linux-kernel - Re: Posix file capabilities in 2.6.24rc2; now 2.6.24-rc3

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Message-ID: <20071120225121.GA24606@sergelap.austin.ibm.com>
Date:	Tue, 20 Nov 2007 16:51:21 -0600
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	Chris Friedhoff <chris@...edhoff.org>
Cc:	"Serge E. Hallyn" <serue@...ibm.com>,
	linux-security-module@...r.kernel.org,
	Stephen Smalley <sds@...ch.ncsc.mil>,
	Andrew Morgan <morgan@...nel.org>,
	Chris Wright <chrisw@...s-sol.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Posix file capabilities in 2.6.24rc2; now 2.6.24-rc3

Quoting Chris Friedhoff (chris@...edhoff.org):
> On Tue, 20 Nov 2007 08:51:06 -0600
> "Serge E. Hallyn" <serue@...ibm.com> wrote:
> 
> > Quoting Chris Friedhoff (chris@...edhoff.org):
> > > On Mon, 19 Nov 2007 17:16:44 -0600
> > > "Serge E. Hallyn" <sergeh@...ibm.com> wrote:
> > > 
> > > > Quoting Chris Friedhoff (chris@...edhoff.org):
> > > > > Hello Serge,
> > > > > 
> > > > > just to let you know: with 2.6.24-rc3 I have the same problem.
> > > > 
> > > > Ok, so here is the flow.
> > > > 
> > > > First off, using runlevel 5 on FC7, using 'log out' correctly brings
> > > > you back to a new login prompt.  Your problem is starting in runlevel
> > > > 3, and typing 'xinit .xinitrc';  when you exit your wm, xinit is not
> > > > allowed to kill X so you don't get back to your console.
> > > 
> > > Yes, I'm booting in a runlevel without a session manager and starting
> > > my X session with xinit.
> > > (slackware: console->runlevel 3; sessionmanager->runlevel 4 )
> > > 
> > > > 
> > > > First comment is, as you point out on your homepage, you could
> > > > 	setfcaps -c cap_kill+p -e /usr/bin/xinit
> > > > Then xinit is allowed to kill X.  Yes xinit forks and execs a
> > > > user-writable script, but of course upon the exec to start the script
> > > > cap_kill is lost, so the user can't abuse this.
> > > > 
> > > > Since you pointed this out on your homepage, I have to assume you've
> > > > decided you don't want to give cap_kill to xinit?
> > > 
> > > No, since I'm using capabilities and I'm very happy with it, I grant
> > > cap_kill to xinit. For myself the problem is solved ...
> > > 
> > > > 
> > > > My other question is - do we want to maintain this signal restriction?
> > > > So long as a privileged process isn't dumpable, is it any more dangerous
> > > > for user hallyn to kill capability-raised process owned by hallyn than
> > > > it is to kill a setuid process started by hallyn?  If we decide no, then
> > > > maybe we should remove cap_task_kill() as well as the cap_task_setnice(),
> > > > cap_task_setioprio(), cap_task_setscheduler()?
> > > > 
> > > > Or maybe i've just forgotten a compelling scenario...
> > > > 
> > > > thanks,
> > > > -serge
> > > 
> > > 
> > > ... but if some user decides to configure capabilities into the 2.6.24
> > > kernel or just uses such a kernel and
> > > 1) is not granting cap_kill to xinit, and
> > > 2) starts X by issuing xinit on the console
> > > 3) ends after some time his X session, to come back to the console
> > > 
> > > he will see a different behavior compared to 2.6.23 exiting his X
> > > session and (I think) believes to have a bug in the X package.
> > > 
> > > Andrew Morton describes the problem here, too:
> > > http://lkml.org/lkml/2006/11/23/15
> > > http://lkml.org/lkml/2006/11/23/19
> > > 
> > > Am I wrong in the assumption, but should one not accept an unchanged
> > > behavior with or without capabilities in the kernel regarding the
> > > behavior of applications, when he is not actually using (by not setting
> > > the xattr capability) capabilities with this application?
> > > 
> > > If I'm wrong, maybe a warning or hint should be given that one has to
> > > grant cap_kill to xinit to come back to the console if the X session
> > > was started by xinit.
> > 
> > Thanks - yes, I see (I tend to get lost in my testruns).  So we're back to
> > trying to do the fix I was trying to do along with the SIGCONT fix a few
> > weeks ago.
> > 
> > The problem is that when you run a setuid binary, its pP and pE are
> > fully raised.  The following patch fixes it for me.  Chris, does it fix
> > your problem?
> 
> Yes, this patch fixes it for me, too.
> 
> Thanks,
> 
> Chris

Thanks for the valuable testing and bugreports, Chris.

If I don't hear any counter-arguments I'll post the patch in its own
thread tomorrow.

-serge
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