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 }
252
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