linux-kernel - Re: [PATCH 2/2] groups: Allow unprivileged processes to use setgroups to drop groups

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Date:	Mon, 17 Nov 2014 14:37:30 -0800
From:	josh@...htriplett.org
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"Eric W.Biederman" <ebiederm@...ssion.com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	linux-man <linux-man@...r.kernel.org>, Ted Ts'o <tytso@....edu>,
	Michael Kerrisk-manpages <mtk.manpages@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux API <linux-api@...r.kernel.org>,
	Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 2/2] groups: Allow unprivileged processes to use
 setgroups to drop groups

On Mon, Nov 17, 2014 at 02:22:59PM -0800, Andy Lutomirski wrote:
> On Mon, Nov 17, 2014 at 2:11 PM, Eric W.Biederman <ebiederm@...ssion.com> wrote:
> >
> >
> > On November 17, 2014 1:07:30 PM EST, Andy Lutomirski <luto@...capital.net> wrote:
> >>On Nov 17, 2014 3:37 AM, "One Thousand Gnomes"
> >><gnomes@...rguk.ukuu.org.uk> wrote:
> >>>
> >>> > optional), I can do that too.  The security model of "having a
> >>group
> >>> > gives you less privilege than not having it" seems crazy, but
> >>> > nonetheless I can see a couple of easy ways that we can avoid
> >>breaking
> >>>
> >>> It's an old pattern of use that makes complete sense in a traditional
> >>> Unix permission world because it's the only way to do "exclude
> >>{list}"
> >>> nicely. Our default IMHO shouldn't break this.
> >>>
> >>> > that pattern, no_new_privs being one of them.  I'd like to make
> >>sure
> >>> > that nobody sees any other real-world corner case that unprivileged
> >>> > setgroups would break.
> >>>
> >>> Barring the usual risk of people doing improper error checking I
> >>don't
> >>> see one immediately.
> >>>
> >>> For containers I think it actually makes sense that the sysctl can be
> >>> applied per container anyway.
> >>
> >>We'll probably need per container sysctls some day.
> >
> > We already have a mess of per network namespace sysctls,
> > as well as few for other namespaces.
> >
> > We have the infrastructure it is just a matter of using it for whatever purpose we need.
> >
> 
> A list of group id ranges that it's permissible to drop would do the
> trick, both for setgroups and for unshare.  The downside would be that
> users in those groups (i.e. everyone by default) would not be able to
> unshare their user ns.
> 
> Better ideas welcome.

Personally, I think that seems like more flexibility than necessary to
achieve the goal.  I think a sysctl turning group-dropping on and off
would suffice; systems that know they don't use groups to exclude
specific users can enable that sysctl.

- Josh Triplett
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