Re: [PATCH] ext4: fix bitmap position validation

[Lukas Czerner <lczerner@redhat.com>]

On Tue, Apr 24, 2018 at 11:43:30AM -0400, Theodore Y. Ts'o wrote:
> On Tue, Apr 24, 2018 at 12:56:54PM +0200, Lukas Czerner wrote:
> > @@ -354,8 +356,8 @@ 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 + sbi->s_itb_per_group) >=
> > +				   EXT4_CLUSTERS_PER_GROUP(sb))
> >  		return blk;
> >  	next_zero_bit = ext4_find_next_zero_bit(bh->b_data,
> >  			EXT4_B2C(sbi, offset + sbi->s_itb_per_group),
> 
> The two checks of offset and offset + sbi->s_itb_per_group are
> necessary because a maliciously crafted file system can take advantage
> of unsigned integer overflow such that offset is a very large number,
> but offset + sbi->s_itb_per_group is a legal offset.  So we have ot
> keep both checks.  As it happens I was working on a similar patch (but
> was slowed down by my attendance at LSF/Mm).  So I've combined your
> patch with mine, and came up with this.

Hi Ted,

maybe I am missing something but offset is signed int, but
s_itb_per_group is unsigned long, so if I recall the arithmetic
conversions correctly the offset will be converted to unsigned long and
the restult will be unsigned long.
Moreover s_itb_per_group value is sanitized when read and can't be very
big so the restult will always fit into unsigned long hence no overflow
is possible. Anyway that was my thought process when I removed the
additional check.

However it we have a maliciously created fs then

blk = ext4_inode_table(sb, desc);

might be either too big or too small and so

offset = blk - group_first_block;

might over/underflow giving us wrong offset that still by chance can land
into the block group. So maybe we need to check that blk is within the file
system and/or that offset does not overflow. Maybe making offset type
ext4_fsblk_t and checking that the result satisfies (offset <= blk) will
be enough ?

Have fun at LSF/MM :)

-Lukas

> 
> 					- Ted
> 
> From 33444e3f7da8ae9840286732c0d7bbf8f9d8471b Mon Sep 17 00:00:00 2001
> From: Lukas Czerner <lczerner@redhat.com>
> Date: Tue, 24 Apr 2018 11:31:44 -0400
> Subject: [PATCH] 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: 7dac4a1726a9 ("ext4: add validity checks for bitmap block numbers")
> Cc: stable@vger.kernel.org
> ---
>  fs/ext4/balloc.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
> index a33d8fb1bf2a..508b905d744d 100644
> --- a/fs/ext4/balloc.c
> +++ b/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),
> -- 
> 2.16.1.72.g5be1f00a9a
>