1 // SPDX-License-Identifier: GPL-2.0
2 /*
3 * Opening fs-verity files
4 *
5 * Copyright 2019 Google LLC
6 */
7
8 #include "fsverity_private.h"
9
10 #include <linux/mm.h>
11 #include <linux/slab.h>
12
13 static struct kmem_cache *fsverity_info_cachep;
14
15 /**
16 * fsverity_init_merkle_tree_params() - initialize Merkle tree parameters
17 * @params: the parameters struct to initialize
18 * @inode: the inode for which the Merkle tree is being built
19 * @hash_algorithm: number of hash algorithm to use
20 * @log_blocksize: log base 2 of block size to use
21 * @salt: pointer to salt (optional)
22 * @salt_size: size of salt, possibly 0
23 *
24 * Validate the hash algorithm and block size, then compute the tree topology
25 * (num levels, num blocks in each level, etc.) and initialize @params.
26 *
27 * Return: 0 on success, -errno on failure
28 */
29 int fsverity_init_merkle_tree_params(struct merkle_tree_params *params,
30 const struct inode *inode,
31 unsigned int hash_algorithm,
32 unsigned int log_blocksize,
33 const u8 *salt, size_t salt_size)
34 {
35 const struct fsverity_hash_alg *hash_alg;
36 int err;
37 u64 blocks;
38 u64 blocks_in_level[FS_VERITY_MAX_LEVELS];
39 u64 offset;
40 int level;
41
42 memset(params, 0, sizeof(*params));
43
44 hash_alg = fsverity_get_hash_alg(inode, hash_algorithm);
45 if (IS_ERR(hash_alg))
46 return PTR_ERR(hash_alg);
47 params->hash_alg = hash_alg;
48 params->digest_size = hash_alg->digest_size;
49
50 params->hashstate = fsverity_prepare_hash_state(hash_alg, salt,
51 salt_size);
52 if (IS_ERR(params->hashstate)) {
53 err = PTR_ERR(params->hashstate);
54 params->hashstate = NULL;
55 fsverity_err(inode, "Error %d preparing hash state", err);
56 goto out_err;
57 }
58
59 /*
60 * fs/verity/ directly assumes that the Merkle tree block size is a
61 * power of 2 less than or equal to PAGE_SIZE. Another restriction
62 * arises from the interaction between fs/verity/ and the filesystems
63 * themselves: filesystems expect to be able to verify a single
64 * filesystem block of data at a time. Therefore, the Merkle tree block
65 * size must also be less than or equal to the filesystem block size.
66 *
67 * The above are the only hard limitations, so in theory the Merkle tree
68 * block size could be as small as twice the digest size. However,
69 * that's not useful, and it would result in some unusually deep and
70 * large Merkle trees. So we currently require that the Merkle tree
71 * block size be at least 1024 bytes. That's small enough to test the
72 * sub-page block case on systems with 4K pages, but not too small.
73 */
74 if (log_blocksize < 10 || log_blocksize > PAGE_SHIFT ||
75 log_blocksize > inode->i_blkbits) {
76 fsverity_warn(inode, "Unsupported log_blocksize: %u",
77 log_blocksize);
78 err = -EINVAL;
79 goto out_err;
80 }
81 params->log_blocksize = log_blocksize;
82 params->block_size = 1 << log_blocksize;
83 params->log_blocks_per_page = PAGE_SHIFT - log_blocksize;
84 params->blocks_per_page = 1 << params->log_blocks_per_page;
85
86 if (WARN_ON_ONCE(!is_power_of_2(params->digest_size))) {
87 err = -EINVAL;
88 goto out_err;
89 }
90 if (params->block_size < 2 * params->digest_size) {
91 fsverity_warn(inode,
92 "Merkle tree block size (%u) too small for hash algorithm \"%s\"",
93 params->block_size, hash_alg->name);
94 err = -EINVAL;
95 goto out_err;
96 }
97 params->log_digestsize = ilog2(params->digest_size);
98 params->log_arity = log_blocksize - params->log_digestsize;
99 params->hashes_per_block = 1 << params->log_arity;
100
101 /*
102 * Compute the number of levels in the Merkle tree and create a map from
103 * level to the starting block of that level. Level 'num_levels - 1' is
104 * the root and is stored first. Level 0 is the level directly "above"
105 * the data blocks and is stored last.
106 */
107
108 /* Compute number of levels and the number of blocks in each level */
109 blocks = ((u64)inode->i_size + params->block_size - 1) >> log_blocksize;
110 while (blocks > 1) {
111 if (params->num_levels >= FS_VERITY_MAX_LEVELS) {
112 fsverity_err(inode, "Too many levels in Merkle tree");
113 err = -EFBIG;
114 goto out_err;
115 }
116 blocks = (blocks + params->hashes_per_block - 1) >>
117 params->log_arity;
118 blocks_in_level[params->num_levels++] = blocks;
119 }
120
121 /* Compute the starting block of each level */
122 offset = 0;
123 for (level = (int)params->num_levels - 1; level >= 0; level--) {
124 params->level_start[level] = offset;
125 offset += blocks_in_level[level];
126 }
127
128 /*
129 * With block_size != PAGE_SIZE, an in-memory bitmap will need to be
130 * allocated to track the "verified" status of hash blocks. Don't allow
131 * this bitmap to get too large. For now, limit it to 1 MiB, which
132 * limits the file size to about 4.4 TB with SHA-256 and 4K blocks.
133 *
134 * Together with the fact that the data, and thus also the Merkle tree,
135 * cannot have more than ULONG_MAX pages, this implies that hash block
136 * indices can always fit in an 'unsigned long'. But to be safe, we
137 * explicitly check for that too. Note, this is only for hash block
138 * indices; data block indices might not fit in an 'unsigned long'.
139 */
140 if ((params->block_size != PAGE_SIZE && offset > 1 << 23) ||
141 offset > ULONG_MAX) {
142 fsverity_err(inode, "Too many blocks in Merkle tree");
143 err = -EFBIG;
144 goto out_err;
145 }
146
147 params->tree_size = offset << log_blocksize;
148 params->tree_pages = PAGE_ALIGN(params->tree_size) >> PAGE_SHIFT;
149 return 0;
150
151 out_err:
152 kfree(params->hashstate);
153 memset(params, 0, sizeof(*params));
154 return err;
155 }
156
157 /*
158 * Compute the file digest by hashing the fsverity_descriptor excluding the
159 * builtin signature and with the sig_size field set to 0.
160 */
161 static int compute_file_digest(const struct fsverity_hash_alg *hash_alg,
162 struct fsverity_descriptor *desc,
163 u8 *file_digest)
164 {
165 __le32 sig_size = desc->sig_size;
166 int err;
167
168 desc->sig_size = 0;
169 err = fsverity_hash_buffer(hash_alg, desc, sizeof(*desc), file_digest);
170 desc->sig_size = sig_size;
171
172 return err;
173 }
174
175 /*
176 * Create a new fsverity_info from the given fsverity_descriptor (with optional
177 * appended builtin signature), and check the signature if present. The
178 * fsverity_descriptor must have already undergone basic validation.
179 */
180 struct fsverity_info *fsverity_create_info(const struct inode *inode,
181 struct fsverity_descriptor *desc)
182 {
183 struct fsverity_info *vi;
184 int err;
185
186 vi = kmem_cache_zalloc(fsverity_info_cachep, GFP_KERNEL);
187 if (!vi)
188 return ERR_PTR(-ENOMEM);
189 vi->inode = inode;
190
191 err = fsverity_init_merkle_tree_params(&vi->tree_params, inode,
192 desc->hash_algorithm,
193 desc->log_blocksize,
194 desc->salt, desc->salt_size);
195 if (err) {
196 fsverity_err(inode,
197 "Error %d initializing Merkle tree parameters",
198 err);
199 goto fail;
200 }
201
202 memcpy(vi->root_hash, desc->root_hash, vi->tree_params.digest_size);
203
204 err = compute_file_digest(vi->tree_params.hash_alg, desc,
205 vi->file_digest);
206 if (err) {
207 fsverity_err(inode, "Error %d computing file digest", err);
208 goto fail;
209 }
210
211 err = fsverity_verify_signature(vi, desc->signature,
212 le32_to_cpu(desc->sig_size));
213 if (err)
214 goto fail;
215
216 if (vi->tree_params.block_size != PAGE_SIZE) {
217 /*
218 * When the Merkle tree block size and page size differ, we use
219 * a bitmap to keep track of which hash blocks have been
220 * verified. This bitmap must contain one bit per hash block,
221 * including alignment to a page boundary at the end.
222 *
223 * Eventually, to support extremely large files in an efficient
224 * way, it might be necessary to make pages of this bitmap
225 * reclaimable. But for now, simply allocating the whole bitmap
226 * is a simple solution that works well on the files on which
227 * fsverity is realistically used. E.g., with SHA-256 and 4K
228 * blocks, a 100MB file only needs a 24-byte bitmap, and the
229 * bitmap for any file under 17GB fits in a 4K page.
230 */
231 unsigned long num_bits =
232 vi->tree_params.tree_pages <<
233 vi->tree_params.log_blocks_per_page;
234
235 vi->hash_block_verified = kvcalloc(BITS_TO_LONGS(num_bits),
236 sizeof(unsigned long),
237 GFP_KERNEL);
238 if (!vi->hash_block_verified) {
239 err = -ENOMEM;
240 goto fail;
241 }
242 }
243
244 return vi;
245
246 fail:
247 fsverity_free_info(vi);
248 return ERR_PTR(err);
249 }
250
251 void fsverity_set_info(struct inode *inode, struct fsverity_info *vi)
252 {
253 /*
254 * Multiple tasks may race to set ->i_verity_info, so use
255 * cmpxchg_release(). This pairs with the smp_load_acquire() in
256 * fsverity_get_info(). I.e., here we publish ->i_verity_info with a
257 * RELEASE barrier so that other tasks can ACQUIRE it.
258 */
259 if (cmpxchg_release(&inode->i_verity_info, NULL, vi) != NULL) {
260 /* Lost the race, so free the fsverity_info we allocated. */
261 fsverity_free_info(vi);
262 /*
263 * Afterwards, the caller may access ->i_verity_info directly,
264 * so make sure to ACQUIRE the winning fsverity_info.
265 */
266 (void)fsverity_get_info(inode);
267 }
268 }
269
270 void fsverity_free_info(struct fsverity_info *vi)
271 {
272 if (!vi)
273 return;
274 kfree(vi->tree_params.hashstate);
275 kvfree(vi->hash_block_verified);
276 kmem_cache_free(fsverity_info_cachep, vi);
277 }
278
279 static bool validate_fsverity_descriptor(struct inode *inode,
280 const struct fsverity_descriptor *desc,
281 size_t desc_size)
282 {
283 if (desc_size < sizeof(*desc)) {
284 fsverity_err(inode, "Unrecognized descriptor size: %zu bytes",
285 desc_size);
286 return false;
287 }
288
289 if (desc->version != 1) {
290 fsverity_err(inode, "Unrecognized descriptor version: %u",
291 desc->version);
292 return false;
293 }
294
295 if (memchr_inv(desc->__reserved, 0, sizeof(desc->__reserved))) {
296 fsverity_err(inode, "Reserved bits set in descriptor");
297 return false;
298 }
299
300 if (desc->salt_size > sizeof(desc->salt)) {
301 fsverity_err(inode, "Invalid salt_size: %u", desc->salt_size);
302 return false;
303 }
304
305 if (le64_to_cpu(desc->data_size) != inode->i_size) {
306 fsverity_err(inode,
307 "Wrong data_size: %llu (desc) != %lld (inode)",
308 le64_to_cpu(desc->data_size), inode->i_size);
309 return false;
310 }
311
312 if (le32_to_cpu(desc->sig_size) > desc_size - sizeof(*desc)) {
313 fsverity_err(inode, "Signature overflows verity descriptor");
314 return false;
315 }
316
317 return true;
318 }
319
320 /*
321 * Read the inode's fsverity_descriptor (with optional appended builtin
322 * signature) from the filesystem, and do basic validation of it.
323 */
324 int fsverity_get_descriptor(struct inode *inode,
325 struct fsverity_descriptor **desc_ret)
326 {
327 int res;
328 struct fsverity_descriptor *desc;
329
330 res = inode->i_sb->s_vop->get_verity_descriptor(inode, NULL, 0);
331 if (res < 0) {
332 fsverity_err(inode,
333 "Error %d getting verity descriptor size", res);
334 return res;
335 }
336 if (res > FS_VERITY_MAX_DESCRIPTOR_SIZE) {
337 fsverity_err(inode, "Verity descriptor is too large (%d bytes)",
338 res);
339 return -EMSGSIZE;
340 }
341 desc = kmalloc(res, GFP_KERNEL);
342 if (!desc)
343 return -ENOMEM;
344 res = inode->i_sb->s_vop->get_verity_descriptor(inode, desc, res);
345 if (res < 0) {
346 fsverity_err(inode, "Error %d reading verity descriptor", res);
347 kfree(desc);
348 return res;
349 }
350
351 if (!validate_fsverity_descriptor(inode, desc, res)) {
352 kfree(desc);
353 return -EINVAL;
354 }
355
356 *desc_ret = desc;
357 return 0;
358 }
359
360 /* Ensure the inode has an ->i_verity_info */
361 static int ensure_verity_info(struct inode *inode)
362 {
363 struct fsverity_info *vi = fsverity_get_info(inode);
364 struct fsverity_descriptor *desc;
365 int err;
366
367 if (vi)
368 return 0;
369
370 err = fsverity_get_descriptor(inode, &desc);
371 if (err)
372 return err;
373
374 vi = fsverity_create_info(inode, desc);
375 if (IS_ERR(vi)) {
376 err = PTR_ERR(vi);
377 goto out_free_desc;
378 }
379
380 fsverity_set_info(inode, vi);
381 err = 0;
382 out_free_desc:
383 kfree(desc);
384 return err;
385 }
386
387 int __fsverity_file_open(struct inode *inode, struct file *filp)
388 {
389 if (filp->f_mode & FMODE_WRITE)
390 return -EPERM;
391 return ensure_verity_info(inode);
392 }
393 EXPORT_SYMBOL_GPL(__fsverity_file_open);
394
395 int __fsverity_prepare_setattr(struct dentry *dentry, struct iattr *attr)
396 {
397 if (attr->ia_valid & ATTR_SIZE)
398 return -EPERM;
399 return 0;
400 }
401 EXPORT_SYMBOL_GPL(__fsverity_prepare_setattr);
402
403 void __fsverity_cleanup_inode(struct inode *inode)
404 {
405 fsverity_free_info(inode->i_verity_info);
406 inode->i_verity_info = NULL;
407 }
408 EXPORT_SYMBOL_GPL(__fsverity_cleanup_inode);
409
410 void __init fsverity_init_info_cache(void)
411 {
412 fsverity_info_cachep = KMEM_CACHE_USERCOPY(
413 fsverity_info,
414 SLAB_RECLAIM_ACCOUNT | SLAB_PANIC,
415 file_digest);
416 }
417
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