linux-kernel - Re: [PATCH] fs: don't let getdents return bogus names

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Date:   Thu, 19 Jul 2018 10:50:00 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     Jann Horn <jannh@...gle.com>, Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        linux-fsdevel@...r.kernel.org,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        linux-alpha@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: don't let getdents return bogus names

On Mon, Jul 16, 2018 at 08:56:57PM +0100, Al Viro wrote:
> On Mon, Jul 16, 2018 at 09:48:43PM +0200, Jann Horn wrote:
> > When you e.g. run `find` on a directory for which getdents returns
> > "filenames" that contain slashes, `find` passes those "filenames" back to
> > the kernel, which then interprets them as paths. That could conceivably
> > cause userspace to do something bad when accessing something like an
> > untrusted USB stick, but I'm not aware of any specific example.
> > 
> > Instead of returning bogus filenames to userspace, return -EUCLEAN.
> 
> Because there's such a lot of userland code that expect and handles that
> error value...

We've been using EUCLEAN ("structure needs cleaning") for indicating
filesystem corruption errors for many years now. e.g.

fs/ext2/ext2.h:#define  EFSCORRUPTED                    EUCLEAN /* Filesystem is corrupted */
fs/ext4/ext4.h:#define EFSCORRUPTED     EUCLEAN         /* Filesystem is corrupted */
fs/hpfs/hpfs_fn.h:#define EFSERROR  EUCLEAN
fs/xfs/xfs_linux.h:#define EFSCORRUPTED EUCLEAN         /* Filesystem is corrupted */
include/linux/jbd2.h:#define EFSCORRUPTED       EUCLEAN         /* Filesystem is corrupted */

There's hundreds of places in the filesystem code that this
specific error is returned to userspace - there's more than 500
individual places this error can be returned from just XFS....

To me it seems like the right error to return if a dirent is
corrupted, because that's exactly what XFS will return if any of the
directory structure around the dirent name itself is corrupt...

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com