Re: [PATCH] ext4: Verify extent header in ext4_find_extent()

[Eric Biggers]

On Mon, Jan 02, 2023 at 02:58:33PM +0000, Shigeru Yoshida wrote:
> syzbot reported use-after-free in ext4_find_extent() [1].  If there is
> a corrupted file system, this can cause invalid memory access.
> 
> This patch fixes the issue by verifying extent header.
> 
> Reported-by: syzbot+bf4bb7731ef73b83a3b4@xxxxxxxxxxxxxxxxxxxxxxxxx
> Link: https://syzkaller.appspot.com/bug?id=cd95cb722bfa1234ac4c78345c8953ee2e7170d0 [1]
> Signed-off-by: Shigeru Yoshida <syoshida@xxxxxxxxxx>
> ---
>  fs/ext4/extents.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index 9de1c9d1a13d..79bfa583ab1d 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -901,6 +901,9 @@ ext4_find_extent(struct inode *inode, ext4_lblk_t block,
>  		ret = -EFSCORRUPTED;
>  		goto err;
>  	}
> +	ret = ext4_ext_check(inode, eh, depth, 0);
> +	if (ret)
> +		goto err;

This patch probably does not address the root cause of the problem.  Please see
the discussion on the very similar patch
https://lore.kernel.org/linux-ext4/20221230062931.2344157-1-tudor.ambarus@xxxxxxxxxx/T/#u

- Eric