[186/272] sched, cgroup: Use exit hook to avoid use-after-free crash

[Greg KH]

2.6.37-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

commit 068c5cc5ac7414a8e9eb7856b4bf3cc4d4744267 upstream.

By not notifying the controller of the on-exit move back to
init_css_set, we fail to move the task out of the previous
cgroup's cfs_rq. This leads to an opportunity for a
cgroup-destroy to come in and free the cgroup (there are no
active tasks left in it after all) to which the not-quite dead
task is still enqueued.

Reported-by: Miklos Vajna <vmiklos@xxxxxxxxxxxxxx>
Fixed-by: Mike Galbraith <efault@xxxxxx>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Mike Galbraith <efault@xxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
LKML-Reference: <1293206353.29444.205.camel@laptop>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

---
 kernel/sched.c |   18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -607,6 +607,9 @@ static inline struct task_group *task_gr
 {
 	struct cgroup_subsys_state *css;
 
+	if (p->flags & PF_EXITING)
+		return &root_task_group;
+
 	css = task_subsys_state_check(p, cpu_cgroup_subsys_id,
 			lockdep_is_held(&task_rq(p)->lock));
 	return container_of(css, struct task_group, css);
@@ -9178,6 +9181,20 @@ cpu_cgroup_attach(struct cgroup_subsys *
 	}
 }
 
+static void
+cpu_cgroup_exit(struct cgroup_subsys *ss, struct task_struct *task)
+{
+	/*
+	 * cgroup_exit() is called in the copy_process() failure path.
+	 * Ignore this case since the task hasn't ran yet, this avoids
+	 * trying to poke a half freed task state from generic code.
+	 */
+	if (!(task->flags & PF_EXITING))
+		return;
+
+	sched_move_task(task);
+}
+
 #ifdef CONFIG_FAIR_GROUP_SCHED
 static int cpu_shares_write_u64(struct cgroup *cgrp, struct cftype *cftype,
 				u64 shareval)
@@ -9250,6 +9267,7 @@ struct cgroup_subsys cpu_cgroup_subsys =
 	.destroy	= cpu_cgroup_destroy,
 	.can_attach	= cpu_cgroup_can_attach,
 	.attach		= cpu_cgroup_attach,
+	.exit		= cpu_cgroup_exit,
 	.populate	= cpu_cgroup_populate,
 	.subsys_id	= cpu_cgroup_subsys_id,
 	.early_init	= 1,


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