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