linux-kernel - Re: [patch] cgroups: disallow attaching kthreadd

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Date:	Sat, 21 Apr 2012 07:31:04 +0200
From:	Mike Galbraith <mgalbraith@...e.de>
To:	Tejun Heo <tj@...nel.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Li Zefan <lizf@...fujitsu.com>,
	David Rientjes <rientjes@...gle.com>,
	Paul Menage <paul@...lmenage.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch] cgroups: disallow attaching kthreadd

On Fri, 2012-04-20 at 10:57 -0700, Tejun Heo wrote: 
> Hello, Mike.
> 
> This is dragging out too long but can you please update the patch so
> that both kthreadd and PF_THREAD_BOUND tasks are disallowed from !root
> cgroups?

cgroups: disallow attaching kthreadd or PF_THREAD_BOUND threads

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 <mgalbraith@...e.de>
Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Acked-by: Thomas Gleixner <tglx@...utronix.de>
---
 kernel/cgroup.c |   12 ++++++++++++
 1 file changed, 12 insertions(+)

--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -60,6 +60,7 @@
 #include <linux/eventfd.h>
 #include <linux/poll.h>
 #include <linux/flex_array.h> /* used in cgroup_attach_proc */
+#include <linux/kthread.h>
 
 #include <linux/atomic.h>
 
@@ -2172,6 +2173,17 @@ static int attach_task_by_pid(struct cgr
 
 	if (threadgroup)
 		tsk = tsk->group_leader;
+
+	/*
+	 * Workqueue threads may acquire PF_THREAD_BOUND and become
+	 * trapped in a cpuset, or RT worker may be born in a cgroup
+	 * with no rt_runtime allocated.  Just say no.
+	 */
+	if (tsk == kthreadd_task || (tsk->flags & PF_THREAD_BOUND)) {
+		ret = -EINVAL;
+		goto out_unlock_cgroup;
+	}
+
 	get_task_struct(tsk);
 	rcu_read_unlock();
 


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