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

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Date:	Sat, 15 Nov 2014 18:35:05 -0800
From:	Josh Triplett <josh@...htriplett.org>
To:	Theodore Ts'o <tytso@....edu>
CC:	Andy Lutomirski <luto@...capital.net>,
	"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 November 15, 2014 6:05:11 PM PST, Theodore Ts'o <tytso@....edu> wrote:
>On Sat, Nov 15, 2014 at 12:20:42PM -0800, Josh Triplett wrote:
>> > However, sudoers seems to allow negative group matches.  So maybe
>> > allowing this only with no_new_privs already set would make sense.
>> 
>> Sigh, bad sudo.  Sure, restricting this to no_new_privs only seems
>fine.
>> I'll do that in v2, and document that in the manpage.
>
>I've also seen use cases (generally back in the bad old days of big
>timesharing VAX 750's :-) where the system admin might assign someone
>to the "games-abusers" group, and then set /usr/games to mode 705
>root:games-abusers --- presumably because it's easier to add a few
>people to the deny list rather than having to add all of the EECS
>department to the games group minus the abusers.
>
>So arbitrarily anyone to drop groups from their supplemental group
>list will result in a change from both existing practice and legacy
>Unix systems, and it could potentially lead to a security exposure.

As Andy pointed out, you can already do that with a user namespace, for any case not involving a setuid or setgid (or otherwise privilege-gaining) program.  And requiring no_new_privs handles that.

Given the combination of those two things, do you still see any problematic cases?

- Josh Triplett

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