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  */
fsverity_get_hash_alg(const struct inode * inode,unsigned int num)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  */
fsverity_prepare_hash_state(const struct fsverity_hash_alg * alg,const u8 * salt,size_t salt_size)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  */
fsverity_hash_block(const struct merkle_tree_params * params,const struct inode * inode,const void * data,u8 * out)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  */
fsverity_hash_buffer(const struct fsverity_hash_alg * alg,const void * data,size_t size,u8 * out)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 
fsverity_check_hash_algs(void)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