linux-kernel - Re: [RESEND][PATCH v4] cgroup: Use CAP_SYS_RESOURCE to allow a process to migrate other tasks between cgroups

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Date:   Tue, 8 Nov 2016 15:41:41 -0800
From:   Kees Cook <keescook@...omium.org>
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 <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>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [RESEND][PATCH v4] cgroup: Use CAP_SYS_RESOURCE to allow a
 process to migrate other tasks between cgroups

On Tue, Nov 8, 2016 at 3:28 PM, John Stultz <john.stultz@...aro.org> wrote:
> This patch adds logic to allows a process to migrate other tasks
> between cgroups if they have CAP_SYS_RESOURCE.
>
> 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.
>
> 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 just merged
> it.
>
> 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
> Acked-by: Serge Hallyn <serge@...lyn.com>
> Signed-off-by: John Stultz <john.stultz@...aro.org>
> ---
> v2: Renamed to just CAP_CGROUP_MIGRATE as recommended by Tejun
> v3: Switched to just using CAP_SYS_RESOURCE as suggested by Michael
> v4: Send out properly folded down version of the patch. :P
> ---
>  kernel/cgroup.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 85bc9be..866059a 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -2856,7 +2856,8 @@ 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))
> +           !uid_eq(cred->euid, tcred->suid) &&
> +           !ns_capable(tcred->user_ns, CAP_SYS_RESOURCE))
>                 ret = -EACCES;
>
>         if (!ret && cgroup_on_dfl(dst_cgrp)) {
> --
> 2.7.4
>

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees

-- 
Kees Cook
Nexus Security