fs/verity/verify.c

   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                         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 }