Re: [RFC][PATCH] cgroup: Add new capability 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 CAP_GROUP_MIGRATE_TASK 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_TASK), and uses it when checking

So realistically, what all can this mean?  Freezing tasks, changing
cpu/memory limits, changing network and disk throughput, forbid forking,
and (most importantly) forbid access to certain devices.

I think that's all ok.  (And we still separately check for inode write
perms.)

If anything I'd say the GLOBAL_ROOT_UID check could be taken out since
otherwise a host-root task effectively cannot drop this capability.

> 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.
> 
> 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: Serge E. Hallyn <serge@hallyn.com>

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

> Signed-off-by: John Stultz <john.stultz@linaro.org>
> ---
>  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..e199ea0 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_TASK	38
> +
> +#define CAP_LAST_CAP         CAP_CGROUP_MIGRATE_TASK
>  
>  #define cap_valid(x) ((x) >= 0 && (x) <= CAP_LAST_CAP)
>  
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 9ba28310..a318956 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -2847,7 +2847,8 @@ 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_TASK))
>  		ret = -EACCES;
>  
>  	if (!ret && cgroup_on_dfl(dst_cgrp)) {
> -- 
> 1.9.1