Re: [PATCH v2] ext4: Fix possible use-after-free in ext4_find_extent

[Eric Biggers]

On Fri, Dec 30, 2022 at 08:29:31AM +0200, Tudor Ambarus wrote:
> syzbot reported a use-after-free Read in ext4_find_extent that is hit when
> using a corrupted file system. The bug was reported on Android 5.15, but
> using the same reproducer triggers the bug on v6.2-rc1 as well.
> 
> Fix the use-after-free by checking the extent header magic. An alternative
> would be to check the values of EXT4_{FIRST,LAST}_{EXTENT,INDEX} used in
> ext4_ext_binsearch() and ext4_ext_binsearch_idx(), so that we make sure
> that pointers returned by EXT4_{FIRST,LAST}_{EXTENT,INDEX} don't exceed the
> bounds of the extent tree node. But this alternative will not squash
> the bug for the cases where eh->eh_entries fit into eh->eh_max. We could
> also try to check the sanity of the path, but costs more than checking just
> the header magic, so stick to the header magic sanity check.
> 
> Link: https://syzkaller.appspot.com/bug?id=be6e90ce70987950e6deb3bac8418344ca8b96cd
> Reported-by: syzbot+0827b4b52b5ebf65f219@xxxxxxxxxxxxxxxxxxxxxxxxx
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Tudor Ambarus <tudor.ambarus@xxxxxxxxxx>
> ---
> v2: drop wrong/uneeded le16_to_cpu() conversion for eh->eh_magic
> 
>  fs/ext4/extents.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index 9de1c9d1a13d..bedc8c098449 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -894,6 +894,12 @@ ext4_find_extent(struct inode *inode, ext4_lblk_t block,
>  		gfp_flags |= __GFP_NOFAIL;
>  
>  	eh = ext_inode_hdr(inode);
> +	if (eh->eh_magic != EXT4_EXT_MAGIC) {
> +		EXT4_ERROR_INODE(inode, "Extent header has invalid magic.");
> +		ret = -EFSCORRUPTED;
> +		goto err;
> +	}
> +

This is (incompletely) validating the extent header in the inode.  Isn't that
supposed to happen when the inode is loaded?  See how __ext4_iget() calls
ext4_ext_check_inode().  Why isn't that working here?

- Eric