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

["Serge E. Hallyn" <serge@hallyn.com>]

Quoting John Stultz (john.stultz@linaro.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@hallyn.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@kernel.org>
> Cc: Li Zefan <lizefan@huawei.com>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: cgroups@vger.kernel.org
> Cc: Android Kernel Team <kernel-team@android.com>
> Cc: Rom Lemarchand <romlem@android.com>
> Cc: Colin Cross <ccross@android.com>
> Cc: Dmitry Shmidt <dimitrysh@google.com>
> Cc: Todd Kjos <tkjos@google.com>
> Cc: Christian Poetzsch <christian.potzsch@imgtec.com>
> Cc: Amit Pundir <amit.pundir@linaro.org>
> Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Serge E. Hallyn <serge@hallyn.com>
> Cc: linux-api@vger.kernel.org
> Signed-off-by: John Stultz <john.stultz@linaro.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