linux-kernel - Re: [PATCH v3 4/4] Allow unprivileged chroot when safe

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Date:	Mon, 30 Jan 2012 17:41:47 -0500
From:	Colin Walters <walters@...bum.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Will Drewry <wad@...omium.org>, linux-kernel@...r.kernel.org,
	Casey Schaufler <casey@...aufler-ca.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jamie Lokier <jamie@...reable.org>, keescook@...omium.org,
	john.johansen@...onical.com, serge.hallyn@...onical.com,
	coreyb@...ux.vnet.ibm.com, pmoore@...hat.com, eparis@...hat.com,
	djm@...drot.org, segoon@...nwall.com, rostedt@...dmis.org,
	jmorris@...ei.org, scarybeasts@...il.com, avi@...hat.com,
	penberg@...helsinki.fi, viro@...iv.linux.org.uk, mingo@...e.hu,
	akpm@...ux-foundation.org, khilman@...com, borislav.petkov@....com,
	amwang@...hat.com, oleg@...hat.com, ak@...ux.intel.com,
	eric.dumazet@...il.com, gregkh@...e.de, dhowells@...hat.com,
	daniel.lezcano@...e.fr, linux-fsdevel@...r.kernel.org,
	linux-security-module@...r.kernel.org, olofj@...omium.org,
	mhalcrow@...gle.com, dlaor@...hat.com, corbet@....net,
	alan@...rguk.ukuu.org.uk
Subject: Re: [PATCH v3 4/4] Allow unprivileged chroot when safe

On Mon, 2012-01-30 at 14:10 -0800, Andy Lutomirski wrote:
> On Mon, Jan 30, 2012 at 1:58 PM, Colin Walters <walters@...bum.org> wrote:
> > On Mon, 2012-01-30 at 08:17 -0800, Andy Lutomirski wrote:
> >> Chroot can easily be used to subvert setuid programs.  If no_new_privs,
> >> then setuid programs don't gain any privilege, so allow chroot.
> >
> > Is this needed/desired by anyone now, or are you just using it to "demo"
> > NO_NEW_PRIVS?  I don't see it as very useful on its own, since in any
> > "container"-type chroot you really want /proc and /dev, and your patch
> > doesn't help with that.
> 
> It's a demo, but it could still be useful for container-ish things.
> If something privileged sets up /proc, /sys, and /dev, then
> unprivileged code can chroot into the container.  This would allow
> much simpler implementations of tools like schroot.

What's the win if you still need a setuid binary?  schroot (and my
linux-user chroot binary) can just as easily call chroot as they can
create bind mounts; I'm not buying a code complexity argument.

> With this change, they wouldn't need to start privileged.
> (Admittedly, this isn't a great argument for this patch.)

Right...I'm not aware of anyone who would find that useful.


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