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

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Date:	Wed, 25 Feb 2009 23:32:34 +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 Roland McGrath (roland@...hat.com):
> > > > > I can't understand why exit_notify() checks capable(CAP_KILL), but this
> > > > > looks just wrong.
> > > >
> > > > I don't know either why it's there.  My guess is that it was not actually
> > > > thought out specifically, just a "unless capable" exception added when the
> > > > security-motivated exclusions (exec_id stuff) were added.
> > > >
> > > > I can't think of any reason not to drop this check.
> > >
> > > Because of the following test?
> > >
> > > #include <stdio.h>
> > > #include <sched.h>
> > > #include <signal.h>
> > > #include <stdlib.h>
> > >
> > > int childfn(void *data)
> > > {
> > > 	printf("hi there, i'm the child\n");
> > > 	sleep(10);
> > > 	exit(0);
> > > }
> > >
> > > int main()
> > > {
> > > 	int stacksize = 4*getpagesize();
> > > 	void *stack, *stacktop;
> > >
> > > 	stack = malloc(stacksize);
> > > 	stacktop = stack + stacksize;
> > >
> > > 	int p = clone(childfn, stacktop, CLONE_PARENT|SIGSTOP, NULL);
> > > 	exit(0);
> > > }
> >
> > 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.

Oleg.

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