linux-kernel - Re: [PATCH 2/2] exit_notify: kill the wrong capable(CAP

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Date:	Thu, 26 Feb 2009 00:16:45 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Serge E. Hallyn" <serge@...lyn.com>
Cc:	Roland McGrath <roland@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Chris Evans <scarybeasts@...il.com>,
	David Howells <dhowells@...hat.com>,
	Don Howard <dhoward@...hat.com>,
	Eugene Teo <eugene@...hat.com>,
	Michael Kerrisk <mtk.manpages@...glemail.com>,
	Tavis Ormandy <taviso@....lonestar.org>,
	Vitaly Mayatskikh <vmayatsk@...hat.com>, stable@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] exit_notify: kill the wrong capable(CAP_KILL) check

On 02/25, Serge E. Hallyn wrote:
>
> Quoting Oleg Nesterov (oleg@...hat.com):
> > On 02/25, Serge E. Hallyn wrote:
> > >
> > > Quoting Oleg Nesterov (oleg@...hat.com):
> > > > On 02/25, Serge E. Hallyn wrote:
> > > > Can't understand... Why do you think CAP_KILL makes things better?
> > > >
> > > > Actually, how can it make any difference in this case?
> > >
> > > Well the check by itself isn't quite right - it seems to me it
> > > should also check whether tsk->euid == parent->uid.  But letting
> > > an unprivileged task send SIGSTOP to a privileged one bc of
> > > some fluke in the task hierarchy doesn't seem right.
> >
> > I think you misread this CAP_KILL check.
> >
> > It does not restrict the unprivileged task to send the signal. Instead,
> > if the exiting task has CAP_KILL, we bypass other security checks.
>
> ?  If the exiting task does not have CAP_KILL,

_and_ (not "or") the execution domains for parent/chils are different,

> we set the signal to
> SIGCHILD (which is deemed safe).

Yes. So why we should not set the signal to SIGCHLD if the task has
CAP_KILL ?

And again, the malicious application can exec the setuid binary before
exit, in this case we never reset ->exit_signal (of course, unless
that binary drops CAP_KILL).

Oleg.

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