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/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 = ((u64)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 digest by hashing the fsverity_descriptor excluding the
 128  * signature and with the sig_size field set to 0.
 129  */
 130 static int compute_file_digest(struct fsverity_hash_alg *hash_alg,
 131                                struct fsverity_descriptor *desc,
 132                                u8 *file_digest)
 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), file_digest);
 139         desc->sig_size = sig_size;
 140
 141         return err;
 142 }
 143
 144 /*
 145  * Create a new fsverity_info from the given fsverity_descriptor (with optional
 146  * appended signature), and check the signature if present.  The
 147  * fsverity_descriptor must have already undergone basic validation.
 148  */
 149 struct fsverity_info *fsverity_create_info(const struct inode *inode,
 150                                            struct fsverity_descriptor *desc,
 151                                            size_t desc_size)
 152 {
 153         struct fsverity_info *vi;
 154         int err;
 155
 156         vi = kmem_cache_zalloc(fsverity_info_cachep, GFP_KERNEL);
 157         if (!vi)
 158                 return ERR_PTR(-ENOMEM);
 159         vi->inode = inode;
 160
 161         err = fsverity_init_merkle_tree_params(&vi->tree_params, inode,
 162                                                desc->hash_algorithm,
 163                                                desc->log_blocksize,
 164                                                desc->salt, desc->salt_size);
 165         if (err) {
 166                 fsverity_err(inode,
 167                              "Error %d initializing Merkle tree parameters",
 168                              err);
 169                 goto out;
 170         }
 171
 172         memcpy(vi->root_hash, desc->root_hash, vi->tree_params.digest_size);
 173
 174         err = compute_file_digest(vi->tree_params.hash_alg, desc,
 175                                   vi->file_digest);
 176         if (err) {
 177                 fsverity_err(inode, "Error %d computing file digest", err);
 178                 goto out;
 179         }
 180         pr_debug("Computed file digest: %s:%*phN\n",
 181                  vi->tree_params.hash_alg->name,
 182                  vi->tree_params.digest_size, vi->file_digest);
 183
 184         err = fsverity_verify_signature(vi, desc->signature,
 185                                         le32_to_cpu(desc->sig_size));
 186 out:
 187         if (err) {
 188                 fsverity_free_info(vi);
 189                 vi = ERR_PTR(err);
 190         }
 191         return vi;
 192 }
 193
 194 void fsverity_set_info(struct inode *inode, struct fsverity_info *vi)
 195 {
 196         /*
 197          * Multiple tasks may race to set ->i_verity_info, so use
 198          * cmpxchg_release().  This pairs with the smp_load_acquire() in
 199          * fsverity_get_info().  I.e., here we publish ->i_verity_info with a
 200          * RELEASE barrier so that other tasks can ACQUIRE it.
 201          */
 202         if (cmpxchg_release(&inode->i_verity_info, NULL, vi) != NULL) {
 203                 /* Lost the race, so free the fsverity_info we allocated. */
 204                 fsverity_free_info(vi);
 205                 /*
 206                  * Afterwards, the caller may access ->i_verity_info directly,
 207                  * so make sure to ACQUIRE the winning fsverity_info.
 208                  */
 209                 (void)fsverity_get_info(inode);
 210         }
 211 }
 212
 213 void fsverity_free_info(struct fsverity_info *vi)
 214 {
 215         if (!vi)
 216                 return;
 217         kfree(vi->tree_params.hashstate);
 218         kmem_cache_free(fsverity_info_cachep, vi);
 219 }
 220
 221 static bool validate_fsverity_descriptor(struct inode *inode,
 222                                          const struct fsverity_descriptor *desc,
 223                                          size_t desc_size)
 224 {
 225         if (desc_size < sizeof(*desc)) {
 226                 fsverity_err(inode, "Unrecognized descriptor size: %zu bytes",
 227                              desc_size);
 228                 return false;
 229         }
 230
 231         if (desc->version != 1) {
 232                 fsverity_err(inode, "Unrecognized descriptor version: %u",
 233                              desc->version);
 234                 return false;
 235         }
 236
 237         if (memchr_inv(desc->__reserved, 0, sizeof(desc->__reserved))) {
 238                 fsverity_err(inode, "Reserved bits set in descriptor");
 239                 return false;
 240         }
 241
 242         if (desc->salt_size > sizeof(desc->salt)) {
 243                 fsverity_err(inode, "Invalid salt_size: %u", desc->salt_size);
 244                 return false;
 245         }
 246
 247         if (le64_to_cpu(desc->data_size) != inode->i_size) {
 248                 fsverity_err(inode,
 249                              "Wrong data_size: %llu (desc) != %lld (inode)",
 250                              le64_to_cpu(desc->data_size), inode->i_size);
 251                 return false;
 252         }
 253
 254         if (le32_to_cpu(desc->sig_size) > desc_size - sizeof(*desc)) {
 255                 fsverity_err(inode, "Signature overflows verity descriptor");
 256                 return false;
 257         }
 258
 259         return true;
 260 }
 261
 262 /*
 263  * Read the inode's fsverity_descriptor (with optional appended signature) from
 264  * the filesystem, and do basic validation of it.
 265  */
 266 int fsverity_get_descriptor(struct inode *inode,
 267                             struct fsverity_descriptor **desc_ret,
 268                             size_t *desc_size_ret)
 269 {
 270         int res;
 271         struct fsverity_descriptor *desc;
 272
 273         res = inode->i_sb->s_vop->get_verity_descriptor(inode, NULL, 0);
 274         if (res < 0) {
 275                 fsverity_err(inode,
 276                              "Error %d getting verity descriptor size", res);
 277                 return res;
 278         }
 279         if (res > FS_VERITY_MAX_DESCRIPTOR_SIZE) {
 280                 fsverity_err(inode, "Verity descriptor is too large (%d bytes)",
 281                              res);
 282                 return -EMSGSIZE;
 283         }
 284         desc = kmalloc(res, GFP_KERNEL);
 285         if (!desc)
 286                 return -ENOMEM;
 287         res = inode->i_sb->s_vop->get_verity_descriptor(inode, desc, res);
 288         if (res < 0) {
 289                 fsverity_err(inode, "Error %d reading verity descriptor", res);
 290                 kfree(desc);
 291                 return res;
 292         }
 293
 294         if (!validate_fsverity_descriptor(inode, desc, res)) {
 295                 kfree(desc);
 296                 return -EINVAL;
 297         }
 298
 299         *desc_ret = desc;
 300         *desc_size_ret = res;
 301         return 0;
 302 }
 303
 304 /* Ensure the inode has an ->i_verity_info */
 305 static int ensure_verity_info(struct inode *inode)
 306 {
 307         struct fsverity_info *vi = fsverity_get_info(inode);
 308         struct fsverity_descriptor *desc;
 309         size_t desc_size;
 310         int err;
 311
 312         if (vi)
 313                 return 0;
 314
 315         err = fsverity_get_descriptor(inode, &desc, &desc_size);
 316         if (err)
 317                 return err;
 318
 319         vi = fsverity_create_info(inode, desc, desc_size);
 320         if (IS_ERR(vi)) {
 321                 err = PTR_ERR(vi);
 322                 goto out_free_desc;
 323         }
 324
 325         fsverity_set_info(inode, vi);
 326         err = 0;
 327 out_free_desc:
 328         kfree(desc);
 329         return err;
 330 }
 331
 332 /**
 333  * fsverity_file_open() - prepare to open a verity file
 334  * @inode: the inode being opened
 335  * @filp: the struct file being set up
 336  *
 337  * When opening a verity file, deny the open if it is for writing.  Otherwise,
 338  * set up the inode's ->i_verity_info if not already done.
 339  *
 340  * When combined with fscrypt, this must be called after fscrypt_file_open().
 341  * Otherwise, we won't have the key set up to decrypt the verity metadata.
 342  *
 343  * Return: 0 on success, -errno on failure
 344  */
 345 int fsverity_file_open(struct inode *inode, struct file *filp)
 346 {
 347         if (!IS_VERITY(inode))
 348                 return 0;
 349
 350         if (filp->f_mode & FMODE_WRITE) {
 351                 pr_debug("Denying opening verity file (ino %lu) for write\n",
 352                          inode->i_ino);
 353                 return -EPERM;
 354         }
 355
 356         return ensure_verity_info(inode);
 357 }
 358 EXPORT_SYMBOL_GPL(fsverity_file_open);
 359
 360 /**
 361  * fsverity_prepare_setattr() - prepare to change a verity inode's attributes
 362  * @dentry: dentry through which the inode is being changed
 363  * @attr: attributes to change
 364  *
 365  * Verity files are immutable, so deny truncates.  This isn't covered by the
 366  * open-time check because sys_truncate() takes a path, not a file descriptor.
 367  *
 368  * Return: 0 on success, -errno on failure
 369  */
 370 int fsverity_prepare_setattr(struct dentry *dentry, struct iattr *attr)
 371 {
 372         if (IS_VERITY(d_inode(dentry)) && (attr->ia_valid & ATTR_SIZE)) {
 373                 pr_debug("Denying truncate of verity file (ino %lu)\n",
 374                          d_inode(dentry)->i_ino);
 375                 return -EPERM;
 376         }
 377         return 0;
 378 }
 379 EXPORT_SYMBOL_GPL(fsverity_prepare_setattr);
 380
 381 /**
 382  * fsverity_cleanup_inode() - free the inode's verity info, if present
 383  * @inode: an inode being evicted
 384  *
 385  * Filesystems must call this on inode eviction to free ->i_verity_info.
 386  */
 387 void fsverity_cleanup_inode(struct inode *inode)
 388 {
 389         fsverity_free_info(inode->i_verity_info);
 390         inode->i_verity_info = NULL;
 391 }
 392 EXPORT_SYMBOL_GPL(fsverity_cleanup_inode);
 393
 394 int __init fsverity_init_info_cache(void)
 395 {
 396         fsverity_info_cachep = KMEM_CACHE_USERCOPY(fsverity_info,
 397                                                    SLAB_RECLAIM_ACCOUNT,
 398                                                    file_digest);
 399         if (!fsverity_info_cachep)
 400                 return -ENOMEM;
 401         return 0;
 402 }
 403
 404 void __init fsverity_exit_info_cache(void)
 405 {
 406         kmem_cache_destroy(fsverity_info_cachep);
 407         fsverity_info_cachep = NULL;
 408 }