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

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Message-ID: <20141116191219.GD5032@thin>
Date:	Sun, 16 Nov 2014 11:12:19 -0800
From:	Josh Triplett <josh@...htriplett.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Theodore Ts'o <tytso@....edu>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Michael Kerrisk-manpages <mtk.manpages@...il.com>,
	Linux API <linux-api@...r.kernel.org>,
	linux-man <linux-man@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] groups: Allow unprivileged processes to use
 setgroups to drop groups

On Sun, Nov 16, 2014 at 07:42:30AM -0800, Andy Lutomirski wrote:
> On Sun, Nov 16, 2014 at 5:32 AM, Theodore Ts'o <tytso@....edu> wrote:
> > On Sat, Nov 15, 2014 at 09:08:07PM -0600, Eric W. Biederman wrote:
> >>
> >> That may be a bug with the user namespace permission check.  Perhaps we
> >> shouldn't allow dropping groups that aren't mapped in the user
> >> namespace.
> >
> > I'm not saying that we can't change the behavior of whether or not a
> > user can drop a group permission.  I'm just saying that we need to do
> > so consciously.  The setgroups()/getgroups() ABI isn't part of
> > POSIX/SuSv3 so we wouldn't be breaking POSIX compatibility, for those
> > people who care about that.
> 
> It may make sense to reach out to some place like oss-security.
> 
> FWIW, I think we should ask, at the same time, about:
> 
>  - Dropping supplementary groups.
>  - Switching gid/egid/sgid to a supplementary group.
>  - Denying ptrace of a process with supplementary groups that the
> tracer doesn't have.

I wonder how crazy it would be to just require either CAP_SYS_PTRACE or
cred1 == cred2 (as in, you have *exactly* the same credentials as the
target)?

> Also, I much prefer a sysctl to a boot option.  Boot options are nasty
> to configure in many distributions.

Agreed.

- Josh Triplett
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