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Date: Thu, 26 Feb 2009 00:16:45 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: "Serge E. Hallyn" <serge@hallyn.com>
Cc: Roland McGrath <roland@redhat.com>,
       Andrew Morton <akpm@linux-foundation.org>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Alan Cox <alan@lxorguk.ukuu.org.uk>,
       Chris Evans <scarybeasts@gmail.com>,
       David Howells <dhowells@redhat.com>, Don Howard <dhoward@redhat.com>,
       Eugene Teo <eugene@redhat.com>,
       Michael Kerrisk <mtk.manpages@googlemail.com>,
       Tavis Ormandy <taviso@sdf.lonestar.org>,
       Vitaly Mayatskikh <vmayatsk@redhat.com>, stable@kernel.org,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] exit_notify: kill the wrong capable(CAP_KILL) check
Message-ID: <20090225231645.GA17868@redhat.com>
References: <20090225190218.GA7453@redhat.com> <20090225194140.D6F6EFC3DA@magilla.sf.frob.com> <20090225215356.GA1442@hallyn.com> <20090225220324.GA14667@redhat.com> <20090225221415.GA1751@hallyn.com> <20090225223234.GA16156@redhat.com> <20090225224751.GA2062@hallyn.com>
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On 02/25, Serge E. Hallyn wrote:
>
> Quoting Oleg Nesterov (oleg@redhat.com):
> > On 02/25, Serge E. Hallyn wrote:
> > >
> > > Quoting Oleg Nesterov (oleg@redhat.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.