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  *
  32  * If the file's fs-verity descriptor includes a signature of the file
  33  * measurement, verify it against the certificates in the fs-verity keyring.
  34  *
  35  * Return: 0 on success (signature valid or not required); -errno on failure
  36  */
  37 int fsverity_verify_signature(const struct fsverity_info *vi,
  38                               const struct fsverity_descriptor *desc,
  39                               size_t desc_size)
  40 {
  41         const struct inode *inode = vi->inode;
  42         const struct fsverity_hash_alg *hash_alg = vi->tree_params.hash_alg;
  43         const u32 sig_size = le32_to_cpu(desc->sig_size);
  44         struct fsverity_signed_digest *d;
  45         int err;
  46
  47         if (sig_size == 0) {
  48                 if (fsverity_require_signatures) {
  49                         fsverity_err(inode,
  50                                      "require_signatures=1, rejecting unsigned file!");
  51                         return -EPERM;
  52                 }
  53                 return 0;
  54         }
  55
  56         if (sig_size > desc_size - sizeof(*desc)) {
  57                 fsverity_err(inode, "Signature overflows verity descriptor");
  58                 return -EBADMSG;
  59         }
  60
  61         if (fsverity_keyring->keys.nr_leaves_on_tree == 0) {
  62                 /*
  63                  * The ".fs-verity" keyring is empty, due to builtin signatures
  64                  * being supported by the kernel but not actually being used.
  65                  * In this case, verify_pkcs7_signature() would always return an
  66                  * error, usually ENOKEY.  It could also be EBADMSG if the
  67                  * PKCS#7 is malformed, but that isn't very important to
  68                  * distinguish.  So, just skip to ENOKEY to avoid the attack
  69                  * surface of the PKCS#7 parser, which would otherwise be
  70                  * reachable by any task able to execute FS_IOC_ENABLE_VERITY.
  71                  */
  72                 fsverity_err(inode,
  73                              "fs-verity keyring is empty, rejecting signed file!");
  74                 return -ENOKEY;
  75         }
  76
  77         d = kzalloc(sizeof(*d) + hash_alg->digest_size, GFP_KERNEL);
  78         if (!d)
  79                 return -ENOMEM;
  80         memcpy(d->magic, "FSVerity", 8);
  81         d->digest_algorithm = cpu_to_le16(hash_alg - fsverity_hash_algs);
  82         d->digest_size = cpu_to_le16(hash_alg->digest_size);
  83         memcpy(d->digest, vi->measurement, hash_alg->digest_size);
  84
  85         err = verify_pkcs7_signature(d, sizeof(*d) + hash_alg->digest_size,
  86                                      desc->signature, sig_size,
  87                                      fsverity_keyring,
  88                                      VERIFYING_UNSPECIFIED_SIGNATURE,
  89                                      NULL, NULL);
  90         kfree(d);
  91
  92         if (err) {
  93                 if (err == -ENOKEY)
  94                         fsverity_err(inode,
  95                                      "File's signing cert isn't in the fs-verity keyring");
  96                 else if (err == -EKEYREJECTED)
  97                         fsverity_err(inode, "Incorrect file signature");
  98                 else if (err == -EBADMSG)
  99                         fsverity_err(inode, "Malformed file signature");
 100                 else
 101                         fsverity_err(inode, "Error %d verifying file signature",
 102                                      err);
 103                 return err;
 104         }
 105
 106         pr_debug("Valid signature for file measurement %s:%*phN\n",
 107                  hash_alg->name, hash_alg->digest_size, vi->measurement);
 108         return 0;
 109 }
 110
 111 #ifdef CONFIG_SYSCTL
 112 static struct ctl_table_header *fsverity_sysctl_header;
 113
 114 static const struct ctl_path fsverity_sysctl_path[] = {
 115         { .procname = "fs", },
 116         { .procname = "verity", },
 117         { }
 118 };
 119
 120 static struct ctl_table fsverity_sysctl_table[] = {
 121         {
 122                 .procname       = "require_signatures",
 123                 .data           = &fsverity_require_signatures,
 124                 .maxlen         = sizeof(int),
 125                 .mode           = 0644,
 126                 .proc_handler   = proc_dointvec_minmax,
 127                 .extra1         = SYSCTL_ZERO,
 128                 .extra2         = SYSCTL_ONE,
 129         },
 130         { }
 131 };
 132
 133 static int __init fsverity_sysctl_init(void)
 134 {
 135         fsverity_sysctl_header = register_sysctl_paths(fsverity_sysctl_path,
 136                                                        fsverity_sysctl_table);
 137         if (!fsverity_sysctl_header) {
 138                 pr_err("sysctl registration failed!\n");
 139                 return -ENOMEM;
 140         }
 141         return 0;
 142 }
 143 #else /* !CONFIG_SYSCTL */
 144 static inline int __init fsverity_sysctl_init(void)
 145 {
 146         return 0;
 147 }
 148 #endif /* !CONFIG_SYSCTL */
 149
 150 int __init fsverity_init_signature(void)
 151 {
 152         struct key *ring;
 153         int err;
 154
 155         ring = keyring_alloc(".fs-verity", KUIDT_INIT(0), KGIDT_INIT(0),
 156                              current_cred(), KEY_POS_SEARCH |
 157                                 KEY_USR_VIEW | KEY_USR_READ | KEY_USR_WRITE |
 158                                 KEY_USR_SEARCH | KEY_USR_SETATTR,
 159                              KEY_ALLOC_NOT_IN_QUOTA, NULL, NULL);
 160         if (IS_ERR(ring))
 161                 return PTR_ERR(ring);
 162
 163         err = fsverity_sysctl_init();
 164         if (err)
 165                 goto err_put_ring;
 166
 167         fsverity_keyring = ring;
 168         return 0;
 169
 170 err_put_ring:
 171         key_put(ring);
 172         return err;
 173 }