ext4: fix bitmap position validation

Currently in ext4_valid_block_bitmap() we expect the bitmap to be
positioned anywhere between 0 and s_blocksize clusters, but that's
wrong because the bitmap can be placed anywhere in the block group. This
causes false positives when validating bitmaps on perfectly valid file
system layouts. Fix it by checking whether the bitmap is within the group
boundary.

The problem can be reproduced using the following

mkfs -t ext3 -E stride=256 /dev/vdb1
mount /dev/vdb1 /mnt/test
cd /mnt/test
wget https://cdn.kernel.org/pub/linux/kernel/v4.x/linux-4.16.3.tar.xz
tar xf linux-4.16.3.tar.xz

This will result in the warnings in the logs

EXT4-fs error (device vdb1): ext4_validate_block_bitmap:399: comm tar: bg 84: block 2774529: invalid block bitmap

[ Changed slightly for clarity and to not drop a overflow test -- TYT ]
Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reported-by: Ilya Dryomov <idryomov@gmail.com>
Fixes: 7dac4a17 ("ext4: add validity checks for bitmap block numbers")
Cc: stable@vger.kernel.org

fs/ext4/balloc.c

@@ -321,6 +321,7 @@ static ext4_fsblk_t ext4_valid_block_bitmap(struct super_block *sb,
 	struct ext4_sb_info *sbi = EXT4_SB(sb);
 	ext4_grpblk_t offset;
 	ext4_grpblk_t next_zero_bit;
+	ext4_grpblk_t max_bit = EXT4_CLUSTERS_PER_GROUP(sb);
 	ext4_fsblk_t blk;
 	ext4_fsblk_t group_first_block;
 
@@ -338,7 +339,7 @@ static ext4_fsblk_t ext4_valid_block_bitmap(struct super_block *sb,
 	/* check whether block bitmap block number is set */
 	blk = ext4_block_bitmap(sb, desc);
 	offset = blk - group_first_block;
-	if (offset < 0 || EXT4_B2C(sbi, offset) >= sb->s_blocksize ||
+	if (offset < 0 || EXT4_B2C(sbi, offset) >= max_bit ||
 	    !ext4_test_bit(EXT4_B2C(sbi, offset), bh->b_data))
 		/* bad block bitmap */
 		return blk;
@@ -346,7 +347,7 @@ static ext4_fsblk_t ext4_valid_block_bitmap(struct super_block *sb,
 	/* check whether the inode bitmap block number is set */
 	blk = ext4_inode_bitmap(sb, desc);
 	offset = blk - group_first_block;
-	if (offset < 0 || EXT4_B2C(sbi, offset) >= sb->s_blocksize ||
+	if (offset < 0 || EXT4_B2C(sbi, offset) >= max_bit ||
 	    !ext4_test_bit(EXT4_B2C(sbi, offset), bh->b_data))
 		/* bad block bitmap */
 		return blk;
@@ -354,8 +355,8 @@ static ext4_fsblk_t ext4_valid_block_bitmap(struct super_block *sb,
 	/* check whether the inode table block number is set */
 	blk = ext4_inode_table(sb, desc);
 	offset = blk - group_first_block;
-	if (offset < 0 || EXT4_B2C(sbi, offset) >= sb->s_blocksize ||
-	    EXT4_B2C(sbi, offset + sbi->s_itb_per_group) >= sb->s_blocksize)
+	if (offset < 0 || EXT4_B2C(sbi, offset) >= max_bit ||
+	    EXT4_B2C(sbi, offset + sbi->s_itb_per_group) >= max_bit)
 		return blk;
 	next_zero_bit = ext4_find_next_zero_bit(bh->b_data,
 			EXT4_B2C(sbi, offset + sbi->s_itb_per_group),