root/fs/verity/verify.c

DEFINITIONS

1. hash_at_level
2. extract_hash
3. cmp_hashes
4. verify_page
5. fsverity_verify_page
6. fsverity_verify_bio
7. fsverity_enqueue_verify_work
8. fsverity_init_workqueue
9. fsverity_exit_workqueue

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * fs/verity/verify.c: 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 {
  89         const struct merkle_tree_params *params = &vi->tree_params;
  90         const unsigned int hsize = params->digest_size;
  91         const pgoff_t index = data_page->index;
  92         int level;
  93         u8 _want_hash[FS_VERITY_MAX_DIGEST_SIZE];
  94         const u8 *want_hash;
  95         u8 real_hash[FS_VERITY_MAX_DIGEST_SIZE];
  96         struct page *hpages[FS_VERITY_MAX_LEVELS];
  97         unsigned int hoffsets[FS_VERITY_MAX_LEVELS];
  98         int err;
  99 
 100         if (WARN_ON_ONCE(!PageLocked(data_page) || PageUptodate(data_page)))
 101                 return false;
 102 
 103         pr_debug_ratelimited("Verifying data page %lu...\n", index);
 104 
 105         /*
 106          * Starting at the leaf level, ascend the tree saving hash pages along
 107          * the way until we find a verified hash page, indicated by PageChecked;
 108          * or until we reach the root.
 109          */
 110         for (level = 0; level < params->num_levels; level++) {
 111                 pgoff_t hindex;
 112                 unsigned int hoffset;
 113                 struct page *hpage;
 114 
 115                 hash_at_level(params, index, level, &hindex, &hoffset);
 116 
 117                 pr_debug_ratelimited("Level %d: hindex=%lu, hoffset=%u\n",
 118                                      level, hindex, hoffset);
 119 
 120                 hpage = inode->i_sb->s_vop->read_merkle_tree_page(inode,
 121                                                                   hindex);
 122                 if (IS_ERR(hpage)) {
 123                         err = PTR_ERR(hpage);
 124                         fsverity_err(inode,
 125                                      "Error %d reading Merkle tree page %lu",
 126                                      err, hindex);
 127                         goto out;
 128                 }
 129 
 130                 if (PageChecked(hpage)) {
 131                         extract_hash(hpage, hoffset, hsize, _want_hash);
 132                         want_hash = _want_hash;
 133                         put_page(hpage);
 134                         pr_debug_ratelimited("Hash page already checked, want %s:%*phN\n",
 135                                              params->hash_alg->name,
 136                                              hsize, want_hash);
 137                         goto descend;
 138                 }
 139                 pr_debug_ratelimited("Hash page not yet checked\n");
 140                 hpages[level] = hpage;
 141                 hoffsets[level] = hoffset;
 142         }
 143 
 144         want_hash = vi->root_hash;
 145         pr_debug("Want root hash: %s:%*phN\n",
 146                  params->hash_alg->name, hsize, want_hash);
 147 descend:
 148         /* Descend the tree verifying hash pages */
 149         for (; level > 0; level--) {
 150                 struct page *hpage = hpages[level - 1];
 151                 unsigned int hoffset = hoffsets[level - 1];
 152 
 153                 err = fsverity_hash_page(params, inode, req, hpage, real_hash);
 154                 if (err)
 155                         goto out;
 156                 err = cmp_hashes(vi, want_hash, real_hash, index, level - 1);
 157                 if (err)
 158                         goto out;
 159                 SetPageChecked(hpage);
 160                 extract_hash(hpage, hoffset, hsize, _want_hash);
 161                 want_hash = _want_hash;
 162                 put_page(hpage);
 163                 pr_debug("Verified hash page at level %d, now want %s:%*phN\n",
 164                          level - 1, params->hash_alg->name, hsize, want_hash);
 165         }
 166 
 167         /* Finally, verify the data page */
 168         err = fsverity_hash_page(params, inode, req, data_page, real_hash);
 169         if (err)
 170                 goto out;
 171         err = cmp_hashes(vi, want_hash, real_hash, index, -1);
 172 out:
 173         for (; level > 0; level--)
 174                 put_page(hpages[level - 1]);
 175 
 176         return err == 0;
 177 }
 178 
 179 /**
 180  * fsverity_verify_page() - verify a data page
 181  *
 182  * Verify a page that has just been read from a verity file.  The page must be a
 183  * pagecache page that is still locked and not yet uptodate.
 184  *
 185  * Return: true if the page is valid, else false.
 186  */
 187 bool fsverity_verify_page(struct page *page)
 188 {
 189         struct inode *inode = page->mapping->host;
 190         const struct fsverity_info *vi = inode->i_verity_info;
 191         struct ahash_request *req;
 192         bool valid;
 193 
 194         req = ahash_request_alloc(vi->tree_params.hash_alg->tfm, GFP_NOFS);
 195         if (unlikely(!req))
 196                 return false;
 197 
 198         valid = verify_page(inode, vi, req, page);
 199 
 200         ahash_request_free(req);
 201 
 202         return valid;
 203 }
 204 EXPORT_SYMBOL_GPL(fsverity_verify_page);
 205 
 206 #ifdef CONFIG_BLOCK
 207 /**
 208  * fsverity_verify_bio() - verify a 'read' bio that has just completed
 209  *
 210  * Verify a set of pages that have just been read from a verity file.  The pages
 211  * must be pagecache pages that are still locked and not yet uptodate.  Pages
 212  * that fail verification are set to the Error state.  Verification is skipped
 213  * for pages already in the Error state, e.g. due to fscrypt decryption failure.
 214  *
 215  * This is a helper function for use by the ->readpages() method of filesystems
 216  * that issue bios to read data directly into the page cache.  Filesystems that
 217  * populate the page cache without issuing bios (e.g. non block-based
 218  * filesystems) must instead call fsverity_verify_page() directly on each page.
 219  * All filesystems must also call fsverity_verify_page() on holes.
 220  */
 221 void fsverity_verify_bio(struct bio *bio)
 222 {
 223         struct inode *inode = bio_first_page_all(bio)->mapping->host;
 224         const struct fsverity_info *vi = inode->i_verity_info;
 225         struct ahash_request *req;
 226         struct bio_vec *bv;
 227         struct bvec_iter_all iter_all;
 228 
 229         req = ahash_request_alloc(vi->tree_params.hash_alg->tfm, GFP_NOFS);
 230         if (unlikely(!req)) {
 231                 bio_for_each_segment_all(bv, bio, iter_all)
 232                         SetPageError(bv->bv_page);
 233                 return;
 234         }
 235 
 236         bio_for_each_segment_all(bv, bio, iter_all) {
 237                 struct page *page = bv->bv_page;
 238 
 239                 if (!PageError(page) && !verify_page(inode, vi, req, page))
 240                         SetPageError(page);
 241         }
 242 
 243         ahash_request_free(req);
 244 }
 245 EXPORT_SYMBOL_GPL(fsverity_verify_bio);
 246 #endif /* CONFIG_BLOCK */
 247 
 248 /**
 249  * fsverity_enqueue_verify_work() - enqueue work on the fs-verity workqueue
 250  *
 251  * Enqueue verification work for asynchronous processing.
 252  */
 253 void fsverity_enqueue_verify_work(struct work_struct *work)
 254 {
 255         queue_work(fsverity_read_workqueue, work);
 256 }
 257 EXPORT_SYMBOL_GPL(fsverity_enqueue_verify_work);
 258 
 259 int __init fsverity_init_workqueue(void)
 260 {
 261         /*
 262          * Use an unbound workqueue to allow bios to be verified in parallel
 263          * even when they happen to complete on the same CPU.  This sacrifices
 264          * locality, but it's worthwhile since hashing is CPU-intensive.
 265          *
 266          * Also use a high-priority workqueue to prioritize verification work,
 267          * which blocks reads from completing, over regular application tasks.
 268          */
 269         fsverity_read_workqueue = alloc_workqueue("fsverity_read_queue",
 270                                                   WQ_UNBOUND | WQ_HIGHPRI,
 271                                                   num_online_cpus());
 272         if (!fsverity_read_workqueue)
 273                 return -ENOMEM;
 274         return 0;
 275 }
 276 
 277 void __init fsverity_exit_workqueue(void)
 278 {
 279         destroy_workqueue(fsverity_read_workqueue);
 280         fsverity_read_workqueue = NULL;
 281 }