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
13static 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 */
29int 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
151out_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 */
161static 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 */
180struct 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
246fail:
247	fsverity_free_info(vi);
248	return ERR_PTR(err);
249}
250
251void 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
270void 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
279static 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 */
324int 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 */
361static 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;
382out_free_desc:
383	kfree(desc);
384	return err;
385}
386
387int __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}
393EXPORT_SYMBOL_GPL(__fsverity_file_open);
394
395int __fsverity_prepare_setattr(struct dentry *dentry, struct iattr *attr)
396{
397	if (attr->ia_valid & ATTR_SIZE)
398		return -EPERM;
399	return 0;
400}
401EXPORT_SYMBOL_GPL(__fsverity_prepare_setattr);
402
403void __fsverity_cleanup_inode(struct inode *inode)
404{
405	fsverity_free_info(inode->i_verity_info);
406	inode->i_verity_info = NULL;
407}
408EXPORT_SYMBOL_GPL(__fsverity_cleanup_inode);
409
410void __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