hash_alg *hash_alg = vi->tree_params.hash_alg;
  45         struct fsverity_formatted_digest *d;
  46         int err;
  47
  48         if (sig_size == 0) {
  49                 if (fsverity_require_signatures) {
  50                         fsverity_err(inode,
  51                                      "require_signatures=1, rejecting unsigned file!");
  52                         return -EPERM;
  53                 }
  54                 return 0;
  55         }
  56
  57         if (fsverity_keyring->keys.nr_leaves_on_tree == 0) {
  58                 /*
  59                  * The ".fs-verity" keyring is empty, due to builtin signatures
  60                  * being supported by the kernel but not actually being used.
  61                  * In this case, verify_pkcs7_signature() would always return an
  62                  * error, usually ENOKEY.  It could also be EBADMSG if the
  63                  * PKCS#7 is malformed, but that isn't very important to
  64                  * distinguish.  So, just skip to ENOKEY to avoid the attack
  65                  * surface of the PKCS#7 parser, which would otherwise be
  66                  * reachable by any task able to execute FS_IOC_ENABLE_VERITY.
  67                  */
  68                 fsverity_err(inode,
  69                              "fs-verity keyring is empty, rejecting signed file!");
  70                 return -ENOKEY;
  71         }
  72
  73         d = kzalloc(sizeof(*d) + hash_alg->digest_size, GFP_KERNEL);
  74         if (!d)
  75                 return -ENOMEM;
  76         memcpy(d->magic, "FSVerity", 8);
  77         d->digest_algorithm = cpu_to_le16(hash_alg - fsverity_hash_algs);
  78         d->digest_size = cpu_to_le16(hash_alg->digest_size);
  79         memcpy(d->digest, vi->file_digest, hash_alg->digest_size);
  80
  81         err = verify_pkcs7_signature(d, sizeof(*d) + hash_alg->digest_size,
  82                                      signature, sig_size, fsverity_keyring,
  83                                      VERIFYING_UNSPECIFIED_SIGNATURE,
  84                                      NULL, NULL);
  85         kfree(d);
  86
  87         if (err) {
  88                 if (err == -ENOKEY)
  89                         fsverity_err(inode,
  90                                      "File's signing cert isn't in the fs-verity keyring");
  91                 else if (err == -EKEYREJECTED)
  92                         fsverity_err(inode, "Incorrect file signature");
  93                 else if (err == -EBADMSG)
  94                         fsverity_err(inode, "Malformed file signature");
  95                 else
  96                         fsverity_err(inode, "Error %d verifying file signature",
  97                                      err);
  98                 return err;
  99         }
 100
 101         pr_debug("Valid signature for file digest %s:%*phN\n",
 102                  hash_alg->name, hash_alg->digest_size, vi->file_digest);
 103         return 0;
 104 }
 105
 106 #ifdef CONFIG_SYSCTL
 107 static struct ctl_table_header *fsverity_sysctl_header;
 108
 109 static const struct ctl_path fsverity_sysctl_path[] = {
 110         { .procname = "fs", },
 111         { .procname = "verity", },
 112         { }
 113 };
 114
 115 static struct ctl_table fsverity_sysctl_table[] = {
 116         {
 117                 .procname       = "require_signatures",
 118                 .data           = &fsverity_require_signatures,
 119                 .maxlen         = sizeof(int),
 120                 .mode           = 0644,
 121                 .proc_handler   = proc_dointvec_minmax,
 122                 .extra1         = SYSCTL_ZERO,
 123                 .extra2         = SYSCTL_ONE,
 124         },
 125         { }
 126 };
 127
 128 static int __init fsverity_sysctl_init(void)
 129 {
 130         fsverity_sysctl_header = register_sysctl_paths(fsverity_sysctl_path,
 131                                                        fsverity_sysctl_table);
 132         if (!fsverity_sysctl_header) {
 133                 pr_err("sysctl registration failed!\n");
 134                 return -ENOMEM;
 135         }
 136         return 0;
 137 }
 138 #else /* !CONFIG_SYSCTL */
 139 static inline int __init fsverity_sysctl_init(void)
 140 {
 141         return 0;
 142 }
 143 #endif /* !CONFIG_SYSCTL */
 144
 145 int __init fsverity_init_signature(void)
 146 {
 147         struct key *ring;
 148         int err;
 149
 150         ring = keyring_alloc(".fs-verity", KUIDT_INIT(0), KGIDT_INIT(0),
 151                              current_cred(), KEY_POS_SEARCH |
 152                                 KEY_USR_VIEW | KEY_USR_READ | KEY_USR_WRITE |
 153                                 KEY_USR_SEARCH | KEY_USR_SETATTR,
 154                              KEY_ALLOC_NOT_IN_QUOTA, NULL, NULL);
 155         if (IS_ERR(ring))
 156                 return PTR_ERR(ring);
 157
 158         err = fsverity_sysctl_init();
 159         if (err)
 160                 goto err_put_ring;
 161
 162         fsverity_keyring = ring;
 163         return 0;
 164
 165 err_put_ring:
 166         key_put(ring);
 167         return err;
 168 }