Re: [PATCH] ext4: verify dirent offset in ext4_readdir()

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Jul 01, 2025 at 05:11:41PM +0300, Dmitry Antipov wrote:
> On a corrupted filesystem, an unexpectedly large invalid value
> returned by 'ext4_rec_len_from_disk()' may cause 'ext4_readdir()'
> to read the next dirent from an area beyond the corresponding
> buffer head's data. At this point, an exact length of the dirent
> is not known yet but it's possible to check whether the shortest
> possible dirent will be read from within the bh's data at least.
> 
> Reported-by: syzbot+5322c5c260eb44d209ed@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=5322c5c260eb44d209ed
> Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
> ---
>  fs/ext4/dir.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/fs/ext4/dir.c b/fs/ext4/dir.c
> index d4164c507a90..8097016f69aa 100644
> --- a/fs/ext4/dir.c
> +++ b/fs/ext4/dir.c
> @@ -258,6 +258,12 @@ static int ext4_readdir(struct file *file, struct dir_context *ctx)
>  
>  		while (ctx->pos < inode->i_size
>  		       && offset < sb->s_blocksize) {
> +			/* Ensure that at least the shortest possible
> +			 * dirent will be read from within the bh's data.
> +			 */
> +			if (offset + offsetof(struct ext4_dir_entry_2, name)
> +			    > bh->b_size)
> +				break;

Why wouldn't you encode this check in __ext4_check_dir_entry and solve
this problem for all the callsites?

--D

>  			de = (struct ext4_dir_entry_2 *) (bh->b_data + offset);
>  			if (ext4_check_dir_entry(inode, file, de, bh,
>  						 bh->b_data, bh->b_size,
> -- 
> 2.50.0
> 
>