fs/verity/open.c

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