udf: Improve table length check to avoid possible overflow

When a partition table length is corrupted to be close to 1 << 32, the
check for its length may overflow on 32-bit systems and we will think
the length is valid. Later on the kernel can crash trying to read beyond
end of buffer. Fix the check to avoid possible overflow.

CC: stable@vger.kernel.org
Reported-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/udf/super.c

@@ -1340,7 +1340,7 @@ static int udf_load_logicalvol(struct super_block *sb, sector_t block,
 	BUG_ON(ident != TAG_IDENT_LVD);
 	lvd = (struct logicalVolDesc *)bh->b_data;
 	table_len = le32_to_cpu(lvd->mapTableLength);
-	if (sizeof(*lvd) + table_len > sb->s_blocksize) {
+	if (table_len > sb->s_blocksize - sizeof(*lvd)) {
 		udf_err(sb, "error loading logical volume descriptor: "
 			"Partition table too long (%u > %lu)\n", table_len,
 			sb->s_blocksize - sizeof(*lvd));