Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934106AbcJQWlJ (ORCPT ); Mon, 17 Oct 2016 18:41:09 -0400 Received: from mail-vk0-f47.google.com ([209.85.213.47]:33249 "EHLO mail-vk0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933153AbcJQWk7 (ORCPT ); Mon, 17 Oct 2016 18:40:59 -0400 MIME-Version: 1.0 In-Reply-To: <1476743724-9104-1-git-send-email-john.stultz@linaro.org> References: <1476743724-9104-1-git-send-email-john.stultz@linaro.org> From: Andy Lutomirski Date: Mon, 17 Oct 2016 15:40:37 -0700 Message-ID: Subject: Re: [PATCH] cgroup: Add new capability to allow a process to migrate other tasks between cgroups To: John Stultz Cc: lkml , Tejun Heo , Li Zefan , Jonathan Corbet , "open list:CONTROL GROUP (CGROUP)" , Android Kernel Team , Rom Lemarchand , Colin Cross , Dmitry Shmidt , Ricky Zhou , Dmitry Torokhov , Todd Kjos , Christian Poetzsch , Amit Pundir , "Serge E . Hallyn" , Linux API Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3905 Lines: 95 On Mon, Oct 17, 2016 at 3:35 PM, John Stultz 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. > > The approach taken there overloads CAP_SYS_NICE a bit much, and > is maybe more complicated then needed. > > So this patch, as suggested by Tejun, simply adds a 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 > Cc: Li Zefan > Cc: Jonathan Corbet > Cc: cgroups@vger.kernel.org > Cc: Android Kernel Team > Cc: Rom Lemarchand > Cc: Colin Cross > Cc: Dmitry Shmidt > Cc: Ricky Zhou > Cc: Dmitry Torokhov > Cc: Todd Kjos > Cc: Christian Poetzsch > Cc: Amit Pundir > Cc: Serge E. Hallyn > Cc: linux-api@vger.kernel.org > Signed-off-by: John Stultz > --- > v2: Renamed to just CAP_CGROUP_MIGRATE as reccomended by Tejun > --- > include/uapi/linux/capability.h | 5 ++++- > kernel/cgroup.c | 3 ++- > 2 files changed, 6 insertions(+), 2 deletions(-) > > diff --git a/include/uapi/linux/capability.h b/include/uapi/linux/capability.h > index 49bc062..44d7ff4 100644 > --- a/include/uapi/linux/capability.h > +++ b/include/uapi/linux/capability.h > @@ -349,8 +349,11 @@ struct vfs_cap_data { > > #define CAP_AUDIT_READ 37 > > +/* Allow migrating tasks between cgroups */ > > -#define CAP_LAST_CAP CAP_AUDIT_READ > +#define CAP_CGROUP_MIGRATE 38 > + > +#define CAP_LAST_CAP CAP_CGROUP_MIGRATE > > #define cap_valid(x) ((x) >= 0 && (x) <= CAP_LAST_CAP) > > diff --git a/kernel/cgroup.c b/kernel/cgroup.c > index 85bc9be..09f84d2 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_CGROUP_MIGRATE)) > ret = -EACCES; This logic seems rather confused to me. Without this patch, a user can write to procs if it's root *or* it matches the target uid *or* it matches the target suid. How does this make sense? How about ptrace_may_access(...) || ns_capable(tcred->user_ns, CAP_CGROUP_MIGRATE)? --Andy