[PATCH v5] cgroup: Add new capability to allow a process to migrate other tasks between cgroups

[John Stultz]

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.

I feel the approach taken there overloads CAP_SYS_NICE a bit much
for non-android environments. Efforts to re-use CAP_SYS_RESOURCE
for this purpose (which Android has since adopted) was also
stymied by concerns about risks from future cgroups that could be
considered "dangerous" by how they might change system semantics.

So to avoid overlapping usage, this patch adds a brand 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: Todd Kjos <tkjos@xxxxxxxxxx>
Cc: Christian Poetzsch <christian.potzsch@xxxxxxxxxx>
Cc: Amit Pundir <amit.pundir@xxxxxxxxxx>
Cc: Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Serge E. Hallyn <serge@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: linux-api@xxxxxxxxxxxxxxx
Acked-by: Serge Hallyn <serge@xxxxxxxxxx>
Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
---
v2: Renamed to just CAP_CGROUP_MIGRATE as reccomended 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
v5: Switch back to CAP_CGROUP_MIGRATE due to concerns from Andy
---
 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..32d3829 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 migration of other 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 2ee9ec3..784f115 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