[PATCH] sched/fair: Fix fault in reweight_entity

[Tadeusz Struk <tadeusz.struk@linaro.org>]

Syzbot found a GPF in reweight_entity. This has been bisected to commit
c85c6fadbef0 ("kernel/sched: Fix sched_fork() access an invalid sched_task_group")
Looks like after this change there is a time window, when
task_struct->se.cfs_rq can be NULL. This can be exploited to trigger
null-ptr-deref by calling setpriority on that task.

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
Cc: Zhang Qiao <zhangqiao22@huawei.com>
Cc: stable@vger.kernel.org
Cc: linux-kernel@vger.kernel.org

Link: https://syzkaller.appspot.com/bug?id=9d9c27adc674e3a7932b22b61c79a02da82cbdc1
Fixes: c85c6fadbef0 ("kernel/sched: Fix sched_fork() access an invalid sched_task_group")
Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org>
---
 kernel/sched/fair.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 095b0aa378df..196f8cee3f9b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3042,6 +3042,9 @@ dequeue_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se) { }
 static void reweight_entity(struct cfs_rq *cfs_rq, struct sched_entity *se,
 			    unsigned long weight)
 {
+	if (!cfs_rq)
+		return;
+
 	if (se->on_rq) {
 		/* commit outstanding execution time */
 		if (cfs_rq->curr == se)
-- 
2.34.1