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

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Date:	Sun, 15 Jan 2012 17:08:38 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Casey Schaufler <casey@...aufler-ca.com>,
	Jamie Lokier <jamie@...reable.org>,
	Will Drewry <wad@...omium.org>, linux-kernel@...r.kernel.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 4/4] Allow unprivileged chroot when safe

On Sun, Jan 15, 2012 at 4:45 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Sun, Jan 15, 2012 at 4:37 PM, Andy Lutomirski <luto@...capital.net> 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.
>>
>> Because chroot is an easy way to break out of chroot jail, CAP_SYS_ADMIN
>> is still required if the caller is already chrooted.
>
> So I think this whole chroot thing needs more people looking at it. I
> brought up chroot as an example, but there may be other reasons why
> you don't want user chrooting things than just the setuid confusion.

Agreed.  There are plenty of security people cc'd.  Thoughts (and
attacks) are welcome!

>
> There's also the whole issue with doing things like local non-root
> bind mounts, which are arguably more useful than chroot, and which are
> disallowed for similar reasons. So I don't think chroot is all that
> special.

They're almost certainly more useful.  Binding the tree of your choice
on top of / is a nice (and more secure) way to emulate chroot.  The
only downside I've thought of in five minutes is that it would prevent
the administrator from blocking access to a directory by bind-mounting
something on to of it -- an unprivileged non-recursive bind mount of
the containing filesystem would get the hidden directory back.  I'm
not sure this is a real problem.

--Andy
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