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

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Date:	Mon, 30 Jan 2012 17:48:49 -0500
From:	Colin Walters <walters@...bum.org>
To:	Will Drewry <wad@...omium.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Andy Lutomirski <luto@...capital.net>,
	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, 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 16:38 -0600, Will Drewry wrote:
> On Mon, Jan 30, 2012 at 4:18 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> > On Mon, 2012-01-30 at 16:58 -0500, Colin Walters 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.
> >>
> >> System daemons that do chroot for a modicum of security already start
> >> privileged, so this doesn't help them either.
> >
> > I thought this was all for sandboxing? If a browers (or user) wants to
> > run some untrusted code, perhaps a chroot is the best way to do so. It
> > just will break if it needs to access /proc or /dev.

I think you'll find your definition of "code" becomes very limited
without /dev/null, /dev/zero and /proc/cpuinfo for example, as used by
glibc.

Personally I find it amazing we're even debating putting new
security-relevant API in the kernel with no known userspace consumer.
It can always go in later if someone actually wants it.

> Interestingly, I believe this change would work for the Chromium
> setuid sandbox[1]. It uses a fancy clone trick (CLONE_FS) to start the
> process then chroot once all its dependencies are loaded. It then
> chroot()s to /proc/self/fd_info (or another empty process-specific
> directory). 

But...it's setuid, so it can call chroot already?  I'm not following how
this change would benefit the helper.


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