ext2: Verify bitmap and itable block numbers before using them

Verify bitmap block numbers and inode table blocks are sane before using
them for checking bits in the block bitmap.

CC: stable@vger.kernel.org
Signed-off-by: Jan Kara <jack@suse.cz>

fs/ext2/balloc.c

@@ -77,26 +77,33 @@ static int ext2_valid_block_bitmap(struct super_block *sb,
 	ext2_grpblk_t next_zero_bit;
 	ext2_fsblk_t bitmap_blk;
 	ext2_fsblk_t group_first_block;
+	ext2_grpblk_t max_bit;
 
 	group_first_block = ext2_group_first_block_no(sb, block_group);
+	max_bit = ext2_group_last_block_no(sb, block_group) - group_first_block;
 
 	/* check whether block bitmap block number is set */
 	bitmap_blk = le32_to_cpu(desc->bg_block_bitmap);
 	offset = bitmap_blk - group_first_block;
-	if (!ext2_test_bit(offset, bh->b_data))
+	if (offset < 0 || offset > max_bit ||
+	    !ext2_test_bit(offset, bh->b_data))
 		/* bad block bitmap */
 		goto err_out;
 
 	/* check whether the inode bitmap block number is set */
 	bitmap_blk = le32_to_cpu(desc->bg_inode_bitmap);
 	offset = bitmap_blk - group_first_block;
-	if (!ext2_test_bit(offset, bh->b_data))
+	if (offset < 0 || offset > max_bit ||
+	    !ext2_test_bit(offset, bh->b_data))
 		/* bad block bitmap */
 		goto err_out;
 
 	/* check whether the inode table block number is set */
 	bitmap_blk = le32_to_cpu(desc->bg_inode_table);
 	offset = bitmap_blk - group_first_block;
+	if (offset < 0 || offset > max_bit ||
+	    offset + EXT2_SB(sb)->s_itb_per_group - 1 > max_bit)
+		goto err_out;
 	next_zero_bit = ext2_find_next_zero_bit(bh->b_data,
 				offset + EXT2_SB(sb)->s_itb_per_group,
 				offset);