fs/ext4/verity.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * fs/ext4/verity.c: fs-verity support for ext4
   4  *
   5  * Copyright 2019 Google LLC
   6  */
   7
   8 /*
   9  * Implementation of fsverity_operations for ext4.
  10  *
  11  * ext4 stores the verity metadata (Merkle tree and fsverity_descriptor) past
  12  * the end of the file, starting at the first 64K boundary beyond i_size.  This
  13  * approach works because (a) verity files are readonly, and (b) pages fully
  14  * beyond i_size aren't visible to userspace but can be read/written internally
  15  * by ext4 with only some relatively small changes to ext4.  This approach
  16  * avoids having to depend on the EA_INODE feature and on rearchitecturing
  17  * ext4's xattr support to support paging multi-gigabyte xattrs into memory, and
  18  * to support encrypting xattrs.  Note that the verity metadata *must* be
  19  * encrypted when the file is, since it contains hashes of the plaintext data.
  20  *
  21  * Using a 64K boundary rather than a 4K one keeps things ready for
  22  * architectures with 64K pages, and it doesn't necessarily waste space on-disk
  23  * since there can be a hole between i_size and the start of the Merkle tree.
  24  */
  25
  26 #include <linux/quotaops.h>
  27
  28 #include "ext4.h"
  29 #include "ext4_extents.h"
  30 #include "ext4_jbd2.h"
  31
  32 static inline loff_t ext4_verity_metadata_pos(const struct inode *inode)
  33 {
  34         return round_up(inode->i_size, 65536);
  35 }
  36
  37 /*
  38  * Read some verity metadata from the inode.  __vfs_read() can't be used because
  39  * we need to read beyond i_size.
  40  */
  41 static int pagecache_read(struct inode *inode, void *buf, size_t count,
  42                           loff_t pos)
  43 {
  44         while (count) {
  45                 size_t n = min_t(size_t, count,
  46                                  PAGE_SIZE - offset_in_page(pos));
  47                 struct page *page;
  48                 void *addr;
  49
  50                 page = read_mapping_page(inode->i_mapping, pos >> PAGE_SHIFT,
  51                                          NULL);
  52                 if (IS_ERR(page))
  53                         return PTR_ERR(page);
  54
  55                 addr = kmap_atomic(page);
  56                 memcpy(buf, addr + offset_in_page(pos), n);
  57                 kunmap_atomic(addr);
  58
  59                 put_page(page);
  60
  61                 buf += n;
  62                 pos += n;
  63                 count -= n;
  64         }
  65         return 0;
  66 }
  67
  68 /*
  69  * Write some verity metadata to the inode for FS_IOC_ENABLE_VERITY.
  70  * kernel_write() can't be used because the file descriptor is readonly.
  71  */
  72 static int pagecache_write(struct inode *inode, const void *buf, size_t count,
  73                            loff_t pos)
  74 {
  75         if (pos + count > inode->i_sb->s_maxbytes)
  76                 return -EFBIG;
  77
  78         while (count) {
  79                 size_t n = min_t(size_t, count,
  80                                  PAGE_SIZE - offset_in_page(pos));
  81                 struct page *page;
  82                 void *fsdata;
  83                 void *addr;
  84                 int res;
  85
  86                 res = pagecache_write_begin(NULL, inode->i_mapping, pos, n, 0,
  87                                             &page, &fsdata);
  88                 if (res)
  89                         return res;
  90
  91                 addr = kmap_atomic(page);
  92                 memcpy(addr + offset_in_page(pos), buf, n);
  93                 kunmap_atomic(addr);
  94
  95                 res = pagecache_write_end(NULL, inode->i_mapping, pos, n, n,
  96                                           page, fsdata);
  97                 if (res < 0)
  98                         return res;
  99                 if (res != n)
 100                         return -EIO;
 101
 102                 buf += n;
 103                 pos += n;
 104                 count -= n;
 105         }
 106         return 0;
 107 }
 108
 109 static int ext4_begin_enable_verity(struct file *filp)
 110 {
 111         struct inode *inode = file_inode(filp);
 112         const int credits = 2; /* superblock and inode for ext4_orphan_add() */
 113         handle_t *handle;
 114         int err;
 115
 116         if (ext4_verity_in_progress(inode))
 117                 return -EBUSY;
 118
 119         /*
 120          * Since the file was opened readonly, we have to initialize the jbd
 121          * inode and quotas here and not rely on ->open() doing it.  This must
 122          * be done before evicting the inline data.
 123          */
 124
 125         err = ext4_inode_attach_jinode(inode);
 126         if (err)
 127                 return err;
 128
 129         err = dquot_initialize(inode);
 130         if (err)
 131                 return err;
 132
 133         err = ext4_convert_inline_data(inode);
 134         if (err)
 135                 return err;
 136
 137         if (!ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS)) {
 138                 ext4_warning_inode(inode,
 139                                    "verity is only allowed on extent-based files");
 140                 return -EOPNOTSUPP;
 141         }
 142
 143         /*
 144          * ext4 uses the last allocated block to find the verity descriptor, so
 145          * we must remove any other blocks past EOF which might confuse things.
 146          */
 147         err = ext4_truncate(inode);
 148         if (err)
 149                 return err;
 150
 151         handle = ext4_journal_start(inode, EXT4_HT_INODE, credits);
 152         if (IS_ERR(handle))
 153                 return PTR_ERR(handle);
 154
 155         err = ext4_orphan_add(handle, inode);
 156         if (err == 0)
 157                 ext4_set_inode_state(inode, EXT4_STATE_VERITY_IN_PROGRESS);
 158
 159         ext4_journal_stop(handle);
 160         return err;
 161 }
 162
 163 /*
 164  * ext4 stores the verity descriptor beginning on the next filesystem block
 165  * boundary after the Merkle tree.  Then, the descriptor size is stored in the
 166  * last 4 bytes of the last allocated filesystem block --- which is either the
 167  * block in which the descriptor ends, or the next block after that if there
 168  * weren't at least 4 bytes remaining.
 169  *
 170  * We can't simply store the descriptor in an xattr because it *must* be
 171  * encrypted when ext4 encryption is used, but ext4 encryption doesn't encrypt
 172  * xattrs.  Also, if the descriptor includes a large signature blob it may be
 173  * too large to store in an xattr without the EA_INODE feature.
 174  */
 175 static int ext4_write_verity_descriptor(struct inode *inode, const void *desc,
 176                                         size_t desc_size, u64 merkle_tree_size)
 177 {
 178         const u64 desc_pos = round_up(ext4_verity_metadata_pos(inode) +
 179                                       merkle_tree_size, i_blocksize(inode));
 180         const u64 desc_end = desc_pos + desc_size;
 181         const __le32 desc_size_disk = cpu_to_le32(desc_size);
 182         const u64 desc_size_pos = round_up(desc_end + sizeof(desc_size_disk),
 183                                            i_blocksize(inode)) -
 184                                   sizeof(desc_size_disk);
 185         int err;
 186
 187         err = pagecache_write(inode, desc, desc_size, desc_pos);
 188         if (err)
 189                 return err;
 190
 191         return pagecache_write(inode, &desc_size_disk, sizeof(desc_size_disk),
 192                                desc_size_pos);
 193 }
 194
 195 static int ext4_end_enable_verity(struct file *filp, const void *desc,
 196                                   size_t desc_size, u64 merkle_tree_size)
 197 {
 198         struct inode *inode = file_inode(filp);
 199         const int credits = 2; /* superblock and inode for ext4_orphan_del() */
 200         handle_t *handle;
 201         int err = 0;
 202         int err2;
 203
 204         if (desc != NULL) {
 205                 /* Succeeded; write the verity descriptor. */
 206                 err = ext4_write_verity_descriptor(inode, desc, desc_size,
 207                                                    merkle_tree_size);
 208
 209                 /* Write all pages before clearing VERITY_IN_PROGRESS. */
 210                 if (!err)
 211                         err = filemap_write_and_wait(inode->i_mapping);
 212         }
 213
 214         /* If we failed, truncate anything we wrote past i_size. */
 215         if (desc == NULL || err)
 216                 ext4_truncate(inode);
 217
 218         /*
 219          * We must always clean up by clearing EXT4_STATE_VERITY_IN_PROGRESS and
 220          * deleting the inode from the orphan list, even if something failed.
 221          * If everything succeeded, we'll also set the verity bit in the same
 222          * transaction.
 223          */
 224
 225         ext4_clear_inode_state(inode, EXT4_STATE_VERITY_IN_PROGRESS);
 226
 227         handle = ext4_journal_start(inode, EXT4_HT_INODE, credits);
 228         if (IS_ERR(handle)) {
 229                 ext4_orphan_del(NULL, inode);
 230                 return PTR_ERR(handle);
 231         }
 232
 233         err2 = ext4_orphan_del(handle, inode);
 234         if (err2)
 235                 goto out_stop;
 236
 237         if (desc != NULL && !err) {
 238                 struct ext4_iloc iloc;
 239
 240                 err = ext4_reserve_inode_write(handle, inode, &iloc);
 241                 if (err)
 242                         goto out_stop;
 243                 ext4_set_inode_flag(inode, EXT4_INODE_VERITY);
 244                 ext4_set_inode_flags(inode);
 245                 err = ext4_mark_iloc_dirty(handle, inode, &iloc);
 246         }
 247 out_stop:
 248         ext4_journal_stop(handle);
 249         return err ?: err2;
 250 }
 251
 252 static int ext4_get_verity_descriptor_location(struct inode *inode,
 253                                                size_t *desc_size_ret,
 254                                                u64 *desc_pos_ret)
 255 {
 256         struct ext4_ext_path *path;
 257         struct ext4_extent *last_extent;
 258         u32 end_lblk;
 259         u64 desc_size_pos;
 260         __le32 desc_size_disk;
 261         u32 desc_size;
 262         u64 desc_pos;
 263         int err;
 264
 265         /*
 266          * Descriptor size is in last 4 bytes of last allocated block.
 267          * See ext4_write_verity_descriptor().
 268          */
 269
 270         if (!ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS)) {
 271                 EXT4_ERROR_INODE(inode, "verity file doesn't use extents");
 272                 return -EFSCORRUPTED;
 273         }
 274
 275         path = ext4_find_extent(inode, EXT_MAX_BLOCKS - 1, NULL, 0);
 276         if (IS_ERR(path))
 277                 return PTR_ERR(path);
 278
 279         last_extent = path[path->p_depth].p_ext;
 280         if (!last_extent) {
 281                 EXT4_ERROR_INODE(inode, "verity file has no extents");
 282                 ext4_ext_drop_refs(path);
 283                 kfree(path);
 284                 return -EFSCORRUPTED;
 285         }
 286
 287         end_lblk = le32_to_cpu(last_extent->ee_block) +
 288                    ext4_ext_get_actual_len(last_extent);
 289         desc_size_pos = (u64)end_lblk << inode->i_blkbits;
 290         ext4_ext_drop_refs(path);
 291         kfree(path);
 292
 293         if (desc_size_pos < sizeof(desc_size_disk))
 294                 goto bad;
 295         desc_size_pos -= sizeof(desc_size_disk);
 296
 297         err = pagecache_read(inode, &desc_size_disk, sizeof(desc_size_disk),
 298                              desc_size_pos);
 299         if (err)
 300                 return err;
 301         desc_size = le32_to_cpu(desc_size_disk);
 302
 303         /*
 304          * The descriptor is stored just before the desc_size_disk, but starting
 305          * on a filesystem block boundary.
 306          */
 307
 308         if (desc_size > INT_MAX || desc_size > desc_size_pos)
 309                 goto bad;
 310
 311         desc_pos = round_down(desc_size_pos - desc_size, i_blocksize(inode));
 312         if (desc_pos < ext4_verity_metadata_pos(inode))
 313                 goto bad;
 314
 315         *desc_size_ret = desc_size;
 316         *desc_pos_ret = desc_pos;
 317         return 0;
 318
 319 bad:
 320         EXT4_ERROR_INODE(inode, "verity file corrupted; can't find descriptor");
 321         return -EFSCORRUPTED;
 322 }
 323
 324 static int ext4_get_verity_descriptor(struct inode *inode, void *buf,
 325                                       size_t buf_size)
 326 {
 327         size_t desc_size = 0;
 328         u64 desc_pos = 0;
 329         int err;
 330
 331         err = ext4_get_verity_descriptor_location(inode, &desc_size, &desc_pos);
 332         if (err)
 333                 return err;
 334
 335         if (buf_size) {
 336                 if (desc_size > buf_size)
 337                         return -ERANGE;
 338                 err = pagecache_read(inode, buf, desc_size, desc_pos);
 339                 if (err)
 340                         return err;
 341         }
 342         return desc_size;
 343 }
 344
 345 /*
 346  * Prefetch some pages from the file's Merkle tree.
 347  *
 348  * This is basically a stripped-down version of __do_page_cache_readahead()
 349  * which works on pages past i_size.
 350  */
 351 static void ext4_merkle_tree_readahead(struct address_space *mapping,
 352                                        pgoff_t start_index, unsigned long count)
 353 {
 354         LIST_HEAD(pages);
 355         unsigned int nr_pages = 0;
 356         struct page *page;
 357         pgoff_t index;
 358         struct blk_plug plug;
 359
 360         for (index = start_index; index < start_index + count; index++) {
 361                 page = xa_load(&mapping->i_pages, index);
 362                 if (!page || xa_is_value(page)) {
 363                         page = __page_cache_alloc(readahead_gfp_mask(mapping));
 364                         if (!page)
 365                                 break;
 366                         page->index = index;
 367                         list_add(&page->lru, &pages);
 368                         nr_pages++;
 369                 }
 370         }
 371         blk_start_plug(&plug);
 372         ext4_mpage_readpages(mapping, &pages, NULL, nr_pages, true);
 373         blk_finish_plug(&plug);
 374 }
 375
 376 static struct page *ext4_read_merkle_tree_page(struct inode *inode,
 377                                                pgoff_t index,
 378                                                unsigned long num_ra_pages)
 379 {
 380         struct page *page;
 381
 382         index += ext4_verity_metadata_pos(inode) >> PAGE_SHIFT;
 383
 384         page = find_get_page_flags(inode->i_mapping, index, FGP_ACCESSED);
 385         if (!page || !PageUptodate(page)) {
 386                 if (page)
 387                         put_page(page);
 388                 else if (num_ra_pages > 1)
 389                         ext4_merkle_tree_readahead(inode->i_mapping, index,
 390                                                    num_ra_pages);
 391                 page = read_mapping_page(inode->i_mapping, index, NULL);
 392         }
 393         return page;
 394 }
 395
 396 static int ext4_write_merkle_tree_block(struct inode *inode, const void *buf,
 397                                         u64 index, int log_blocksize)
 398 {
 399         loff_t pos = ext4_verity_metadata_pos(inode) + (index << log_blocksize);
 400
 401         return pagecache_write(inode, buf, 1 << log_blocksize, pos);
 402 }
 403
 404 const struct fsverity_operations ext4_verityops = {
 405         .begin_enable_verity    = ext4_begin_enable_verity,
 406         .end_enable_verity      = ext4_end_enable_verity,
 407         .get_verity_descriptor  = ext4_get_verity_descriptor,
 408         .read_merkle_tree_page  = ext4_read_merkle_tree_page,
 409         .write_merkle_tree_block = ext4_write_merkle_tree_block,
 410 };