Commit message: read_boot(): Handle excessive FAT size specifications

The variable used for storing the FAT size (in bytes) was an unsigned
int. Since the size in sectors read from the BPB was not sufficiently
checked, this could end up being zero after multiplying it with the
sector size while some offsets still stayed excessive. Ultimately it
would cause segfaults when accessing FAT entries for which no memory
was allocated.

Make it more robust by changing the types used to store FAT size to
off_t and abort if there is no room for data clusters. Additionally
check that FAT size is not specified as zero.

Fixes #25 and fixes #26.

Reported-by: Hanno Böck
Signed-off-by: Andreas Bombe <aeb@debian.org>
CVE: CVE-2016-4804
CWE: CWE-119
Vulnerability description: The read_boot function in boot.c in dosfstools before 4.0 allows attackers to cause a denial of service (crash) via a crafted filesystem, which triggers a heap-based buffer overflow in the (1) read_fat function or an out-of-bounds heap read in (2) get_fat function.

Project: dosfstools
File: boot.c

void read_boot(DOS_FS * fs)
{
     struct boot_sector b;
     unsigned total_sectors;
     unsigned short logical_sector_size, sectors;
    unsigned fat_length;
     unsigned total_fat_entries;
     off_t data_size;
 
    fs_read(0, sizeof(b), &b);
    logical_sector_size = GET_UNALIGNED_W(b.sector_size);
    if (!logical_sector_size)
	die("Logical sector size is zero.");

    /* This was moved up because it's the first thing that will fail */
    /* if the platform needs special handling of unaligned multibyte accesses */
    /* but such handling isn't being provided. See GET_UNALIGNED_W() above. */
    if (logical_sector_size & (SECTOR_SIZE - 1))
	die("Logical sector size (%d bytes) is not a multiple of the physical "
	    "sector size.", logical_sector_size);

    fs->cluster_size = b.cluster_size * logical_sector_size;
    if (!fs->cluster_size)
	die("Cluster size is zero.");
    if (b.fats != 2 && b.fats != 1)
	die("Currently, only 1 or 2 FATs are supported, not %d.\n", b.fats);
    fs->nfats = b.fats;
    sectors = GET_UNALIGNED_W(b.sectors);
    total_sectors = sectors ? sectors : le32toh(b.total_sect);
    if (verbose)
	printf("Checking we can access the last sector of the filesystem\n");
     /* Can't access last odd sector anyway, so round down */
     fs_test((off_t)((total_sectors & ~1) - 1) * logical_sector_size,
 	    logical_sector_size);
     fat_length = le16toh(b.fat_length) ?
 	le16toh(b.fat_length) : le32toh(b.fat32_length);
     fs->fat_start = (off_t)le16toh(b.reserved) * logical_sector_size;
     fs->root_start = ((off_t)le16toh(b.reserved) + b.fats * fat_length) *
 	logical_sector_size;
     fs->root_entries = GET_UNALIGNED_W(b.dir_entries);
     fs->data_start = fs->root_start + ROUND_TO_MULTIPLE(fs->root_entries <<
 							MSDOS_DIR_BITS,
 							logical_sector_size);
     data_size = (off_t)total_sectors * logical_sector_size - fs->data_start;
     fs->data_clusters = data_size / fs->cluster_size;
     fs->root_cluster = 0;	/* indicates standard, pre-FAT32 root dir */
     fs->fsinfo_start = 0;	/* no FSINFO structure */
    fs->free_clusters = -1;	/* unknown */
    if (!b.fat_length && b.fat32_length) {
	fs->fat_bits = 32;
	fs->root_cluster = le32toh(b.root_cluster);
	if (!fs->root_cluster && fs->root_entries)
	    /* M$ hasn't specified this, but it looks reasonable: If
	     * root_cluster is 0 but there is a separate root dir
	     * (root_entries != 0), we handle the root dir the old way. Give a
	     * warning, but convertig to a root dir in a cluster chain seems
	     * to complex for now... */
	    printf("Warning: FAT32 root dir not in cluster chain! "
		   "Compatibility mode...\n");
	else if (!fs->root_cluster && !fs->root_entries)
	    die("No root directory!");
	else if (fs->root_cluster && fs->root_entries)
	    printf("Warning: FAT32 root dir is in a cluster chain, but "
		   "a separate root dir\n"
		   "  area is defined. Cannot fix this easily.\n");
	if (fs->data_clusters < FAT16_THRESHOLD)
	    printf("Warning: Filesystem is FAT32 according to fat_length "
		   "and fat32_length fields,\n"
		   "  but has only %lu clusters, less than the required "
		   "minimum of %d.\n"
		   "  This may lead to problems on some systems.\n",
		   (unsigned long)fs->data_clusters, FAT16_THRESHOLD);

	check_fat_state_bit(fs, &b);
	fs->backupboot_start = le16toh(b.backup_boot) * logical_sector_size;
	check_backup_boot(fs, &b, logical_sector_size);

	read_fsinfo(fs, &b, logical_sector_size);
    } else if (!atari_format) {
	/* On real MS-DOS, a 16 bit FAT is used whenever there would be too
	 * much clusers otherwise. */
	fs->fat_bits = (fs->data_clusters >= FAT12_THRESHOLD) ? 16 : 12;
	if (fs->data_clusters >= FAT16_THRESHOLD)
	    die("Too many clusters (%lu) for FAT16 filesystem.", fs->data_clusters);
	check_fat_state_bit(fs, &b);
    } else {
	/* On Atari, things are more difficult: GEMDOS always uses 12bit FATs
	 * on floppies, and always 16 bit on harddisks. */
	fs->fat_bits = 16;	/* assume 16 bit FAT for now */
	/* If more clusters than fat entries in 16-bit fat, we assume
	 * it's a real MSDOS FS with 12-bit fat. */
	if (fs->data_clusters + 2 > fat_length * logical_sector_size * 8 / 16 ||
	    /* if it has one of the usual floppy sizes -> 12bit FAT  */
	    (total_sectors == 720 || total_sectors == 1440 ||
	     total_sectors == 2880))
	    fs->fat_bits = 12;
    }
    /* On FAT32, the high 4 bits of a FAT entry are reserved */
    fs->eff_fat_bits = (fs->fat_bits == 32) ? 28 : fs->fat_bits;
    fs->fat_size = fat_length * logical_sector_size;

    fs->label = calloc(12, sizeof(uint8_t));
    if (fs->fat_bits == 12 || fs->fat_bits == 16) {
	struct boot_sector_16 *b16 = (struct boot_sector_16 *)&b;
	if (b16->extended_sig == 0x29)
	    memmove(fs->label, b16->label, 11);
	else
	    fs->label = NULL;
    } else if (fs->fat_bits == 32) {
	if (b.extended_sig == 0x29)
	    memmove(fs->label, &b.label, 11);
	else
	    fs->label = NULL;
    }

    total_fat_entries = (uint64_t)fs->fat_size * 8 / fs->fat_bits;
    if (fs->data_clusters > total_fat_entries - 2)
	die("Filesystem has %u clusters but only space for %u FAT entries.",
	    fs->data_clusters, total_fat_entries - 2);
    if (!fs->root_entries && !fs->root_cluster)
	die("Root directory has zero size.");
    if (fs->root_entries & (MSDOS_DPS - 1))
	die("Root directory (%d entries) doesn't span an integral number of "
	    "sectors.", fs->root_entries);
    if (logical_sector_size & (SECTOR_SIZE - 1))
	die("Logical sector size (%d bytes) is not a multiple of the physical "
	    "sector size.", logical_sector_size);
#if 0				/* linux kernel doesn't check that either */
    /* ++roman: On Atari, these two fields are often left uninitialized */
    if (!atari_format && (!b.secs_track || !b.heads))
	die("Invalid disk format in boot sector.");
#endif
    if (verbose)
	dump_boot(fs, &b, logical_sector_size);
}