Code: void set_fat(DOS_FS * fs, uint32_t cluster, int32_t new)
{
    unsigned char *data = NULL;
    int size;
    loff_t offs;

    if (new == -1)
	new = FAT_EOF(fs);
    else if ((long)new == -2)
	new = FAT_BAD(fs);
    switch (fs->fat_bits) {
    case 12:
	data = fs->fat + cluster * 3 / 2;
	offs = fs->fat_start + cluster * 3 / 2;
	if (cluster & 1) {
	    FAT_ENTRY prevEntry;
	    get_fat(&prevEntry, fs->fat, cluster - 1, fs);
	    data[0] = ((new & 0xf) << 4) | (prevEntry.value >> 8);
 	    data[1] = new >> 4;
 	} else {
 	    FAT_ENTRY subseqEntry;
	    if (cluster != fs->clusters - 1)
 		get_fat(&subseqEntry, fs->fat, cluster + 1, fs);
 	    else
 		subseqEntry.value = 0;
	    data[0] = new & 0xff;
	    data[1] = (new >> 8) | ((0xff & subseqEntry.value) << 4);
	}
	size = 2;
	break;
    case 16:
	data = fs->fat + cluster * 2;
	offs = fs->fat_start + cluster * 2;
	*(unsigned short *)data = htole16(new);
	size = 2;
	break;
    case 32:
	{
	    FAT_ENTRY curEntry;
	    get_fat(&curEntry, fs->fat, cluster, fs);

	    data = fs->fat + cluster * 4;
	    offs = fs->fat_start + cluster * 4;
	    /* According to M$, the high 4 bits of a FAT32 entry are reserved and
	     * are not part of the cluster number. So we never touch them. */
	    *(uint32_t *)data = htole32((new & 0xfffffff) |
					     (curEntry.reserved << 28));
	    size = 4;
	}
	break;
    default:
	die("Bad FAT entry size: %d bits.", fs->fat_bits);
    }
    fs_write(offs, size, data);
    if (fs->nfats > 1) {
	fs_write(offs + fs->fat_size, size, data);
    }
}

Vulnerability Type: DoS 
CWE ID: CWE-189
Summary: The set_fat function in fat.c in dosfstools before 4.0 might allow attackers to corrupt a FAT12 filesystem or cause a denial of service (invalid memory read and crash) by writing an odd number of clusters to the third to last entry on a FAT12 filesystem, which triggers an *off-by-two error.*
Commit Message: set_fat(): Fix off-by-2 error leading to corruption in FAT12

In FAT12 two 12 bit entries are combined to a 24 bit value (three
bytes). Therefore, when an even numbered FAT entry is set in FAT12, it
must be be combined with the following entry. To prevent accessing
beyond the end of the FAT array, it must be checked that the cluster is
not the last one.

Previously, the check tested that the requested cluster was equal to
fs->clusters - 1. However, fs->clusters is the number of data clusters
not including the two reserved FAT entries at the start so the test
triggered two clusters early.

If the third to last entry was written on a FAT12 filesystem with an
odd number of clusters, the second to last entry would be corrupted.
This corruption may also lead to invalid memory accesses when the
corrupted entry becomes out of bounds and is used later.

Change the test to fs->clusters + 1 to fix.

Reported-by: Hanno Böck
Signed-off-by: Andreas Bombe <aeb@debian.org>