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

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Date:   Mon, 12 Dec 2016 17:40:50 -0800
From:   John Stultz <john.stultz@...aro.org>
To:     lkml <linux-kernel@...r.kernel.org>
Cc:     John Stultz <john.stultz@...aro.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 Mon, Dec 12, 2016 at 5:39 PM, John Stultz <john.stultz@...aro.org> wrote:
> This patch adds CAP_GROUP_MIGRATE and logic to allows a process
> 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!
>
> Cc: Tejun Heo <tj@...nel.org>
> Cc: Li Zefan <lizefan@...wei.com>
> Cc: Jonathan Corbet <corbet@....net>
> Cc: cgroups@...r.kernel.org
> Cc: Android Kernel Team <kernel-team@...roid.com>
> Cc: Rom Lemarchand <romlem@...roid.com>
> Cc: Colin Cross <ccross@...roid.com>
> Cc: Dmitry Shmidt <dimitrysh@...gle.com>
> Cc: Todd Kjos <tkjos@...gle.com>
> Cc: Christian Poetzsch <christian.potzsch@...tec.com>
> Cc: Amit Pundir <amit.pundir@...aro.org>
> Cc: Dmitry Torokhov <dmitry.torokhov@...il.com>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Serge E. Hallyn <serge@...lyn.com>
> Cc: Andy Lutomirski <luto@...capital.net>
> Cc: linux-api@...r.kernel.org
> Acked-by: Serge Hallyn <serge@...lyn.com>

After sending this I just realized that this is changed enough I
should probably remove Serge's Acked-by here. Apologies.

But otherwise feedback on this would be appreciated!

thanks
-john