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Linux/fs/verity/hash_algs.c

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  1 // SPDX-License-Identifier: GPL-2.0
  2 /*
  3  * fs/verity/hash_algs.c: fs-verity hash algorithms
  4  *
  5  * Copyright 2019 Google LLC
  6  */
  7 
  8 #include "fsverity_private.h"
  9 
 10 #include <crypto/hash.h>
 11 #include <linux/scatterlist.h>
 12 
 13 /* The hash algorithms supported by fs-verity */
 14 struct fsverity_hash_alg fsverity_hash_algs[] = {
 15         [FS_VERITY_HASH_ALG_SHA256] = {
 16                 .name = "sha256",
 17                 .digest_size = SHA256_DIGEST_SIZE,
 18                 .block_size = SHA256_BLOCK_SIZE,
 19         },
 20         [FS_VERITY_HASH_ALG_SHA512] = {
 21                 .name = "sha512",
 22                 .digest_size = SHA512_DIGEST_SIZE,
 23                 .block_size = SHA512_BLOCK_SIZE,
 24         },
 25 };
 26 
 27 static DEFINE_MUTEX(fsverity_hash_alg_init_mutex);
 28 
 29 /**
 30  * fsverity_get_hash_alg() - validate and prepare a hash algorithm
 31  * @inode: optional inode for logging purposes
 32  * @num: the hash algorithm number
 33  *
 34  * Get the struct fsverity_hash_alg for the given hash algorithm number, and
 35  * ensure it has a hash transform ready to go.  The hash transforms are
 36  * allocated on-demand so that we don't waste resources unnecessarily, and
 37  * because the crypto modules may be initialized later than fs/verity/.
 38  *
 39  * Return: pointer to the hash alg on success, else an ERR_PTR()
 40  */
 41 struct fsverity_hash_alg *fsverity_get_hash_alg(const struct inode *inode,
 42                                                 unsigned int num)
 43 {
 44         struct fsverity_hash_alg *alg;
 45         struct crypto_ahash *tfm;
 46         int err;
 47 
 48         if (num >= ARRAY_SIZE(fsverity_hash_algs) ||
 49             !fsverity_hash_algs[num].name) {
 50                 fsverity_warn(inode, "Unknown hash algorithm number: %u", num);
 51                 return ERR_PTR(-EINVAL);
 52         }
 53         alg = &fsverity_hash_algs[num];
 54 
 55         /* pairs with smp_store_release() below */
 56         if (likely(smp_load_acquire(&alg->tfm) != NULL))
 57                 return alg;
 58 
 59         mutex_lock(&fsverity_hash_alg_init_mutex);
 60 
 61         if (alg->tfm != NULL)
 62                 goto out_unlock;
 63 
 64         /*
 65          * Using the shash API would make things a bit simpler, but the ahash
 66          * API is preferable as it allows the use of crypto accelerators.
 67          */
 68         tfm = crypto_alloc_ahash(alg->name, 0, 0);
 69         if (IS_ERR(tfm)) {
 70                 if (PTR_ERR(tfm) == -ENOENT) {
 71                         fsverity_warn(inode,
 72                                       "Missing crypto API support for hash algorithm \"%s\"",
 73                                       alg->name);
 74                         alg = ERR_PTR(-ENOPKG);
 75                         goto out_unlock;
 76                 }
 77                 fsverity_err(inode,
 78                              "Error allocating hash algorithm \"%s\": %ld",
 79                              alg->name, PTR_ERR(tfm));
 80                 alg = ERR_CAST(tfm);
 81                 goto out_unlock;
 82         }
 83 
 84         err = -EINVAL;
 85         if (WARN_ON(alg->digest_size != crypto_ahash_digestsize(tfm)))
 86                 goto err_free_tfm;
 87         if (WARN_ON(alg->block_size != crypto_ahash_blocksize(tfm)))
 88                 goto err_free_tfm;
 89 
 90         err = mempool_init_kmalloc_pool(&alg->req_pool, 1,
 91                                         sizeof(struct ahash_request) +
 92                                         crypto_ahash_reqsize(tfm));
 93         if (err)
 94                 goto err_free_tfm;
 95 
 96         pr_info("%s using implementation \"%s\"\n",
 97                 alg->name, crypto_ahash_driver_name(tfm));
 98 
 99         /* pairs with smp_load_acquire() above */
100         smp_store_release(&alg->tfm, tfm);
101         goto out_unlock;
102 
103 err_free_tfm:
104         crypto_free_ahash(tfm);
105         alg = ERR_PTR(err);
106 out_unlock:
107         mutex_unlock(&fsverity_hash_alg_init_mutex);
108         return alg;
109 }
110 
111 /**
112  * fsverity_alloc_hash_request() - allocate a hash request object
113  * @alg: the hash algorithm for which to allocate the request
114  * @gfp_flags: memory allocation flags
115  *
116  * This is mempool-backed, so this never fails if __GFP_DIRECT_RECLAIM is set in
117  * @gfp_flags.  However, in that case this might need to wait for all
118  * previously-allocated requests to be freed.  So to avoid deadlocks, callers
119  * must never need multiple requests at a time to make forward progress.
120  *
121  * Return: the request object on success; NULL on failure (but see above)
122  */
123 struct ahash_request *fsverity_alloc_hash_request(struct fsverity_hash_alg *alg,
124                                                   gfp_t gfp_flags)
125 {
126         struct ahash_request *req = mempool_alloc(&alg->req_pool, gfp_flags);
127 
128         if (req)
129                 ahash_request_set_tfm(req, alg->tfm);
130         return req;
131 }
132 
133 /**
134  * fsverity_free_hash_request() - free a hash request object
135  * @alg: the hash algorithm
136  * @req: the hash request object to free
137  */
138 void fsverity_free_hash_request(struct fsverity_hash_alg *alg,
139                                 struct ahash_request *req)
140 {
141         if (req) {
142                 ahash_request_zero(req);
143                 mempool_free(req, &alg->req_pool);
144         }
145 }
146 
147 /**
148  * fsverity_prepare_hash_state() - precompute the initial hash state
149  * @alg: hash algorithm
150  * @salt: a salt which is to be prepended to all data to be hashed
151  * @salt_size: salt size in bytes, possibly 0
152  *
153  * Return: NULL if the salt is empty, otherwise the kmalloc()'ed precomputed
154  *         initial hash state on success or an ERR_PTR() on failure.
155  */
156 const u8 *fsverity_prepare_hash_state(struct fsverity_hash_alg *alg,
157                                       const u8 *salt, size_t salt_size)
158 {
159         u8 *hashstate = NULL;
160         struct ahash_request *req = NULL;
161         u8 *padded_salt = NULL;
162         size_t padded_salt_size;
163         struct scatterlist sg;
164         DECLARE_CRYPTO_WAIT(wait);
165         int err;
166 
167         if (salt_size == 0)
168                 return NULL;
169 
170         hashstate = kmalloc(crypto_ahash_statesize(alg->tfm), GFP_KERNEL);
171         if (!hashstate)
172                 return ERR_PTR(-ENOMEM);
173 
174         /* This allocation never fails, since it's mempool-backed. */
175         req = fsverity_alloc_hash_request(alg, GFP_KERNEL);
176 
177         /*
178          * Zero-pad the salt to the next multiple of the input size of the hash
179          * algorithm's compression function, e.g. 64 bytes for SHA-256 or 128
180          * bytes for SHA-512.  This ensures that the hash algorithm won't have
181          * any bytes buffered internally after processing the salt, thus making
182          * salted hashing just as fast as unsalted hashing.
183          */
184         padded_salt_size = round_up(salt_size, alg->block_size);
185         padded_salt = kzalloc(padded_salt_size, GFP_KERNEL);
186         if (!padded_salt) {
187                 err = -ENOMEM;
188                 goto err_free;
189         }
190         memcpy(padded_salt, salt, salt_size);
191 
192         sg_init_one(&sg, padded_salt, padded_salt_size);
193         ahash_request_set_callback(req, CRYPTO_TFM_REQ_MAY_SLEEP |
194                                         CRYPTO_TFM_REQ_MAY_BACKLOG,
195                                    crypto_req_done, &wait);
196         ahash_request_set_crypt(req, &sg, NULL, padded_salt_size);
197 
198         err = crypto_wait_req(crypto_ahash_init(req), &wait);
199         if (err)
200                 goto err_free;
201 
202         err = crypto_wait_req(crypto_ahash_update(req), &wait);
203         if (err)
204                 goto err_free;
205 
206         err = crypto_ahash_export(req, hashstate);
207         if (err)
208                 goto err_free;
209 out:
210         fsverity_free_hash_request(alg, req);
211         kfree(padded_salt);
212         return hashstate;
213 
214 err_free:
215         kfree(hashstate);
216         hashstate = ERR_PTR(err);
217         goto out;
218 }
219 
220 /**
221  * fsverity_hash_page() - hash a single data or hash page
222  * @params: the Merkle tree's parameters
223  * @inode: inode for which the hashing is being done
224  * @req: preallocated hash request
225  * @page: the page to hash
226  * @out: output digest, size 'params->digest_size' bytes
227  *
228  * Hash a single data or hash block, assuming block_size == PAGE_SIZE.
229  * The hash is salted if a salt is specified in the Merkle tree parameters.
230  *
231  * Return: 0 on success, -errno on failure
232  */
233 int fsverity_hash_page(const struct merkle_tree_params *params,
234                        const struct inode *inode,
235                        struct ahash_request *req, struct page *page, u8 *out)
236 {
237         struct scatterlist sg;
238         DECLARE_CRYPTO_WAIT(wait);
239         int err;
240 
241         if (WARN_ON(params->block_size != PAGE_SIZE))
242                 return -EINVAL;
243 
244         sg_init_table(&sg, 1);
245         sg_set_page(&sg, page, PAGE_SIZE, 0);
246         ahash_request_set_callback(req, CRYPTO_TFM_REQ_MAY_SLEEP |
247                                         CRYPTO_TFM_REQ_MAY_BACKLOG,
248                                    crypto_req_done, &wait);
249         ahash_request_set_crypt(req, &sg, out, PAGE_SIZE);
250 
251         if (params->hashstate) {
252                 err = crypto_ahash_import(req, params->hashstate);
253                 if (err) {
254                         fsverity_err(inode,
255                                      "Error %d importing hash state", err);
256                         return err;
257                 }
258                 err = crypto_ahash_finup(req);
259         } else {
260                 err = crypto_ahash_digest(req);
261         }
262 
263         err = crypto_wait_req(err, &wait);
264         if (err)
265                 fsverity_err(inode, "Error %d computing page hash", err);
266         return err;
267 }
268 
269 /**
270  * fsverity_hash_buffer() - hash some data
271  * @alg: the hash algorithm to use
272  * @data: the data to hash
273  * @size: size of data to hash, in bytes
274  * @out: output digest, size 'alg->digest_size' bytes
275  *
276  * Hash some data which is located in physically contiguous memory (i.e. memory
277  * allocated by kmalloc(), not by vmalloc()).  No salt is used.
278  *
279  * Return: 0 on success, -errno on failure
280  */
281 int fsverity_hash_buffer(struct fsverity_hash_alg *alg,
282                          const void *data, size_t size, u8 *out)
283 {
284         struct ahash_request *req;
285         struct scatterlist sg;
286         DECLARE_CRYPTO_WAIT(wait);
287         int err;
288 
289         /* This allocation never fails, since it's mempool-backed. */
290         req = fsverity_alloc_hash_request(alg, GFP_KERNEL);
291 
292         sg_init_one(&sg, data, size);
293         ahash_request_set_callback(req, CRYPTO_TFM_REQ_MAY_SLEEP |
294                                         CRYPTO_TFM_REQ_MAY_BACKLOG,
295                                    crypto_req_done, &wait);
296         ahash_request_set_crypt(req, &sg, out, size);
297 
298         err = crypto_wait_req(crypto_ahash_digest(req), &wait);
299 
300         fsverity_free_hash_request(alg, req);
301         return err;
302 }
303 
304 void __init fsverity_check_hash_algs(void)
305 {
306         size_t i;
307 
308         /*
309          * Sanity check the hash algorithms (could be a build-time check, but
310          * they're in an array)
311          */
312         for (i = 0; i < ARRAY_SIZE(fsverity_hash_algs); i++) {
313                 const struct fsverity_hash_alg *alg = &fsverity_hash_algs[i];
314 
315                 if (!alg->name)
316                         continue;
317 
318                 BUG_ON(alg->digest_size > FS_VERITY_MAX_DIGEST_SIZE);
319 
320                 /*
321                  * For efficiency, the implementation currently assumes the
322                  * digest and block sizes are powers of 2.  This limitation can
323                  * be lifted if the code is updated to handle other values.
324                  */
325                 BUG_ON(!is_power_of_2(alg->digest_size));
326                 BUG_ON(!is_power_of_2(alg->block_size));
327         }
328 }
329 

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