From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S934912AbcLMRcv (ORCPT <rfc822;w@1wt.eu>);
        Tue, 13 Dec 2016 12:32:51 -0500
Received: from mail-oi0-f52.google.com ([209.85.218.52]:32784 "EHLO
        mail-oi0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S934776AbcLMRcs (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Tue, 13 Dec 2016 12:32:48 -0500
MIME-Version: 1.0
In-Reply-To: <f7ccb39f-bad8-4a78-c796-90927b839494@schaufler-ca.com>
References: <1481593143-18756-1-git-send-email-john.stultz@linaro.org>
 <CAKgNAkhGSYmtqLnCyXH8-dbmBJreo9mbVGRMyd_9hZ5D1F7q4g@mail.gmail.com>
 <221e80bd-3d99-6c35-dcd3-b2547f0abb11@schaufler-ca.com> <CALAqxLWaJT=a1Cc2Ja8ZuazjJYi3a2WW9ATB0NFP8670Dyq6bg@mail.gmail.com>
 <f7ccb39f-bad8-4a78-c796-90927b839494@schaufler-ca.com>
From: John Stultz <john.stultz@linaro.org>
Date: Tue, 13 Dec 2016 09:24:37 -0800
Message-ID: <CALAqxLUgmeNxt5B5k720YRM=pUh=a-b2cg+mCM1n1QG-un2ofQ@mail.gmail.com>
Subject: Re: [PATCH v5] cgroup: Add new capability to allow a process to
 migrate other tasks between cgroups
To: Casey Schaufler <casey@schaufler-ca.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>,
        lkml <linux-kernel@vger.kernel.org>, Tejun Heo <tj@kernel.org>,
        Li Zefan <lizefan@huawei.com>, Jonathan Corbet <corbet@lwn.net>,
        "open list:CONTROL GROUP (CGROUP)" <cgroups@vger.kernel.org>,
        Android Kernel Team <kernel-team@android.com>,
        Rom Lemarchand <romlem@android.com>, Colin Cross <ccross@android.com>,
        Dmitry Shmidt <dimitrysh@google.com>, Todd Kjos <tkjos@google.com>,
        Christian Poetzsch <christian.potzsch@imgtec.com>,
        Amit Pundir <amit.pundir@linaro.org>,
        Dmitry Torokhov <dmitry.torokhov@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        "Serge E . Hallyn" <serge@hallyn.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Linux API <linux-api@vger.kernel.org>
Content-Type: text/plain; charset=UTF-8
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Tue, Dec 13, 2016 at 9:17 AM, Casey Schaufler <casey@schaufler-ca.com> wrote:
> On 12/13/2016 8:49 AM, John Stultz wrote:
>> On Tue, Dec 13, 2016 at 8:39 AM, Casey Schaufler <casey@schaufler-ca.com> wrote:
>>> On 12/13/2016 1:47 AM, Michael Kerrisk (man-pages) wrote:
>>>> How about CAP_CGROUP_CONTROL or some such, with the idea that this
>>>> might be a capability that allows the holder to step outside usual
>>>> cgroup rules? At the moment, that capability would allow only one such
>>>> step, but maybe there would be others in the future.
>>> I agree, but want to put it more strongly. The granularity of
>>> capabilities can never be fine enough for some people, and this
>>> is an example of a case where you're going a bit too far. If the
>>> use case is Android as you say, you don't need this. As my friends
>>> on the far side of the aisle would say, "just write SELinux policy"
>>> to correctly control access as required.
>> So.. The trouble is that while selinux is good for restricting
>> permissions, the in-kernel permission checks here are already too
>> restrictive.
>
> Why did the original authors of cgroups make it that restrictive?
> If there isn't a good reason, loosen it up. If there is a good
> reason, then pay heed to it.

That's what this patch is proposing. And I agree with Michael that the
newly proposed cap was a bit to narrowly focused on my immediate use
case, and broadening it to CGROUP_CONTROL is smart. Then that
capability could be further restricted w/ selinux policy, as you
suggest.

thanks
-john