[PATCH] sched/numa: fix unsafe get_task_struct() in task_numa_assign()

[Oleg Nesterov]

The lockless get_task_struct(tsk) is only safe if tsk == current
and didn't pass exit_notify(), or if this tsk was found on a rcu
protected list (say, for_each_process() or find_task_by_vpid()).
IOW, it is only safe if release_task() was not called before we
take rcu_read_lock(), in this case we can rely on the fact that
delayed_put_pid() can not drop the (potentially) last reference
until rcu_read_unlock().

And as Kirill pointed out task_numa_compare()->task_numa_assign()
path does get_task_struct(dst_rq->curr) and this is not safe. The
task_struct itself can't go away, but rcu_read_lock() can't save
us from the final put_task_struct() in finish_task_switch(); this
reference goes away without rcu gp.

Reported-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
---
 kernel/sched/fair.c |    8 +++++++-
 1 files changed, 7 insertions(+), 1 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0090e8c..52049b9 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1158,7 +1158,13 @@ static void task_numa_compare(struct task_numa_env *env,
 
 	rcu_read_lock();
 	cur = ACCESS_ONCE(dst_rq->curr);
-	if (cur->pid == 0) /* idle */
+	/*
+	 * No need to move the exiting task, and this ensures that ->curr
+	 * wasn't reaped and thus get_task_struct() in task_numa_assign()
+	 * is safe; note that rcu_read_lock() can't protect from the final
+	 * put_task_struct() after the last schedule().
+	 */
+	if (is_idle_task(cur) || (cur->flags & PF_EXITING))
 		cur = NULL;
 
 	/*
-- 
1.5.5.1


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