aboutsummaryrefslogtreecommitdiff
path: root/fs/verity/verify.c
blob: 0adb970f4e7368a3c138edfe66a46a714d47b3d0 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
// SPDX-License-Identifier: GPL-2.0
/*
 * Data verification functions, i.e. hooks for ->readpages()
 *
 * Copyright 2019 Google LLC
 */

#include "fsverity_private.h"

#include <crypto/hash.h>
#include <linux/bio.h>
#include <linux/ratelimit.h>

static struct workqueue_struct *fsverity_read_workqueue;

/**
 * hash_at_level() - compute the location of the block's hash at the given level
 *
 * @params:	(in) the Merkle tree parameters
 * @dindex:	(in) the index of the data block being verified
 * @level:	(in) the level of hash we want (0 is leaf level)
 * @hindex:	(out) the index of the hash block containing the wanted hash
 * @hoffset:	(out) the byte offset to the wanted hash within the hash block
 */
static void hash_at_level(const struct merkle_tree_params *params,
			  pgoff_t dindex, unsigned int level, pgoff_t *hindex,
			  unsigned int *hoffset)
{
	pgoff_t position;

	/* Offset of the hash within the level's region, in hashes */
	position = dindex >> (level * params->log_arity);

	/* Index of the hash block in the tree overall */
	*hindex = params->level_start[level] + (position >> params->log_arity);

	/* Offset of the wanted hash (in bytes) within the hash block */
	*hoffset = (position & ((1 << params->log_arity) - 1)) <<
		   (params->log_blocksize - params->log_arity);
}

/* Extract a hash from a hash page */
static void extract_hash(struct page *hpage, unsigned int hoffset,
			 unsigned int hsize, u8 *out)
{
	void *virt = kmap_atomic(hpage);

	memcpy(out, virt + hoffset, hsize);
	kunmap_atomic(virt);
}

static inline int cmp_hashes(const struct fsverity_info *vi,
			     const u8 *want_hash, const u8 *real_hash,
			     pgoff_t index, int level)
{
	const unsigned int hsize = vi->tree_params.digest_size;

	if (memcmp(want_hash, real_hash, hsize) == 0)
		return 0;

	fsverity_err(vi->inode,
		     "FILE CORRUPTED! index=%lu, level=%d, want_hash=%s:%*phN, real_hash=%s:%*phN",
		     index, level,
		     vi->tree_params.hash_alg->name, hsize, want_hash,
		     vi->tree_params.hash_alg->name, hsize, real_hash);
	return -EBADMSG;
}

/*
 * Verify a single data page against the file's Merkle tree.
 *
 * In principle, we need to verify the entire path to the root node.  However,
 * for efficiency the filesystem may cache the hash pages.  Therefore we need
 * only ascend the tree until an already-verified page is seen, as indicated by
 * the PageChecked bit being set; then verify the path to that page.
 *
 * This code currently only supports the case where the verity block size is
 * equal to PAGE_SIZE.  Doing otherwise would be possible but tricky, since we
 * wouldn't be able to use the PageChecked bit.
 *
 * Note that multiple processes may race to verify a hash page and mark it
 * Checked, but it doesn't matter; the result will be the same either way.
 *
 * Return: true if the page is valid, else false.
 */
static bool verify_page(struct inode *inode, const struct fsverity_info *vi,
			struct ahash_request *req, struct page *data_page,
			unsigned long level0_ra_pages)
{
	const struct merkle_tree_params *params = &vi->tree_params;
	const unsigned int hsize = params->digest_size;
	const pgoff_t index = data_page->index;
	int level;
	u8 _want_hash[FS_VERITY_MAX_DIGEST_SIZE];
	const u8 *want_hash;
	u8 real_hash[FS_VERITY_MAX_DIGEST_SIZE];
	struct page *hpages[FS_VERITY_MAX_LEVELS];
	unsigned int hoffsets[FS_VERITY_MAX_LEVELS];
	int err;

	if (WARN_ON_ONCE(!PageLocked(data_page) || PageUptodate(data_page)))
		return false;

	pr_debug_ratelimited("Verifying data page %lu...\n", index);

	/*
	 * Starting at the leaf level, ascend the tree saving hash pages along
	 * the way until we find a verified hash page, indicated by PageChecked;
	 * or until we reach the root.
	 */
	for (level = 0; level < params->num_levels; level++) {
		pgoff_t hindex;
		unsigned int hoffset;
		struct page *hpage;

		hash_at_level(params, index, level, &hindex, &hoffset);

		pr_debug_ratelimited("Level %d: hindex=%lu, hoffset=%u\n",
				     level, hindex, hoffset);

		hpage = inode->i_sb->s_vop->read_merkle_tree_page(inode, hindex,
				level == 0 ? level0_ra_pages : 0);
		if (IS_ERR(hpage)) {
			err = PTR_ERR(hpage);
			fsverity_err(inode,
				     "Error %d reading Merkle tree page %lu",
				     err, hindex);
			goto out;
		}

		if (PageChecked(hpage)) {
			extract_hash(hpage, hoffset, hsize, _want_hash);
			want_hash = _want_hash;
			put_page(hpage);
			pr_debug_ratelimited("Hash page already checked, want %s:%*phN\n",
					     params->hash_alg->name,
					     hsize, want_hash);
			goto descend;
		}
		pr_debug_ratelimited("Hash page not yet checked\n");
		hpages[level] = hpage;
		hoffsets[level] = hoffset;
	}

	want_hash = vi->root_hash;
	pr_debug("Want root hash: %s:%*phN\n",
		 params->hash_alg->name, hsize, want_hash);
descend:
	/* Descend the tree verifying hash pages */
	for (; level > 0; level--) {
		struct page *hpage = hpages[level - 1];
		unsigned int hoffset = hoffsets[level - 1];

		err = fsverity_hash_page(params, inode, req, hpage, real_hash);
		if (err)
			goto out;
		err = cmp_hashes(vi, want_hash, real_hash, index, level - 1);
		if (err)
			goto out;
		SetPageChecked(hpage);
		extract_hash(hpage, hoffset, hsize, _want_hash);
		want_hash = _want_hash;
		put_page(hpage);
		pr_debug("Verified hash page at level %d, now want %s:%*phN\n",
			 level - 1, params->hash_alg->name, hsize, want_hash);
	}

	/* Finally, verify the data page */
	err = fsverity_hash_page(params, inode, req, data_page, real_hash);
	if (err)
		goto out;
	err = cmp_hashes(vi, want_hash, real_hash, index, -1);
out:
	for (; level > 0; level--)
		put_page(hpages[level - 1]);

	return err == 0;
}

/**
 * fsverity_verify_page() - verify a data page
 * @page: the page to verity
 *
 * Verify a page that has just been read from a verity file.  The page must be a
 * pagecache page that is still locked and not yet uptodate.
 *
 * Return: true if the page is valid, else false.
 */
bool fsverity_verify_page(struct page *page)
{
	struct inode *inode = page->mapping->host;
	const struct fsverity_info *vi = inode->i_verity_info;
	struct ahash_request *req;
	bool valid;

	/* This allocation never fails, since it's mempool-backed. */
	req = fsverity_alloc_hash_request(vi->tree_params.hash_alg, GFP_NOFS);

	valid = verify_page(inode, vi, req, page, 0);

	fsverity_free_hash_request(vi->tree_params.hash_alg, req);

	return valid;
}
EXPORT_SYMBOL_GPL(fsverity_verify_page);

#ifdef CONFIG_BLOCK
/**
 * fsverity_verify_bio() - verify a 'read' bio that has just completed
 * @bio: the bio to verify
 *
 * Verify a set of pages that have just been read from a verity file.  The pages
 * must be pagecache pages that are still locked and not yet uptodate.  Pages
 * that fail verification are set to the Error state.  Verification is skipped
 * for pages already in the Error state, e.g. due to fscrypt decryption failure.
 *
 * This is a helper function for use by the ->readpages() method of filesystems
 * that issue bios to read data directly into the page cache.  Filesystems that
 * populate the page cache without issuing bios (e.g. non block-based
 * filesystems) must instead call fsverity_verify_page() directly on each page.
 * All filesystems must also call fsverity_verify_page() on holes.
 */
void fsverity_verify_bio(struct bio *bio)
{
	struct inode *inode = bio_first_page_all(bio)->mapping->host;
	const struct fsverity_info *vi = inode->i_verity_info;
	const struct merkle_tree_params *params = &vi->tree_params;
	struct ahash_request *req;
	struct bio_vec *bv;
	struct bvec_iter_all iter_all;
	unsigned long max_ra_pages = 0;

	/* This allocation never fails, since it's mempool-backed. */
	req = fsverity_alloc_hash_request(params->hash_alg, GFP_NOFS);

	if (bio->bi_opf & REQ_RAHEAD) {
		/*
		 * If this bio is for data readahead, then we also do readahead
		 * of the first (largest) level of the Merkle tree.  Namely,
		 * when a Merkle tree page is read, we also try to piggy-back on
		 * some additional pages -- up to 1/4 the number of data pages.
		 *
		 * This improves sequential read performance, as it greatly
		 * reduces the number of I/O requests made to the Merkle tree.
		 */
		bio_for_each_segment_all(bv, bio, iter_all)
			max_ra_pages++;
		max_ra_pages /= 4;
	}

	bio_for_each_segment_all(bv, bio, iter_all) {
		struct page *page = bv->bv_page;
		unsigned long level0_index = page->index >> params->log_arity;
		unsigned long level0_ra_pages =
			min(max_ra_pages, params->level0_blocks - level0_index);

		if (!PageError(page) &&
		    !verify_page(inode, vi, req, page, level0_ra_pages))
			SetPageError(page);
	}

	fsverity_free_hash_request(params->hash_alg, req);
}
EXPORT_SYMBOL_GPL(fsverity_verify_bio);
#endif /* CONFIG_BLOCK */

/**
 * fsverity_enqueue_verify_work() - enqueue work on the fs-verity workqueue
 * @work: the work to enqueue
 *
 * Enqueue verification work for asynchronous processing.
 */
void fsverity_enqueue_verify_work(struct work_struct *work)
{
	queue_work(fsverity_read_workqueue, work);
}
EXPORT_SYMBOL_GPL(fsverity_enqueue_verify_work);

int __init fsverity_init_workqueue(void)
{
	/*
	 * Use an unbound workqueue to allow bios to be verified in parallel
	 * even when they happen to complete on the same CPU.  This sacrifices
	 * locality, but it's worthwhile since hashing is CPU-intensive.
	 *
	 * Also use a high-priority workqueue to prioritize verification work,
	 * which blocks reads from completing, over regular application tasks.
	 */
	fsverity_read_workqueue = alloc_workqueue("fsverity_read_queue",
						  WQ_UNBOUND | WQ_HIGHPRI,
						  num_online_cpus());
	if (!fsverity_read_workqueue)
		return -ENOMEM;
	return 0;
}

void __init fsverity_exit_workqueue(void)
{
	destroy_workqueue(fsverity_read_workqueue);
	fsverity_read_workqueue = NULL;
}