ext4: add sanity checking to count_overhead()

The commit "ext4: sanity check the block and cluster size at mount
time" should prevent any problems, but in case the superblock is
modified while the file system is mounted, add an extra safety check
to make sure we won't overrun the allocated buffer.
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org

fs/ext4/super.c

@@ -3195,10 +3195,15 @@ static int count_overhead(struct super_block *sb, ext4_group_t grp,
 			ext4_set_bit(s++, buf);
 			count++;
 		}
-		for (j = ext4_bg_num_gdb(sb, grp); j > 0; j--) {
+		j = ext4_bg_num_gdb(sb, grp);
+		if (s + j > EXT4_BLOCKS_PER_GROUP(sb)) {
+			ext4_error(sb, "Invalid number of block group "
+				   "descriptor blocks: %d", j);
+			j = EXT4_BLOCKS_PER_GROUP(sb) - s;
+		}
+		count += j;
+		for (; j > 0; j--)
 			ext4_set_bit(EXT4_B2C(sbi, s++), buf);
-			count++;
-		}
 	}
 	if (!count)
 		return 0;