linux-kernel - Re: [PATCH v5] cgroup: Add new capability to allow a process to migrate other tasks between cgroups

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Message-ID: <CALAqxLUgmeNxt5B5k720YRM=pUh=a-b2cg+mCM1n1QG-un2ofQ@mail.gmail.com>
Date:   Tue, 13 Dec 2016 09:24:37 -0800
From:   John Stultz <john.stultz@...aro.org>
To:     Casey Schaufler <casey@...aufler-ca.com>
Cc:     Michael Kerrisk <mtk.manpages@...il.com>,
        lkml <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
        Li Zefan <lizefan@...wei.com>,
        Jonathan Corbet <corbet@....net>,
        "open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>,
        Android Kernel Team <kernel-team@...roid.com>,
        Rom Lemarchand <romlem@...roid.com>,
        Colin Cross <ccross@...roid.com>,
        Dmitry Shmidt <dimitrysh@...gle.com>,
        Todd Kjos <tkjos@...gle.com>,
        Christian Poetzsch <christian.potzsch@...tec.com>,
        Amit Pundir <amit.pundir@...aro.org>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Kees Cook <keescook@...omium.org>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Andy Lutomirski <luto@...capital.net>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v5] cgroup: Add new capability to allow a process to
 migrate other tasks between cgroups

On Tue, Dec 13, 2016 at 9:17 AM, Casey Schaufler <casey@...aufler-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@...aufler-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