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

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Date:   Tue, 13 Dec 2016 08:49:03 -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 8:39 AM, Casey Schaufler <casey@...aufler-ca.com> wrote:
> On 12/13/2016 1:47 AM, Michael Kerrisk (man-pages) wrote:
>> Hi John,
>>
>> On 13 December 2016 at 02:39, John Stultz <john.stultz@...aro.org> wrote:
>>> This patch adds CAP_GROUP_MIGRATE and logic to allows a process
>> s/CAP_GROUP_MIGRATE/CAP_CGROUP_MIGRATE/
>>
>>> to migrate other tasks between cgroups.
>>>
>>> In Android (where this feature originated), the ActivityManager
>>> tracks various application states (TOP_APP, FOREGROUND,
>>> BACKGROUND, SYSTEM, etc), and then as applications change
>>> states, the SchedPolicy logic will migrate the application tasks
>>> between different cgroups used to control the different
>>> application states (for example, there is a background cpuset
>>> cgroup which can limit background tasks to stay on one low-power
>>> cpu, and the bg_non_interactive cpuctrl cgroup can then further
>>> limit those background tasks to a small percentage of that one
>>> cpu's cpu time).
>>>
>>> However, for security reasons, Android doesn't want to make the
>>> system_server (the process that runs the ActivityManager and
>>> SchedPolicy logic), run as root. So in the Android common.git
>>> kernel, they have some logic to allow cgroups to loosen their
>>> permissions so CAP_SYS_NICE tasks can migrate other tasks between
>>> cgroups.
>>>
>>> I feel the approach taken there overloads CAP_SYS_NICE a bit much
>>> for non-android environments. Efforts to re-use CAP_SYS_RESOURCE
>>> for this purpose (which Android has since adopted) was also
>>> stymied by concerns about risks from future cgroups that could be
>>> considered "dangerous" by how they might change system semantics.
>>>
>>> So to avoid overlapping usage, this patch adds a brand new
>>> process capability flag (CAP_CGROUP_MIGRATE), and uses it when
>>> checking if a task can migrate other tasks between cgroups.
>>>
>>> I've tested this with AOSP master (though its a bit hacked in as
>>> I still need to properly get the selinux bits aware of the new
>>> capability bit) with selinux set to permissive and it seems to be
>>> working well.
>>>
>>> Thoughts and feedback would be appreciated!
>> So, back to the discussion of silos. I understand the argument for
>> wanting a new silo. But, in that case can we at least try not to make
>> it a single-use silo?
>>
>> 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. It seems one must first loosen things up before we can
tighten it with selinux rules. Or are you suggesting the system_server
run as root + further selinux limitations? I worry, the Android
developers may still be hesitant to do that.

thanks
-john