Re: [PATCH 7/9] xfs: kill suid/sgid through the truncate path.

Ben Myers
2013-05-30T17:07:52Z

On Thu, May 30, 2013 at 12:02:40PM -0400, Brian Foster wrote:
> On 05/30/2013 11:52 AM, Ben Myers wrote:
> > Hey Brian,
> >
> > On Thu, May 30, 2013 at 10:17:30AM -0400, Brian Foster wrote:
> >> On 05/27/2013 02:38 AM, Dave Chinner wrote:
> >>> From: Dave Chinner <dchinner@redhat.com>
> >>>
> >>> XFS has failed to kill suid/sgid bits correctly when truncating
> >>> files of non-zero size since commit c4ed4243 ("xfs: split
> >>> xfs_setattr") introduced in the 3.1 kernel. Fix it.
> >>>
> >>
> >> The code makes sense and I can easily hit an assert when truncating
> >> (extending) a suid file on a debug kernel without this patch (and I see
> >> the suid dropped with the patch).
> >
> > What commands did you use? It seems like this is dealing with S_ISGID, correct?
> >
>
> Hi Ben,
>
> Yeah, that confused me at first as well. I believe the vfs interprets
> the ATTR_KILL_SUID/SGIT bits prior to the setattr call and wipes out the
> associated mode bits if necessary.
>
> What I did was basically create a zero sized file as root, chmod to a+s
> and a+rwx and then as a regular user, truncate that file to something
> larger than zero. Without the patch I hit the assert and with the patch
> the assert doesn't fire and the setuid bit is dropped.

Perfect, thanks. I'm able to hit this on 3.10-rc3, but not 3.10-rc1.
Interesting.

-Ben