linux-kernel - Re: [RFC][PATCH v3] cgroup: Use CAP_SYS_RESOURCE to allow a process to migrate other tasks between cgroups

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Message-ID: <20161021134157.GA15619@mail.hallyn.com>
Date:   Fri, 21 Oct 2016 08:41:57 -0500
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     John Stultz <john.stultz@...aro.org>
Cc:     lkml <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
        Li Zefan <lizefan@...wei.com>,
        Jonathan Corbet <corbet@....net>, 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>, linux-api@...r.kernel.org
Subject: Re: [RFC][PATCH v3] cgroup: Use CAP_SYS_RESOURCE to allow a process
 to migrate other tasks between cgroups

Quoting John Stultz (john.stultz@...aro.org):
> This patch adds logic to allows a process to migrate other tasks
> between cgroups if they have CAP_SYS_RESOURCE.

Hi,

fwiw this seems to me a reasonable choice, that is, giving a task
CAP_SYS_RESOURCE to be allow it to write to the procs file doesn't
seem like giving it too much.  +1 from me.  So long as the Android
folks don't have a reason why this won't work for them,

Acked-by: Serge Hallyn <serge@...lyn.com>

-serge

> 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,
> and is maybe more complicated then needed.
> 
> So this patch, as suggested by Michael Kerrisk, simply adds a
> check for CAP_SYS_RESOURCE.
> 
> I've tested this with AOSP master, and this seems to work well
> as Zygote and system_server already use CAP_SYS_RESOURCE. I've
> also submitted patches against the android-4.4 kernel to change
> it to use CAP_SYS_RESOURCE, and the Android developers have
> seemed ok with this change.
> 
> Thouhts 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: linux-api@...r.kernel.org
> Signed-off-by: John Stultz <john.stultz@...aro.org>
> ---
> v2: Renamed to just CAP_CGROUP_MIGRATE as reccomended by Tejun
> v3: Switched to just using CAP_SYS_RESOURCE as suggested by Michael
> ---
>  kernel/cgroup.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 09f84d2..866059a 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -2857,7 +2857,7 @@ static int cgroup_procs_write_permission(struct task_struct *task,
>  	if (!uid_eq(cred->euid, GLOBAL_ROOT_UID) &&
>  	    !uid_eq(cred->euid, tcred->uid) &&
>  	    !uid_eq(cred->euid, tcred->suid) &&
> -	    !ns_capable(tcred->user_ns, CAP_CGROUP_MIGRATE))
> +	    !ns_capable(tcred->user_ns, CAP_SYS_RESOURCE))
>  		ret = -EACCES;
>  
>  	if (!ret && cgroup_on_dfl(dst_cgrp)) {
> -- 
> 2.7.4