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 <[email protected]> Cc: Li Zefan <[email protected]> Cc: Jonathan Corbet <[email protected]> Cc: [email protected] Cc: Android Kernel Team <[email protected]> Cc: Rom Lemarchand <[email protected]> Cc: Colin Cross <[email protected]> Cc: Dmitry Shmidt <[email protected]> Cc: Ricky Zhou <[email protected]> Cc: Dmitry Torokhov <[email protected]> Cc: Todd Kjos <[email protected]> Cc: Christian Poetzsch <[email protected]> Cc: Amit Pundir <[email protected]> Cc: Serge E. Hallyn <[email protected]> Cc: [email protected] Signed-off-by: John Stultz <[email protected]> --- 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; if (!ret && cgroup_on_dfl(dst_cgrp)) { -- 2.7.4
