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

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Date:	Mon, 30 Jan 2012 15:15:02 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Colin Walters <walters@...bum.org>
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, Jan 30, 2012 at 3:10 PM, Colin Walters <walters@...bum.org> wrote:
> On Mon, 2012-01-30 at 14:43 -0800, Andy Lutomirski wrote:
>
>> You don't need a setuid binary.  Just have an initscript set up the bind mounts.
>
> The point is that dchroot is already setuid root, and calls chroot, so
> it gains nothing from the ability to do it unprivileged.
>
> (And wow, I just looked at the source, it's a setuid C++ binary!  Using
> boost.  Ugh...)

Exactly!

You can accomplish the same thing *without a scary setuid binary*.
The use case doesn't even need a new complicated userspace tool.  You
would set up an initscript or some /etc/fstab entries and then:

no_new_privs chroot /var/chroot/ubuntu_oneiric/ /bin/bash

et voila.  (Where no_new_privs would be a really simple tool that does
PR_SET_NO_NEW_PRIVS and then execs its argument.)

Maybe it's just me, but I think this is useful and I would, in fact,
use it in my regular workflow.

--Andy
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