Re: [PATCH] cgroup: Add new capability to allow a process to migrate other tasks between cgroups

[Andy Lutomirski]

On Mon, Oct 17, 2016 at 3:35 PM, John Stultz <john.stultz@xxxxxxxxxx> 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 <tj@xxxxxxxxxx>
> Cc: Li Zefan <lizefan@xxxxxxxxxx>
> Cc: Jonathan Corbet <corbet@xxxxxxx>
> Cc: cgroups@xxxxxxxxxxxxxxx
> Cc: Android Kernel Team <kernel-team@xxxxxxxxxxx>
> Cc: Rom Lemarchand <romlem@xxxxxxxxxxx>
> Cc: Colin Cross <ccross@xxxxxxxxxxx>
> Cc: Dmitry Shmidt <dimitrysh@xxxxxxxxxx>
> Cc: Ricky Zhou <rickyz@xxxxxxxxxxxx>
> Cc: Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx>
> Cc: Todd Kjos <tkjos@xxxxxxxxxx>
> Cc: Christian Poetzsch <christian.potzsch@xxxxxxxxxx>
> Cc: Amit Pundir <amit.pundir@xxxxxxxxxx>
> Cc: Serge E. Hallyn <serge@xxxxxxxxxx>
> Cc: linux-api@xxxxxxxxxxxxxxx
> Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
> ---
> 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