fs/verity/enable.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * Ioctl to enable verity on a file
   4  *
   5  * Copyright 2019 Google LLC
   6  */
   7
   8 #include "fsverity_private.h"
   9
  10 #include <crypto/hash.h>
  11 #include <linux/mount.h>
  12 #include <linux/sched/signal.h>
  13 #include <linux/uaccess.h>
  14
  15 struct block_buffer {
  16         u32 filled;
  17         bool is_root_hash;
  18         u8 *data;
  19 };
  20
  21 /* Hash a block, writing the result to the next level's pending block buffer. */
  22 static int hash_one_block(struct inode *inode,
  23                           const struct merkle_tree_params *params,
  24                           struct block_buffer *cur)
  25 {
  26         struct block_buffer *next = cur + 1;
  27         int err;
  28
  29         /*
  30          * Safety check to prevent a buffer overflow in case of a filesystem bug
  31          * that allows the file size to change despite deny_write_access(), or a
  32          * bug in the Merkle tree logic itself
  33          */
  34         if (WARN_ON_ONCE(next->is_root_hash && next->filled != 0))
  35                 return -EINVAL;
  36
  37         /* Zero-pad the block if it's shorter than the block size. */
  38         memset(&cur->data[cur->filled], 0, params->block_size - cur->filled);
  39
  40         err = fsverity_hash_block(params, inode, cur->data,
  41                                   &next->data[next->filled]);
  42         if (err)
  43                 return err;
  44         next->filled += params->digest_size;
  45         cur->filled = 0;
  46         return 0;
  47 }
  48
  49 static int write_merkle_tree_block(struct inode *inode, const u8 *buf,
  50                                    unsigned long index,
  51                                    const struct merkle_tree_params *params)
  52 {
  53         u64 pos = (u64)index << params->log_blocksize;
  54         int err;
  55
  56         err = inode->i_sb->s_vop->write_merkle_tree_block(inode, buf, pos,
  57                                                           params->block_size);
  58         if (err)
  59                 fsverity_err(inode, "Error %d writing Merkle tree block %lu",
  60                              err, index);
  61         return err;
  62 }
  63
  64 /*
  65  * Build the Merkle tree for the given file using the given parameters, and
  66  * return the root hash in @root_hash.
  67  *
  68  * The tree is written to a filesystem-specific location as determined by the
  69  * ->write_merkle_tree_block() method.  However, the blocks that comprise the
  70  * tree are the same for all filesystems.
  71  */
  72 static int build_merkle_tree(struct file *filp,
  73                              const struct merkle_tree_params *params,
  74                              u8 *root_hash)
  75 {
  76         struct inode *inode = file_inode(filp);
  77         const u64 data_size = inode->i_size;
  78         const int num_levels = params->num_levels;
  79         struct block_buffer _buffers[1 + FS_VERITY_MAX_LEVELS + 1] = {};
  80         struct block_buffer *buffers = &_buffers[1];
  81         unsigned long level_offset[FS_VERITY_MAX_LEVELS];
  82         int level;
  83         u64 offset;
  84         int err;
  85
  86         if (data_size == 0) {
  87                 /* Empty file is a special case; root hash is all 0's */
  88                 memset(root_hash, 0, params->digest_size);
  89                 return 0;
  90         }
  91
  92         /*
  93          * Allocate the block buffers.  Buffer "-1" is for data blocks.
  94          * Buffers 0 <= level < num_levels are for the actual tree levels.
  95          * Buffer 'num_levels' is for the root hash.
  96          */
  97         for (level = -1; level < num_levels; level++) {
  98                 buffers[level].data = kzalloc(params->block_size, GFP_KERNEL);
  99                 if (!buffers[level].data) {
 100                         err = -ENOMEM;
 101                         goto out;
 102                 }
 103         }
 104         buffers[num_levels].data = root_hash;
 105         buffers[num_levels].is_root_hash = true;
 106
 107         BUILD_BUG_ON(sizeof(level_offset) != sizeof(params->level_start));
 108         memcpy(level_offset, params->level_start, sizeof(level_offset));
 109
 110         /* Hash each data block, also hashing the tree blocks as they fill up */
 111         for (offset = 0; offset < data_size; offset += params->block_size) {
 112                 ssize_t bytes_read;
 113                 loff_t pos = offset;
 114
 115                 buffers[-1].filled = min_t(u64, params->block_size,
 116                                            data_size - offset);
 117                 bytes_read = __kernel_read(filp, buffers[-1].data,
 118                                            buffers[-1].filled, &pos);
 119                 if (bytes_read < 0) {
 120                         err = bytes_read;
 121                         fsverity_err(inode, "Error %d reading file data", err);
 122                         goto out;
 123                 }
 124                 if (bytes_read != buffers[-1].filled) {
 125                         err = -EINVAL;
 126                         fsverity_err(inode, "Short read of file data");
 127                         goto out;
 128                 }
 129                 err = hash_one_block(inode, params, &buffers[-1]);
 130                 if (err)
 131                         goto out;
 132                 for (level = 0; level < num_levels; level++) {
 133                         if (buffers[level].filled + params->digest_size <=
 134                             params->block_size) {
 135                                 /* Next block at @level isn't full yet */
 136                                 break;
 137                         }
 138                         /* Next block at @level is full */
 139
 140                         err = hash_one_block(inode, params, &buffers[level]);
 141                         if (err)
 142                                 goto out;
 143                         err = write_merkle_tree_block(inode,
 144                                                       buffers[level].data,
 145                                                       level_offset[level],
 146                                                       params);
 147                         if (err)
 148                                 goto out;
 149                         level_offset[level]++;
 150                 }
 151                 if (fatal_signal_pending(current)) {
 152                         err = -EINTR;
 153                         goto out;
 154                 }
 155                 cond_resched();
 156         }
 157         /* Finish all nonempty pending tree blocks. */
 158         for (level = 0; level < num_levels; level++) {
 159                 if (buffers[level].filled != 0) {
 160                         err = hash_one_block(inode, params, &buffers[level]);
 161                         if (err)
 162                                 goto out;
 163                         err = write_merkle_tree_block(inode,
 164                                                       buffers[level].data,
 165                                                       level_offset[level],
 166                                                       params);
 167                         if (err)
 168                                 goto out;
 169                 }
 170         }
 171         /* The root hash was filled by the last call to hash_one_block(). */
 172         if (WARN_ON_ONCE(buffers[num_levels].filled != params->digest_size)) {
 173                 err = -EINVAL;
 174                 goto out;
 175         }
 176         err = 0;
 177 out:
 178         for (level = -1; level < num_levels; level++)
 179                 kfree(buffers[level].data);
 180         return err;
 181 }
 182
 183 static int enable_verity(struct file *filp,
 184                          const struct fsverity_enable_arg *arg)
 185 {
 186         struct inode *inode = file_inode(filp);
 187         const struct fsverity_operations *vops = inode->i_sb->s_vop;
 188         struct merkle_tree_params params = { };
 189         struct fsverity_descriptor *desc;
 190         size_t desc_size = struct_size(desc, signature, arg->sig_size);
 191         struct fsverity_info *vi;
 192         int err;
 193
 194         /* Start initializing the fsverity_descriptor */
 195         desc = kzalloc(desc_size, GFP_KERNEL);
 196         if (!desc)
 197                 return -ENOMEM;
 198         desc->version = 1;
 199         desc->hash_algorithm = arg->hash_algorithm;
 200         desc->log_blocksize = ilog2(arg->block_size);
 201
 202         /* Get the salt if the user provided one */
 203         if (arg->salt_size &&
 204             copy_from_user(desc->salt, u64_to_user_ptr(arg->salt_ptr),
 205                            arg->salt_size)) {
 206                 err = -EFAULT;
 207                 goto out;
 208         }
 209         desc->salt_size = arg->salt_size;
 210
 211         /* Get the builtin signature if the user provided one */
 212         if (arg->sig_size &&
 213             copy_from_user(desc->signature, u64_to_user_ptr(arg->sig_ptr),
 214                            arg->sig_size)) {
 215                 err = -EFAULT;
 216                 goto out;
 217         }
 218         desc->sig_size = cpu_to_le32(arg->sig_size);
 219
 220         desc->data_size = cpu_to_le64(inode->i_size);
 221
 222         /* Prepare the Merkle tree parameters */
 223         err = fsverity_init_merkle_tree_params(&params, inode,
 224                                                arg->hash_algorithm,
 225                                                desc->log_blocksize,
 226                                                desc->salt, desc->salt_size);
 227         if (err)
 228                 goto out;
 229
 230         /*
 231          * Start enabling verity on this file, serialized by the inode lock.
 232          * Fail if verity is already enabled or is already being enabled.
 233          */
 234         inode_lock(inode);
 235         if (IS_VERITY(inode))
 236                 err = -EEXIST;
 237         else
 238                 err = vops->begin_enable_verity(filp);
 239         inode_unlock(inode);
 240         if (err)
 241                 goto out;
 242
 243         /*
 244          * Build the Merkle tree.  Don't hold the inode lock during this, since
 245          * on huge files this may take a very long time and we don't want to
 246          * force unrelated syscalls like chown() to block forever.  We don't
 247          * need the inode lock here because deny_write_access() already prevents
 248          * the file from being written to or truncated, and we still serialize
 249          * ->begin_enable_verity() and ->end_enable_verity() using the inode
 250          * lock and only allow one process to be here at a time on a given file.
 251          */
 252         BUILD_BUG_ON(sizeof(desc->root_hash) < FS_VERITY_MAX_DIGEST_SIZE);
 253         err = build_merkle_tree(filp, &params, desc->root_hash);
 254         if (err) {
 255                 fsverity_err(inode, "Error %d building Merkle tree", err);
 256                 goto rollback;
 257         }
 258
 259         /*
 260          * Create the fsverity_info.  Don't bother trying to save work by
 261          * reusing the merkle_tree_params from above.  Instead, just create the
 262          * fsverity_info from the fsverity_descriptor as if it were just loaded
 263          * from disk.  This is simpler, and it serves as an extra check that the
 264          * metadata we're writing is valid before actually enabling verity.
 265          */
 266         vi = fsverity_create_info(inode, desc);
 267         if (IS_ERR(vi)) {
 268                 err = PTR_ERR(vi);
 269                 goto rollback;
 270         }
 271
 272         /*
 273          * Tell the filesystem to finish enabling verity on the file.
 274          * Serialized with ->begin_enable_verity() by the inode lock.
 275          */
 276         inode_lock(inode);
 277         err = vops->end_enable_verity(filp, desc, desc_size, params.tree_size);
 278         inode_unlock(inode);
 279         if (err) {
 280                 fsverity_err(inode, "%ps() failed with err %d",
 281                              vops->end_enable_verity, err);
 282                 fsverity_free_info(vi);
 283         } else if (WARN_ON_ONCE(!IS_VERITY(inode))) {
 284                 err = -EINVAL;
 285                 fsverity_free_info(vi);
 286         } else {
 287                 /* Successfully enabled verity */
 288
 289                 /*
 290                  * Readers can start using ->i_verity_info immediately, so it
 291                  * can't be rolled back once set.  So don't set it until just
 292                  * after the filesystem has successfully enabled verity.
 293                  */
 294                 fsverity_set_info(inode, vi);
 295         }
 296 out:
 297         kfree(params.hashstate);
 298         kfree(desc);
 299         return err;
 300
 301 rollback:
 302         inode_lock(inode);
 303         (void)vops->end_enable_verity(filp, NULL, 0, params.tree_size);
 304         inode_unlock(inode);
 305         goto out;
 306 }
 307
 308 /**
 309  * fsverity_ioctl_enable() - enable verity on a file
 310  * @filp: file to enable verity on
 311  * @uarg: user pointer to fsverity_enable_arg
 312  *
 313  * Enable fs-verity on a file.  See the "FS_IOC_ENABLE_VERITY" section of
 314  * Documentation/filesystems/fsverity.rst for the documentation.
 315  *
 316  * Return: 0 on success, -errno on failure
 317  */
 318 int fsverity_ioctl_enable(struct file *filp, const void __user *uarg)
 319 {
 320         struct inode *inode = file_inode(filp);
 321         struct fsverity_enable_arg arg;
 322         int err;
 323
 324         if (copy_from_user(&arg, uarg, sizeof(arg)))
 325                 return -EFAULT;
 326
 327         if (arg.version != 1)
 328                 return -EINVAL;
 329
 330         if (arg.__reserved1 ||
 331             memchr_inv(arg.__reserved2, 0, sizeof(arg.__reserved2)))
 332                 return -EINVAL;
 333
 334         if (!is_power_of_2(arg.block_size))
 335                 return -EINVAL;
 336
 337         if (arg.salt_size > sizeof_field(struct fsverity_descriptor, salt))
 338                 return -EMSGSIZE;
 339
 340         if (arg.sig_size > FS_VERITY_MAX_SIGNATURE_SIZE)
 341                 return -EMSGSIZE;
 342
 343         /*
 344          * Require a regular file with write access.  But the actual fd must
 345          * still be readonly so that we can lock out all writers.  This is
 346          * needed to guarantee that no writable fds exist to the file once it
 347          * has verity enabled, and to stabilize the data being hashed.
 348          */
 349
 350         err = file_permission(filp, MAY_WRITE);
 351         if (err)
 352                 return err;
 353         /*
 354          * __kernel_read() is used while building the Merkle tree.  So, we can't
 355          * allow file descriptors that were opened for ioctl access only, using
 356          * the special nonstandard access mode 3.  O_RDONLY only, please!
 357          */
 358         if (!(filp->f_mode & FMODE_READ))
 359                 return -EBADF;
 360
 361         if (IS_APPEND(inode))
 362                 return -EPERM;
 363
 364         if (S_ISDIR(inode->i_mode))
 365                 return -EISDIR;
 366
 367         if (!S_ISREG(inode->i_mode))
 368                 return -EINVAL;
 369
 370         err = mnt_want_write_file(filp);
 371         if (err) /* -EROFS */
 372                 return err;
 373
 374         err = deny_write_access(filp);
 375         if (err) /* -ETXTBSY */
 376                 goto out_drop_write;
 377
 378         err = enable_verity(filp, &arg);
 379
 380         /*
 381          * We no longer drop the inode's pagecache after enabling verity.  This
 382          * used to be done to try to avoid a race condition where pages could be
 383          * evicted after being used in the Merkle tree construction, then
 384          * re-instantiated by a concurrent read.  Such pages are unverified, and
 385          * the backing storage could have filled them with different content, so
 386          * they shouldn't be used to fulfill reads once verity is enabled.
 387          *
 388          * But, dropping the pagecache has a big performance impact, and it
 389          * doesn't fully solve the race condition anyway.  So for those reasons,
 390          * and also because this race condition isn't very important relatively
 391          * speaking (especially for small-ish files, where the chance of a page
 392          * being used, evicted, *and* re-instantiated all while enabling verity
 393          * is quite small), we no longer drop the inode's pagecache.
 394          */
 395
 396         /*
 397          * allow_write_access() is needed to pair with deny_write_access().
 398          * Regardless, the filesystem won't allow writing to verity files.
 399          */
 400         allow_write_access(filp);
 401 out_drop_write:
 402         mnt_drop_write_file(filp);
 403         return err;
 404 }
 405 EXPORT_SYMBOL_GPL(fsverity_ioctl_enable);