fs/verity/open.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * fs/verity/open.c: opening fs-verity files
   4  *
   5  * Copyright 2019 Google LLC
   6  */
   7
   8 #include "fsverity_private.h"
   9
  10 #include <linux/slab.h>
  11
  12 static struct kmem_cache *fsverity_info_cachep;
  13
  14 /**
  15  * fsverity_init_merkle_tree_params() - initialize Merkle tree parameters
  16  * @params: the parameters struct to initialize
  17  * @inode: the inode for which the Merkle tree is being built
  18  * @hash_algorithm: number of hash algorithm to use
  19  * @log_blocksize: log base 2 of block size to use
  20  * @salt: pointer to salt (optional)
  21  * @salt_size: size of salt, possibly 0
  22  *
  23  * Validate the hash algorithm and block size, then compute the tree topology
  24  * (num levels, num blocks in each level, etc.) and initialize @params.
  25  *
  26  * Return: 0 on success, -errno on failure
  27  */
  28 int fsverity_init_merkle_tree_params(struct merkle_tree_params *params,
  29                                      const struct inode *inode,
  30                                      unsigned int hash_algorithm,
  31                                      unsigned int log_blocksize,
  32                                      const u8 *salt, size_t salt_size)
  33 {
  34         struct fsverity_hash_alg *hash_alg;
  35         int err;
  36         u64 blocks;
  37         u64 offset;
  38         int level;
  39
  40         memset(params, 0, sizeof(*params));
  41
  42         hash_alg = fsverity_get_hash_alg(inode, hash_algorithm);
  43         if (IS_ERR(hash_alg))
  44                 return PTR_ERR(hash_alg);
  45         params->hash_alg = hash_alg;
  46         params->digest_size = hash_alg->digest_size;
  47
  48         params->hashstate = fsverity_prepare_hash_state(hash_alg, salt,
  49                                                         salt_size);
  50         if (IS_ERR(params->hashstate)) {
  51                 err = PTR_ERR(params->hashstate);
  52                 params->hashstate = NULL;
  53                 fsverity_err(inode, "Error %d preparing hash state", err);
  54                 goto out_err;
  55         }
  56
  57         if (log_blocksize != PAGE_SHIFT) {
  58                 fsverity_warn(inode, "Unsupported log_blocksize: %u",
  59                               log_blocksize);
  60                 err = -EINVAL;
  61                 goto out_err;
  62         }
  63         params->log_blocksize = log_blocksize;
  64         params->block_size = 1 << log_blocksize;
  65
  66         if (WARN_ON(!is_power_of_2(params->digest_size))) {
  67                 err = -EINVAL;
  68                 goto out_err;
  69         }
  70         if (params->block_size < 2 * params->digest_size) {
  71                 fsverity_warn(inode,
  72                               "Merkle tree block size (%u) too small for hash algorithm \"%s\"",
  73                               params->block_size, hash_alg->name);
  74                 err = -EINVAL;
  75                 goto out_err;
  76         }
  77         params->log_arity = params->log_blocksize - ilog2(params->digest_size);
  78         params->hashes_per_block = 1 << params->log_arity;
  79
  80         pr_debug("Merkle tree uses %s with %u-byte blocks (%u hashes/block), salt=%*phN\n",
  81                  hash_alg->name, params->block_size, params->hashes_per_block,
  82                  (int)salt_size, salt);
  83
  84         /*
  85          * Compute the number of levels in the Merkle tree and create a map from
  86          * level to the starting block of that level.  Level 'num_levels - 1' is
  87          * the root and is stored first.  Level 0 is the level directly "above"
  88          * the data blocks and is stored last.
  89          */
  90
  91         /* Compute number of levels and the number of blocks in each level */
  92         blocks = (inode->i_size + params->block_size - 1) >> log_blocksize;
  93         pr_debug("Data is %lld bytes (%llu blocks)\n", inode->i_size, blocks);
  94         while (blocks > 1) {
  95                 if (params->num_levels >= FS_VERITY_MAX_LEVELS) {
  96                         fsverity_err(inode, "Too many levels in Merkle tree");
  97                         err = -EINVAL;
  98                         goto out_err;
  99                 }
 100                 blocks = (blocks + params->hashes_per_block - 1) >>
 101                          params->log_arity;
 102                 /* temporarily using level_start[] to store blocks in level */
 103                 params->level_start[params->num_levels++] = blocks;
 104         }
 105         params->level0_blocks = params->level_start[0];
 106
 107         /* Compute the starting block of each level */
 108         offset = 0;
 109         for (level = (int)params->num_levels - 1; level >= 0; level--) {
 110                 blocks = params->level_start[level];
 111                 params->level_start[level] = offset;
 112                 pr_debug("Level %d is %llu blocks starting at index %llu\n",
 113                          level, blocks, offset);
 114                 offset += blocks;
 115         }
 116
 117         params->tree_size = offset << log_blocksize;
 118         return 0;
 119
 120 out_err:
 121         kfree(params->hashstate);
 122         memset(params, 0, sizeof(*params));
 123         return err;
 124 }
 125
 126 /*
 127  * Compute the file measurement by hashing the fsverity_descriptor excluding the
 128  * signature and with the sig_size field set to 0.
 129  */
 130 static int compute_file_measurement(struct fsverity_hash_alg *hash_alg,
 131                                     struct fsverity_descriptor *desc,
 132                                     u8 *measurement)
 133 {
 134         __le32 sig_size = desc->sig_size;
 135         int err;
 136
 137         desc->sig_size = 0;
 138         err = fsverity_hash_buffer(hash_alg, desc, sizeof(*desc), measurement);
 139         desc->sig_size = sig_size;
 140
 141         return err;
 142 }
 143
 144 /*
 145  * Validate the given fsverity_descriptor and create a new fsverity_info from
 146  * it.  The signature (if present) is also checked.
 147  */
 148 struct fsverity_info *fsverity_create_info(const struct inode *inode,
 149                                            void *_desc, size_t desc_size)
 150 {
 151         struct fsverity_descriptor *desc = _desc;
 152         struct fsverity_info *vi;
 153         int err;
 154
 155         if (desc_size < sizeof(*desc)) {
 156                 fsverity_err(inode, "Unrecognized descriptor size: %zu bytes",
 157                              desc_size);
 158                 return ERR_PTR(-EINVAL);
 159         }
 160
 161         if (desc->version != 1) {
 162                 fsverity_err(inode, "Unrecognized descriptor version: %u",
 163                              desc->version);
 164                 return ERR_PTR(-EINVAL);
 165         }
 166
 167         if (memchr_inv(desc->__reserved, 0, sizeof(desc->__reserved))) {
 168                 fsverity_err(inode, "Reserved bits set in descriptor");
 169                 return ERR_PTR(-EINVAL);
 170         }
 171
 172         if (desc->salt_size > sizeof(desc->salt)) {
 173                 fsverity_err(inode, "Invalid salt_size: %u", desc->salt_size);
 174                 return ERR_PTR(-EINVAL);
 175         }
 176
 177         if (le64_to_cpu(desc->data_size) != inode->i_size) {
 178                 fsverity_err(inode,
 179                              "Wrong data_size: %llu (desc) != %lld (inode)",
 180                              le64_to_cpu(desc->data_size), inode->i_size);
 181                 return ERR_PTR(-EINVAL);
 182         }
 183
 184         vi = kmem_cache_zalloc(fsverity_info_cachep, GFP_KERNEL);
 185         if (!vi)
 186                 return ERR_PTR(-ENOMEM);
 187         vi->inode = inode;
 188
 189         err = fsverity_init_merkle_tree_params(&vi->tree_params, inode,
 190                                                desc->hash_algorithm,
 191                                                desc->log_blocksize,
 192                                                desc->salt, desc->salt_size);
 193         if (err) {
 194                 fsverity_err(inode,
 195                              "Error %d initializing Merkle tree parameters",
 196                              err);
 197                 goto out;
 198         }
 199
 200         memcpy(vi->root_hash, desc->root_hash, vi->tree_params.digest_size);
 201
 202         err = compute_file_measurement(vi->tree_params.hash_alg, desc,
 203                                        vi->measurement);
 204         if (err) {
 205                 fsverity_err(inode, "Error %d computing file measurement", err);
 206                 goto out;
 207         }
 208         pr_debug("Computed file measurement: %s:%*phN\n",
 209                  vi->tree_params.hash_alg->name,
 210                  vi->tree_params.digest_size, vi->measurement);
 211
 212         err = fsverity_verify_signature(vi, desc, desc_size);
 213 out:
 214         if (err) {
 215                 fsverity_free_info(vi);
 216                 vi = ERR_PTR(err);
 217         }
 218         return vi;
 219 }
 220
 221 void fsverity_set_info(struct inode *inode, struct fsverity_info *vi)
 222 {
 223         /*
 224          * Multiple tasks may race to set ->i_verity_info, so use
 225          * cmpxchg_release().  This pairs with the smp_load_acquire() in
 226          * fsverity_get_info().  I.e., here we publish ->i_verity_info with a
 227          * RELEASE barrier so that other tasks can ACQUIRE it.
 228          */
 229         if (cmpxchg_release(&inode->i_verity_info, NULL, vi) != NULL) {
 230                 /* Lost the race, so free the fsverity_info we allocated. */
 231                 fsverity_free_info(vi);
 232                 /*
 233                  * Afterwards, the caller may access ->i_verity_info directly,
 234                  * so make sure to ACQUIRE the winning fsverity_info.
 235                  */
 236                 (void)fsverity_get_info(inode);
 237         }
 238 }
 239
 240 void fsverity_free_info(struct fsverity_info *vi)
 241 {
 242         if (!vi)
 243                 return;
 244         kfree(vi->tree_params.hashstate);
 245         kmem_cache_free(fsverity_info_cachep, vi);
 246 }
 247
 248 /* Ensure the inode has an ->i_verity_info */
 249 static int ensure_verity_info(struct inode *inode)
 250 {
 251         struct fsverity_info *vi = fsverity_get_info(inode);
 252         struct fsverity_descriptor *desc;
 253         int res;
 254
 255         if (vi)
 256                 return 0;
 257
 258         res = inode->i_sb->s_vop->get_verity_descriptor(inode, NULL, 0);
 259         if (res < 0) {
 260                 fsverity_err(inode,
 261                              "Error %d getting verity descriptor size", res);
 262                 return res;
 263         }
 264         if (res > FS_VERITY_MAX_DESCRIPTOR_SIZE) {
 265                 fsverity_err(inode, "Verity descriptor is too large (%d bytes)",
 266                              res);
 267                 return -EMSGSIZE;
 268         }
 269         desc = kmalloc(res, GFP_KERNEL);
 270         if (!desc)
 271                 return -ENOMEM;
 272         res = inode->i_sb->s_vop->get_verity_descriptor(inode, desc, res);
 273         if (res < 0) {
 274                 fsverity_err(inode, "Error %d reading verity descriptor", res);
 275                 goto out_free_desc;
 276         }
 277
 278         vi = fsverity_create_info(inode, desc, res);
 279         if (IS_ERR(vi)) {
 280                 res = PTR_ERR(vi);
 281                 goto out_free_desc;
 282         }
 283
 284         fsverity_set_info(inode, vi);
 285         res = 0;
 286 out_free_desc:
 287         kfree(desc);
 288         return res;
 289 }
 290
 291 /**
 292  * fsverity_file_open() - prepare to open a verity file
 293  * @inode: the inode being opened
 294  * @filp: the struct file being set up
 295  *
 296  * When opening a verity file, deny the open if it is for writing.  Otherwise,
 297  * set up the inode's ->i_verity_info if not already done.
 298  *
 299  * When combined with fscrypt, this must be called after fscrypt_file_open().
 300  * Otherwise, we won't have the key set up to decrypt the verity metadata.
 301  *
 302  * Return: 0 on success, -errno on failure
 303  */
 304 int fsverity_file_open(struct inode *inode, struct file *filp)
 305 {
 306         if (!IS_VERITY(inode))
 307                 return 0;
 308
 309         if (filp->f_mode & FMODE_WRITE) {
 310                 pr_debug("Denying opening verity file (ino %lu) for write\n",
 311                          inode->i_ino);
 312                 return -EPERM;
 313         }
 314
 315         return ensure_verity_info(inode);
 316 }
 317 EXPORT_SYMBOL_GPL(fsverity_file_open);
 318
 319 /**
 320  * fsverity_prepare_setattr() - prepare to change a verity inode's attributes
 321  * @dentry: dentry through which the inode is being changed
 322  * @attr: attributes to change
 323  *
 324  * Verity files are immutable, so deny truncates.  This isn't covered by the
 325  * open-time check because sys_truncate() takes a path, not a file descriptor.
 326  *
 327  * Return: 0 on success, -errno on failure
 328  */
 329 int fsverity_prepare_setattr(struct dentry *dentry, struct iattr *attr)
 330 {
 331         if (IS_VERITY(d_inode(dentry)) && (attr->ia_valid & ATTR_SIZE)) {
 332                 pr_debug("Denying truncate of verity file (ino %lu)\n",
 333                          d_inode(dentry)->i_ino);
 334                 return -EPERM;
 335         }
 336         return 0;
 337 }
 338 EXPORT_SYMBOL_GPL(fsverity_prepare_setattr);
 339
 340 /**
 341  * fsverity_cleanup_inode() - free the inode's verity info, if present
 342  * @inode: an inode being evicted
 343  *
 344  * Filesystems must call this on inode eviction to free ->i_verity_info.
 345  */
 346 void fsverity_cleanup_inode(struct inode *inode)
 347 {
 348         fsverity_free_info(inode->i_verity_info);
 349         inode->i_verity_info = NULL;
 350 }
 351 EXPORT_SYMBOL_GPL(fsverity_cleanup_inode);
 352
 353 int __init fsverity_init_info_cache(void)
 354 {
 355         fsverity_info_cachep = KMEM_CACHE_USERCOPY(fsverity_info,
 356                                                    SLAB_RECLAIM_ACCOUNT,
 357                                                    measurement);
 358         if (!fsverity_info_cachep)
 359                 return -ENOMEM;
 360         return 0;
 361 }
 362
 363 void __init fsverity_exit_info_cache(void)
 364 {
 365         kmem_cache_destroy(fsverity_info_cachep);
 366         fsverity_info_cachep = NULL;
 367 }