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  */
fsverity_init_merkle_tree_params(struct merkle_tree_params * params,const struct inode * inode,unsigned int hash_algorithm,unsigned int log_blocksize,const u8 * salt,size_t salt_size)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  */
compute_file_digest(const struct fsverity_hash_alg * hash_alg,struct fsverity_descriptor * desc,u8 * file_digest)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  */
fsverity_create_info(const struct inode * inode,struct fsverity_descriptor * desc)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 		spin_lock_init(&vi->hash_page_init_lock);
243 	}
244 
245 	return vi;
246 
247 fail:
248 	fsverity_free_info(vi);
249 	return ERR_PTR(err);
250 }
251 
fsverity_set_info(struct inode * inode,struct fsverity_info * vi)252 void fsverity_set_info(struct inode *inode, struct fsverity_info *vi)
253 {
254 	/*
255 	 * Multiple tasks may race to set ->i_verity_info, so use
256 	 * cmpxchg_release().  This pairs with the smp_load_acquire() in
257 	 * fsverity_get_info().  I.e., here we publish ->i_verity_info with a
258 	 * RELEASE barrier so that other tasks can ACQUIRE it.
259 	 */
260 	if (cmpxchg_release(&inode->i_verity_info, NULL, vi) != NULL) {
261 		/* Lost the race, so free the fsverity_info we allocated. */
262 		fsverity_free_info(vi);
263 		/*
264 		 * Afterwards, the caller may access ->i_verity_info directly,
265 		 * so make sure to ACQUIRE the winning fsverity_info.
266 		 */
267 		(void)fsverity_get_info(inode);
268 	}
269 }
270 
fsverity_free_info(struct fsverity_info * vi)271 void fsverity_free_info(struct fsverity_info *vi)
272 {
273 	if (!vi)
274 		return;
275 	kfree(vi->tree_params.hashstate);
276 	kvfree(vi->hash_block_verified);
277 	kmem_cache_free(fsverity_info_cachep, vi);
278 }
279 
validate_fsverity_descriptor(struct inode * inode,const struct fsverity_descriptor * desc,size_t desc_size)280 static bool validate_fsverity_descriptor(struct inode *inode,
281 					 const struct fsverity_descriptor *desc,
282 					 size_t desc_size)
283 {
284 	if (desc_size < sizeof(*desc)) {
285 		fsverity_err(inode, "Unrecognized descriptor size: %zu bytes",
286 			     desc_size);
287 		return false;
288 	}
289 
290 	if (desc->version != 1) {
291 		fsverity_err(inode, "Unrecognized descriptor version: %u",
292 			     desc->version);
293 		return false;
294 	}
295 
296 	if (memchr_inv(desc->__reserved, 0, sizeof(desc->__reserved))) {
297 		fsverity_err(inode, "Reserved bits set in descriptor");
298 		return false;
299 	}
300 
301 	if (desc->salt_size > sizeof(desc->salt)) {
302 		fsverity_err(inode, "Invalid salt_size: %u", desc->salt_size);
303 		return false;
304 	}
305 
306 	if (le64_to_cpu(desc->data_size) != inode->i_size) {
307 		fsverity_err(inode,
308 			     "Wrong data_size: %llu (desc) != %lld (inode)",
309 			     le64_to_cpu(desc->data_size), inode->i_size);
310 		return false;
311 	}
312 
313 	if (le32_to_cpu(desc->sig_size) > desc_size - sizeof(*desc)) {
314 		fsverity_err(inode, "Signature overflows verity descriptor");
315 		return false;
316 	}
317 
318 	return true;
319 }
320 
321 /*
322  * Read the inode's fsverity_descriptor (with optional appended builtin
323  * signature) from the filesystem, and do basic validation of it.
324  */
fsverity_get_descriptor(struct inode * inode,struct fsverity_descriptor ** desc_ret)325 int fsverity_get_descriptor(struct inode *inode,
326 			    struct fsverity_descriptor **desc_ret)
327 {
328 	int res;
329 	struct fsverity_descriptor *desc;
330 
331 	res = inode->i_sb->s_vop->get_verity_descriptor(inode, NULL, 0);
332 	if (res < 0) {
333 		fsverity_err(inode,
334 			     "Error %d getting verity descriptor size", res);
335 		return res;
336 	}
337 	if (res > FS_VERITY_MAX_DESCRIPTOR_SIZE) {
338 		fsverity_err(inode, "Verity descriptor is too large (%d bytes)",
339 			     res);
340 		return -EMSGSIZE;
341 	}
342 	desc = kmalloc(res, GFP_KERNEL);
343 	if (!desc)
344 		return -ENOMEM;
345 	res = inode->i_sb->s_vop->get_verity_descriptor(inode, desc, res);
346 	if (res < 0) {
347 		fsverity_err(inode, "Error %d reading verity descriptor", res);
348 		kfree(desc);
349 		return res;
350 	}
351 
352 	if (!validate_fsverity_descriptor(inode, desc, res)) {
353 		kfree(desc);
354 		return -EINVAL;
355 	}
356 
357 	*desc_ret = desc;
358 	return 0;
359 }
360 
361 /* Ensure the inode has an ->i_verity_info */
ensure_verity_info(struct inode * inode)362 static int ensure_verity_info(struct inode *inode)
363 {
364 	struct fsverity_info *vi = fsverity_get_info(inode);
365 	struct fsverity_descriptor *desc;
366 	int err;
367 
368 	if (vi)
369 		return 0;
370 
371 	err = fsverity_get_descriptor(inode, &desc);
372 	if (err)
373 		return err;
374 
375 	vi = fsverity_create_info(inode, desc);
376 	if (IS_ERR(vi)) {
377 		err = PTR_ERR(vi);
378 		goto out_free_desc;
379 	}
380 
381 	fsverity_set_info(inode, vi);
382 	err = 0;
383 out_free_desc:
384 	kfree(desc);
385 	return err;
386 }
387 
__fsverity_file_open(struct inode * inode,struct file * filp)388 int __fsverity_file_open(struct inode *inode, struct file *filp)
389 {
390 	if (filp->f_mode & FMODE_WRITE)
391 		return -EPERM;
392 	return ensure_verity_info(inode);
393 }
394 EXPORT_SYMBOL_GPL(__fsverity_file_open);
395 
__fsverity_prepare_setattr(struct dentry * dentry,struct iattr * attr)396 int __fsverity_prepare_setattr(struct dentry *dentry, struct iattr *attr)
397 {
398 	if (attr->ia_valid & ATTR_SIZE)
399 		return -EPERM;
400 	return 0;
401 }
402 EXPORT_SYMBOL_GPL(__fsverity_prepare_setattr);
403 
__fsverity_cleanup_inode(struct inode * inode)404 void __fsverity_cleanup_inode(struct inode *inode)
405 {
406 	fsverity_free_info(inode->i_verity_info);
407 	inode->i_verity_info = NULL;
408 }
409 EXPORT_SYMBOL_GPL(__fsverity_cleanup_inode);
410 
fsverity_init_info_cache(void)411 void __init fsverity_init_info_cache(void)
412 {
413 	fsverity_info_cachep = KMEM_CACHE_USERCOPY(
414 					fsverity_info,
415 					SLAB_RECLAIM_ACCOUNT | SLAB_PANIC,
416 					file_digest);
417 }
418