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 17:41:33 -0500
From:	"Eric W.Biederman" <ebiederm@...ssion.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Casey Schaufler <casey@...aufler-ca.com>
CC:	Josh Triplett <josh@...htriplett.org>,
	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 17, 2014 1:46:59 PM EST, Andy Lutomirski <luto@...capital.net> wrote:
>On Mon, Nov 17, 2014 at 10:31 AM, Andy Lutomirski <luto@...capital.net>
>wrote:
>> On Mon, Nov 17, 2014 at 10:06 AM, Casey Schaufler
>> <casey@...aufler-ca.com> wrote:
>>> On 11/15/2014 1:01 AM, Josh Triplett wrote:
>>>> Currently, unprivileged processes (without CAP_SETGID) cannot call
>>>> setgroups at all.  In particular, processes with a set of
>supplementary
>>>> groups cannot further drop permissions without obtaining elevated
>>>> permissions first.
>>>
>>> Has anyone put any thought into how this will interact with
>>> POSIX ACLs? I don't see that anywhere in the discussion.
>>
>> That means that user namespaces are a problem, too, and we need to
>fix
>> it.  Or we should add some control to turn unprivileged user
>namespace
>> creation on and off and document that turning it on defeats POSIX
>ACLs
>> with a group entry that is more restrictive than the other entry.
>>
>
>This is a significant enough issue that I posted it to oss-security:
>
>http://www.openwall.com/lists/oss-security/2014/11/17/19
>
>It's not at all obvious to me how to fix it.  We could disallow userns
>creation of any supplementary groups don't match fsuid, or we could
>keep negative-only groups around in the userns.
>
>It may be worth adding a sysctl to change the behavior, too.  IMO it's
>absurd to use groups to deny permissions that are otherwise available.

There is an obvious user namespace fix.  Don't allow dropping supplemental groups that are not mapped.

That will require a little bit of fancy footwork if you want to play with supplemental groups in your unprivileged user namespace.   I would like to get a grip on what hoops would be required before we add the additional restriction.  Possibly something as simple as calling sg.

I also want to look at what Tizen and any other concrete pieces of code I can find using this negative permission pattern are actually doing.   Bugs definitely exist, but I have this erie feeling that the bugs may be in instances of userspace using this negative group permission pattern.  I think we may have a hideous case of one setuid binary defeating a privilege check of another piece of code.

This issue looks like it is worth a full scale investigation.  Sigh.

Eric
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