fs/verity/hash_algs.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * fs-verity hash algorithms
   4  *
   5  * Copyright 2019 Google LLC
   6  */
   7
   8 #include "fsverity_private.h"
   9
  10 #include <crypto/hash.h>
  11
  12 /* The hash algorithms supported by fs-verity */
  13 struct fsverity_hash_alg fsverity_hash_algs[] = {
  14         [FS_VERITY_HASH_ALG_SHA256] = {
  15                 .name = "sha256",
  16                 .digest_size = SHA256_DIGEST_SIZE,
  17                 .block_size = SHA256_BLOCK_SIZE,
  18                 .algo_id = HASH_ALGO_SHA256,
  19         },
  20         [FS_VERITY_HASH_ALG_SHA512] = {
  21                 .name = "sha512",
  22                 .digest_size = SHA512_DIGEST_SIZE,
  23                 .block_size = SHA512_BLOCK_SIZE,
  24                 .algo_id = HASH_ALGO_SHA512,
  25         },
  26 };
  27
  28 static DEFINE_MUTEX(fsverity_hash_alg_init_mutex);
  29
  30 /**
  31  * fsverity_get_hash_alg() - validate and prepare a hash algorithm
  32  * @inode: optional inode for logging purposes
  33  * @num: the hash algorithm number
  34  *
  35  * Get the struct fsverity_hash_alg for the given hash algorithm number, and
  36  * ensure it has a hash transform ready to go.  The hash transforms are
  37  * allocated on-demand so that we don't waste resources unnecessarily, and
  38  * because the crypto modules may be initialized later than fs/verity/.
  39  *
  40  * Return: pointer to the hash alg on success, else an ERR_PTR()
  41  */
  42 const struct fsverity_hash_alg *fsverity_get_hash_alg(const struct inode *inode,
  43                                                       unsigned int num)
  44 {
  45         struct fsverity_hash_alg *alg;
  46         struct crypto_shash *tfm;
  47         int err;
  48
  49         if (num >= ARRAY_SIZE(fsverity_hash_algs) ||
  50             !fsverity_hash_algs[num].name) {
  51                 fsverity_warn(inode, "Unknown hash algorithm number: %u", num);
  52                 return ERR_PTR(-EINVAL);
  53         }
  54         alg = &fsverity_hash_algs[num];
  55
  56         /* pairs with smp_store_release() below */
  57         if (likely(smp_load_acquire(&alg->tfm) != NULL))
  58                 return alg;
  59
  60         mutex_lock(&fsverity_hash_alg_init_mutex);
  61
  62         if (alg->tfm != NULL)
  63                 goto out_unlock;
  64
  65         tfm = crypto_alloc_shash(alg->name, 0, 0);
  66         if (IS_ERR(tfm)) {
  67                 if (PTR_ERR(tfm) == -ENOENT) {
  68                         fsverity_warn(inode,
  69                                       "Missing crypto API support for hash algorithm \"%s\"",
  70                                       alg->name);
  71                         alg = ERR_PTR(-ENOPKG);
  72                         goto out_unlock;
  73                 }
  74                 fsverity_err(inode,
  75                              "Error allocating hash algorithm \"%s\": %ld",
  76                              alg->name, PTR_ERR(tfm));
  77                 alg = ERR_CAST(tfm);
  78                 goto out_unlock;
  79         }
  80
  81         err = -EINVAL;
  82         if (WARN_ON_ONCE(alg->digest_size != crypto_shash_digestsize(tfm)))
  83                 goto err_free_tfm;
  84         if (WARN_ON_ONCE(alg->block_size != crypto_shash_blocksize(tfm)))
  85                 goto err_free_tfm;
  86
  87         pr_info("%s using implementation \"%s\"\n",
  88                 alg->name, crypto_shash_driver_name(tfm));
  89
  90         /* pairs with smp_load_acquire() above */
  91         smp_store_release(&alg->tfm, tfm);
  92         goto out_unlock;
  93
  94 err_free_tfm:
  95         crypto_free_shash(tfm);
  96         alg = ERR_PTR(err);
  97 out_unlock:
  98         mutex_unlock(&fsverity_hash_alg_init_mutex);
  99         return alg;
 100 }
 101
 102 /**
 103  * fsverity_prepare_hash_state() - precompute the initial hash state
 104  * @alg: hash algorithm
 105  * @salt: a salt which is to be prepended to all data to be hashed
 106  * @salt_size: salt size in bytes, possibly 0
 107  *
 108  * Return: NULL if the salt is empty, otherwise the kmalloc()'ed precomputed
 109  *         initial hash state on success or an ERR_PTR() on failure.
 110  */
 111 const u8 *fsverity_prepare_hash_state(const struct fsverity_hash_alg *alg,
 112                                       const u8 *salt, size_t salt_size)
 113 {
 114         u8 *hashstate = NULL;
 115         SHASH_DESC_ON_STACK(desc, alg->tfm);
 116         u8 *padded_salt = NULL;
 117         size_t padded_salt_size;
 118         int err;
 119
 120         desc->tfm = alg->tfm;
 121
 122         if (salt_size == 0)
 123                 return NULL;
 124
 125         hashstate = kmalloc(crypto_shash_statesize(alg->tfm), GFP_KERNEL);
 126         if (!hashstate)
 127                 return ERR_PTR(-ENOMEM);
 128
 129         /*
 130          * Zero-pad the salt to the next multiple of the input size of the hash
 131          * algorithm's compression function, e.g. 64 bytes for SHA-256 or 128
 132          * bytes for SHA-512.  This ensures that the hash algorithm won't have
 133          * any bytes buffered internally after processing the salt, thus making
 134          * salted hashing just as fast as unsalted hashing.
 135          */
 136         padded_salt_size = round_up(salt_size, alg->block_size);
 137         padded_salt = kzalloc(padded_salt_size, GFP_KERNEL);
 138         if (!padded_salt) {
 139                 err = -ENOMEM;
 140                 goto err_free;
 141         }
 142         memcpy(padded_salt, salt, salt_size);
 143         err = crypto_shash_init(desc);
 144         if (err)
 145                 goto err_free;
 146
 147         err = crypto_shash_update(desc, padded_salt, padded_salt_size);
 148         if (err)
 149                 goto err_free;
 150
 151         err = crypto_shash_export(desc, hashstate);
 152         if (err)
 153                 goto err_free;
 154 out:
 155         kfree(padded_salt);
 156         return hashstate;
 157
 158 err_free:
 159         kfree(hashstate);
 160         hashstate = ERR_PTR(err);
 161         goto out;
 162 }
 163
 164 /**
 165  * fsverity_hash_block() - hash a single data or hash block
 166  * @params: the Merkle tree's parameters
 167  * @inode: inode for which the hashing is being done
 168  * @data: virtual address of a buffer containing the block to hash
 169  * @out: output digest, size 'params->digest_size' bytes
 170  *
 171  * Hash a single data or hash block.  The hash is salted if a salt is specified
 172  * in the Merkle tree parameters.
 173  *
 174  * Return: 0 on success, -errno on failure
 175  */
 176 int fsverity_hash_block(const struct merkle_tree_params *params,
 177                         const struct inode *inode, const void *data, u8 *out)
 178 {
 179         SHASH_DESC_ON_STACK(desc, params->hash_alg->tfm);
 180         int err;
 181
 182         desc->tfm = params->hash_alg->tfm;
 183
 184         if (params->hashstate) {
 185                 err = crypto_shash_import(desc, params->hashstate);
 186                 if (err) {
 187                         fsverity_err(inode,
 188                                      "Error %d importing hash state", err);
 189                         return err;
 190                 }
 191                 err = crypto_shash_finup(desc, data, params->block_size, out);
 192         } else {
 193                 err = crypto_shash_digest(desc, data, params->block_size, out);
 194         }
 195         if (err)
 196                 fsverity_err(inode, "Error %d computing block hash", err);
 197         return err;
 198 }
 199
 200 /**
 201  * fsverity_hash_buffer() - hash some data
 202  * @alg: the hash algorithm to use
 203  * @data: the data to hash
 204  * @size: size of data to hash, in bytes
 205  * @out: output digest, size 'alg->digest_size' bytes
 206  *
 207  * Return: 0 on success, -errno on failure
 208  */
 209 int fsverity_hash_buffer(const struct fsverity_hash_alg *alg,
 210                          const void *data, size_t size, u8 *out)
 211 {
 212         return crypto_shash_tfm_digest(alg->tfm, data, size, out);
 213 }
 214
 215 void __init fsverity_check_hash_algs(void)
 216 {
 217         size_t i;
 218
 219         /*
 220          * Sanity check the hash algorithms (could be a build-time check, but
 221          * they're in an array)
 222          */
 223         for (i = 0; i < ARRAY_SIZE(fsverity_hash_algs); i++) {
 224                 const struct fsverity_hash_alg *alg = &fsverity_hash_algs[i];
 225
 226                 if (!alg->name)
 227                         continue;
 228
 229                 /*
 230                  * 0 must never be allocated as an FS_VERITY_HASH_ALG_* value,
 231                  * as it is reserved for users that use 0 to mean unspecified or
 232                  * a default value.  fs/verity/ itself doesn't care and doesn't
 233                  * have a default algorithm, but some users make use of this.
 234                  */
 235                 BUG_ON(i == 0);
 236
 237                 BUG_ON(alg->digest_size > FS_VERITY_MAX_DIGEST_SIZE);
 238
 239                 /*
 240                  * For efficiency, the implementation currently assumes the
 241                  * digest and block sizes are powers of 2.  This limitation can
 242                  * be lifted if the code is updated to handle other values.
 243                  */
 244                 BUG_ON(!is_power_of_2(alg->digest_size));
 245                 BUG_ON(!is_power_of_2(alg->block_size));
 246
 247                 /* Verify that there is a valid mapping to HASH_ALGO_*. */
 248                 BUG_ON(alg->algo_id == 0);
 249                 BUG_ON(alg->digest_size != hash_digest_size[alg->algo_id]);
 250         }
 251 }