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@suse.de>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index b2f203f25ec8..ad8eae5bb801 100644
--- 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>
 
@@ -2225,6 +2226,18 @@ retry_find_task:
 
         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;
+                rcu_read_unlock();
+                goto out_unlock_cgroup;
+        }
+
         get_task_struct(tsk);
         rcu_read_unlock();