From d96052172265f59c51b95e954514f2f8aab79015 Mon Sep 17 00:00:00 2001
From: Mike Galbraith <efault@gmx.de>
Date: Fri, 16 Dec 2011 15:50:30 +1100
Subject: [PATCH] cpusets, cgroups: disallow attaching kthreadd

Allowing kthreadd to be moved to a non-root group makes no sense, it being
a global resource, and needlessly leads unsuspecting users toward trouble.

1. An RT workqueue worker thread spawned in a task group with no
   rt_runtime allocated is not schedulable.  Simple user error, but
   harmful to the box.

2. A worker thread which acquires PF_THREAD_BOUND can never leave a
   cpuset, rendering the cpuset immortal.

Save the user some unexpected trouble, just say no.

Signed-off-by: Mike Galbraith <efault@gmx.de>
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: Paul Menage <paul@paulmenage.org>
Cc: Tejun Heo <htejun@gmail.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
 kernel/cpuset.c     | 7 +++++--
 kernel/sched/core.c | 9 +++++++++
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index a09ac2b9a661..ca85d353e0fe 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -59,6 +59,7 @@
 #include <linux/mutex.h>
 #include <linux/workqueue.h>
 #include <linux/cgroup.h>
+#include <linux/kthread.h>
 
 /*
  * Workqueue for cpuset related tasks.
@@ -1417,9 +1418,11 @@ static int cpuset_can_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
 		 * unnecessary.  Thus, cpusets are not applicable for such
 		 * threads.  This prevents checking for success of
 		 * set_cpus_allowed_ptr() on all attached tasks before
-		 * cpus_allowed may be changed.
+		 * cpus_allowed may be changed.  We also disallow attaching
+		 * kthreadd, to prevent its child from becoming trapped should
+		 * it then acquire PF_THREAD_BOUND.
 		 */
-		if (task->flags & PF_THREAD_BOUND)
+		if (task->flags & PF_THREAD_BOUND || task == kthreadd_task)
 			return -EINVAL;
 		if ((ret = security_task_setscheduler(task)))
 			return ret;
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9775b8d44336..211cdc595d60 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -71,6 +71,7 @@
 #include <linux/ftrace.h>
 #include <linux/slab.h>
 #include <linux/init_task.h>
+#include <linux/kthread.h>
 
 #include <asm/tlb.h>
 #include <asm/irq_regs.h>
@@ -7535,6 +7536,14 @@ static int cpu_cgroup_can_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
 	struct task_struct *task;
 
 	cgroup_taskset_for_each(task, cgrp, tset) {
+		/*
+		 * kthreadd can fork workers for an RT workqueue in a cgroup
+		 * which may or may not have rt_runtime allocated.  Just say no,
+		 * as attaching a global resource to a non-root group  doesn't
+		 * make any sense anyway.
+		 */
+		if (task == kthreadd_task)
+			return -EINVAL;
 #ifdef CONFIG_RT_GROUP_SCHED
 		if (!sched_rt_can_attach(cgroup_tg(cgrp), task))
 			return -EINVAL;
-- 
2.39.5