xref: /openbmc/linux/fs/verity/verify.c (revision 0ad53fe3)
1 // SPDX-License-Identifier: GPL-2.0
2 /*
3  * Data verification functions, i.e. hooks for ->readpages()
4  *
5  * Copyright 2019 Google LLC
6  */
7 
8 #include "fsverity_private.h"
9 
10 #include <crypto/hash.h>
11 #include <linux/bio.h>
12 #include <linux/ratelimit.h>
13 
14 static struct workqueue_struct *fsverity_read_workqueue;
15 
16 /**
17  * hash_at_level() - compute the location of the block's hash at the given level
18  *
19  * @params:	(in) the Merkle tree parameters
20  * @dindex:	(in) the index of the data block being verified
21  * @level:	(in) the level of hash we want (0 is leaf level)
22  * @hindex:	(out) the index of the hash block containing the wanted hash
23  * @hoffset:	(out) the byte offset to the wanted hash within the hash block
24  */
25 static void hash_at_level(const struct merkle_tree_params *params,
26 			  pgoff_t dindex, unsigned int level, pgoff_t *hindex,
27 			  unsigned int *hoffset)
28 {
29 	pgoff_t position;
30 
31 	/* Offset of the hash within the level's region, in hashes */
32 	position = dindex >> (level * params->log_arity);
33 
34 	/* Index of the hash block in the tree overall */
35 	*hindex = params->level_start[level] + (position >> params->log_arity);
36 
37 	/* Offset of the wanted hash (in bytes) within the hash block */
38 	*hoffset = (position & ((1 << params->log_arity) - 1)) <<
39 		   (params->log_blocksize - params->log_arity);
40 }
41 
42 /* Extract a hash from a hash page */
43 static void extract_hash(struct page *hpage, unsigned int hoffset,
44 			 unsigned int hsize, u8 *out)
45 {
46 	void *virt = kmap_atomic(hpage);
47 
48 	memcpy(out, virt + hoffset, hsize);
49 	kunmap_atomic(virt);
50 }
51 
52 static inline int cmp_hashes(const struct fsverity_info *vi,
53 			     const u8 *want_hash, const u8 *real_hash,
54 			     pgoff_t index, int level)
55 {
56 	const unsigned int hsize = vi->tree_params.digest_size;
57 
58 	if (memcmp(want_hash, real_hash, hsize) == 0)
59 		return 0;
60 
61 	fsverity_err(vi->inode,
62 		     "FILE CORRUPTED! index=%lu, level=%d, want_hash=%s:%*phN, real_hash=%s:%*phN",
63 		     index, level,
64 		     vi->tree_params.hash_alg->name, hsize, want_hash,
65 		     vi->tree_params.hash_alg->name, hsize, real_hash);
66 	return -EBADMSG;
67 }
68 
69 /*
70  * Verify a single data page against the file's Merkle tree.
71  *
72  * In principle, we need to verify the entire path to the root node.  However,
73  * for efficiency the filesystem may cache the hash pages.  Therefore we need
74  * only ascend the tree until an already-verified page is seen, as indicated by
75  * the PageChecked bit being set; then verify the path to that page.
76  *
77  * This code currently only supports the case where the verity block size is
78  * equal to PAGE_SIZE.  Doing otherwise would be possible but tricky, since we
79  * wouldn't be able to use the PageChecked bit.
80  *
81  * Note that multiple processes may race to verify a hash page and mark it
82  * Checked, but it doesn't matter; the result will be the same either way.
83  *
84  * Return: true if the page is valid, else false.
85  */
86 static bool verify_page(struct inode *inode, const struct fsverity_info *vi,
87 			struct ahash_request *req, struct page *data_page,
88 			unsigned long level0_ra_pages)
89 {
90 	const struct merkle_tree_params *params = &vi->tree_params;
91 	const unsigned int hsize = params->digest_size;
92 	const pgoff_t index = data_page->index;
93 	int level;
94 	u8 _want_hash[FS_VERITY_MAX_DIGEST_SIZE];
95 	const u8 *want_hash;
96 	u8 real_hash[FS_VERITY_MAX_DIGEST_SIZE];
97 	struct page *hpages[FS_VERITY_MAX_LEVELS];
98 	unsigned int hoffsets[FS_VERITY_MAX_LEVELS];
99 	int err;
100 
101 	if (WARN_ON_ONCE(!PageLocked(data_page) || PageUptodate(data_page)))
102 		return false;
103 
104 	pr_debug_ratelimited("Verifying data page %lu...\n", index);
105 
106 	/*
107 	 * Starting at the leaf level, ascend the tree saving hash pages along
108 	 * the way until we find a verified hash page, indicated by PageChecked;
109 	 * or until we reach the root.
110 	 */
111 	for (level = 0; level < params->num_levels; level++) {
112 		pgoff_t hindex;
113 		unsigned int hoffset;
114 		struct page *hpage;
115 
116 		hash_at_level(params, index, level, &hindex, &hoffset);
117 
118 		pr_debug_ratelimited("Level %d: hindex=%lu, hoffset=%u\n",
119 				     level, hindex, hoffset);
120 
121 		hpage = inode->i_sb->s_vop->read_merkle_tree_page(inode, hindex,
122 				level == 0 ? level0_ra_pages : 0);
123 		if (IS_ERR(hpage)) {
124 			err = PTR_ERR(hpage);
125 			fsverity_err(inode,
126 				     "Error %d reading Merkle tree page %lu",
127 				     err, hindex);
128 			goto out;
129 		}
130 
131 		if (PageChecked(hpage)) {
132 			extract_hash(hpage, hoffset, hsize, _want_hash);
133 			want_hash = _want_hash;
134 			put_page(hpage);
135 			pr_debug_ratelimited("Hash page already checked, want %s:%*phN\n",
136 					     params->hash_alg->name,
137 					     hsize, want_hash);
138 			goto descend;
139 		}
140 		pr_debug_ratelimited("Hash page not yet checked\n");
141 		hpages[level] = hpage;
142 		hoffsets[level] = hoffset;
143 	}
144 
145 	want_hash = vi->root_hash;
146 	pr_debug("Want root hash: %s:%*phN\n",
147 		 params->hash_alg->name, hsize, want_hash);
148 descend:
149 	/* Descend the tree verifying hash pages */
150 	for (; level > 0; level--) {
151 		struct page *hpage = hpages[level - 1];
152 		unsigned int hoffset = hoffsets[level - 1];
153 
154 		err = fsverity_hash_page(params, inode, req, hpage, real_hash);
155 		if (err)
156 			goto out;
157 		err = cmp_hashes(vi, want_hash, real_hash, index, level - 1);
158 		if (err)
159 			goto out;
160 		SetPageChecked(hpage);
161 		extract_hash(hpage, hoffset, hsize, _want_hash);
162 		want_hash = _want_hash;
163 		put_page(hpage);
164 		pr_debug("Verified hash page at level %d, now want %s:%*phN\n",
165 			 level - 1, params->hash_alg->name, hsize, want_hash);
166 	}
167 
168 	/* Finally, verify the data page */
169 	err = fsverity_hash_page(params, inode, req, data_page, real_hash);
170 	if (err)
171 		goto out;
172 	err = cmp_hashes(vi, want_hash, real_hash, index, -1);
173 out:
174 	for (; level > 0; level--)
175 		put_page(hpages[level - 1]);
176 
177 	return err == 0;
178 }
179 
180 /**
181  * fsverity_verify_page() - verify a data page
182  * @page: the page to verity
183  *
184  * Verify a page that has just been read from a verity file.  The page must be a
185  * pagecache page that is still locked and not yet uptodate.
186  *
187  * Return: true if the page is valid, else false.
188  */
189 bool fsverity_verify_page(struct page *page)
190 {
191 	struct inode *inode = page->mapping->host;
192 	const struct fsverity_info *vi = inode->i_verity_info;
193 	struct ahash_request *req;
194 	bool valid;
195 
196 	/* This allocation never fails, since it's mempool-backed. */
197 	req = fsverity_alloc_hash_request(vi->tree_params.hash_alg, GFP_NOFS);
198 
199 	valid = verify_page(inode, vi, req, page, 0);
200 
201 	fsverity_free_hash_request(vi->tree_params.hash_alg, req);
202 
203 	return valid;
204 }
205 EXPORT_SYMBOL_GPL(fsverity_verify_page);
206 
207 #ifdef CONFIG_BLOCK
208 /**
209  * fsverity_verify_bio() - verify a 'read' bio that has just completed
210  * @bio: the bio to verify
211  *
212  * Verify a set of pages that have just been read from a verity file.  The pages
213  * must be pagecache pages that are still locked and not yet uptodate.  Pages
214  * that fail verification are set to the Error state.  Verification is skipped
215  * for pages already in the Error state, e.g. due to fscrypt decryption failure.
216  *
217  * This is a helper function for use by the ->readpages() method of filesystems
218  * that issue bios to read data directly into the page cache.  Filesystems that
219  * populate the page cache without issuing bios (e.g. non block-based
220  * filesystems) must instead call fsverity_verify_page() directly on each page.
221  * All filesystems must also call fsverity_verify_page() on holes.
222  */
223 void fsverity_verify_bio(struct bio *bio)
224 {
225 	struct inode *inode = bio_first_page_all(bio)->mapping->host;
226 	const struct fsverity_info *vi = inode->i_verity_info;
227 	const struct merkle_tree_params *params = &vi->tree_params;
228 	struct ahash_request *req;
229 	struct bio_vec *bv;
230 	struct bvec_iter_all iter_all;
231 	unsigned long max_ra_pages = 0;
232 
233 	/* This allocation never fails, since it's mempool-backed. */
234 	req = fsverity_alloc_hash_request(params->hash_alg, GFP_NOFS);
235 
236 	if (bio->bi_opf & REQ_RAHEAD) {
237 		/*
238 		 * If this bio is for data readahead, then we also do readahead
239 		 * of the first (largest) level of the Merkle tree.  Namely,
240 		 * when a Merkle tree page is read, we also try to piggy-back on
241 		 * some additional pages -- up to 1/4 the number of data pages.
242 		 *
243 		 * This improves sequential read performance, as it greatly
244 		 * reduces the number of I/O requests made to the Merkle tree.
245 		 */
246 		bio_for_each_segment_all(bv, bio, iter_all)
247 			max_ra_pages++;
248 		max_ra_pages /= 4;
249 	}
250 
251 	bio_for_each_segment_all(bv, bio, iter_all) {
252 		struct page *page = bv->bv_page;
253 		unsigned long level0_index = page->index >> params->log_arity;
254 		unsigned long level0_ra_pages =
255 			min(max_ra_pages, params->level0_blocks - level0_index);
256 
257 		if (!PageError(page) &&
258 		    !verify_page(inode, vi, req, page, level0_ra_pages))
259 			SetPageError(page);
260 	}
261 
262 	fsverity_free_hash_request(params->hash_alg, req);
263 }
264 EXPORT_SYMBOL_GPL(fsverity_verify_bio);
265 #endif /* CONFIG_BLOCK */
266 
267 /**
268  * fsverity_enqueue_verify_work() - enqueue work on the fs-verity workqueue
269  * @work: the work to enqueue
270  *
271  * Enqueue verification work for asynchronous processing.
272  */
273 void fsverity_enqueue_verify_work(struct work_struct *work)
274 {
275 	queue_work(fsverity_read_workqueue, work);
276 }
277 EXPORT_SYMBOL_GPL(fsverity_enqueue_verify_work);
278 
279 int __init fsverity_init_workqueue(void)
280 {
281 	/*
282 	 * Use an unbound workqueue to allow bios to be verified in parallel
283 	 * even when they happen to complete on the same CPU.  This sacrifices
284 	 * locality, but it's worthwhile since hashing is CPU-intensive.
285 	 *
286 	 * Also use a high-priority workqueue to prioritize verification work,
287 	 * which blocks reads from completing, over regular application tasks.
288 	 */
289 	fsverity_read_workqueue = alloc_workqueue("fsverity_read_queue",
290 						  WQ_UNBOUND | WQ_HIGHPRI,
291 						  num_online_cpus());
292 	if (!fsverity_read_workqueue)
293 		return -ENOMEM;
294 	return 0;
295 }
296 
297 void __init fsverity_exit_workqueue(void)
298 {
299 	destroy_workqueue(fsverity_read_workqueue);
300 	fsverity_read_workqueue = NULL;
301 }
302