linux-kernel - Re: [Patch] allow file truncations when both suid and write permissions set

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Date:	Wed, 01 Jul 2009 16:15:41 -0400
From:	Eric Paris <eparis@...hat.com>
To:	Amerigo Wang <amwang@...hat.com>
Cc:	Eric Sandeen <sandeen@...hat.com>, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, Eugene Teo <eteo@...hat.com>,
	viro@...iv.linux.org.uk
Subject: Re: [Patch] allow file truncations when both suid and write 
 permissions set

On Wed, 2009-07-01 at 15:29 -0400, Eric Paris wrote:
> On Wed, Jul 1, 2009 at 3:27 PM, Eric Sandeen<sandeen@...hat.com> wrote:
> > Eric Sandeen wrote:
> >> Amerigo Wang wrote:
> >>> When suid is set and the non-owner user has write permission,
> >>> any writing into this file should be allowed and suid should be
> >>> removed after that.
> >>>
> >>> However, current kernel only allows writing without truncations,
> >>> when we do truncations on that file, we get EPERM. This is a bug.
> >
> > ...
> >
> >> So I think the main problem here is simply that we didn't set
> >> ATTR_FORCE, right...
> >>
> >> Seems a little odd to |= with ret, -then- check if it's non-0.  Maybe:
> >>
> >>       /* Remove suid/sgid on truncate too */
> >> -     newattrs.ia_valid |= should_remove_suid(dentry);
> >> +     ret = should_remove_suid(dentry);
> >> +     if (ret)
> >> +             newattrs.ia_valid |= (ret | ATTR_FORCE);
> >>
> >
> > On second thought, and after talking w/ eparis, I think this probably
> > needs a security_inode_killpriv() too... it seems like it might be best
> > to change file_remove_suid(*file) to dentry_remove_suid(*dentry) and
> > just call that from do_truncate()?
> >
> > -Eric
> 
> All of this stuff seems horribly complex....   I'm trying to wrap my
> head around everything going on here as well....

So NAK on both Amerigo's and Eric's patch.  ATTR_FORCE is wrong.
ATTR_FORCE here is going to force the security system to accept ALL of
the attr changes, not just the ATTR_KILL_SUID and/or ATTR_KILL_SGID you
|= in from should_remove_suid.

You need to follow esandeen's recommendation, change file_remove_suid()
to dentry_remove_suid() and then use dentry_remove_suid() in do_truncate
instead of what you are currently trying to do (and I think that's
supposed to be done under the i_mutex right?)

-Eric

-Eric

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