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Subject[186/272] sched, cgroup: Use exit hook to avoid use-after-free crash
2.6.37-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Peter Zijlstra <peterz@infradead.org>

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@frugalware.org>
Fixed-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
LKML-Reference: <1293206353.29444.205.camel@laptop>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
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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