Re: [patch] cgroups: disallow attaching kthreadd

From: Mike Galbraith
Date: Sat Apr 21 2012 - 01:31:46 EST

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@xxxxxxx>
Acked-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Acked-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
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;
+ }

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