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

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Date:	Mon, 30 Jan 2012 16:38:28 -0600
From:	Will Drewry <wad@...omium.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Colin Walters <walters@...bum.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, 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. And perhaps we
> don't want untrusted code accessing /proc and /dev.

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).  Of course, pid namespacing still wouldn't be there, but
it'd be nice to have a fallback if someone doesn't want the sandboxing
setup code to have privileges (or can only install unpriv'd code).

cheers!
will
1 - http://code.google.com/p/setuid-sandbox/source/browse/privdrop.c
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