[PATCH] sched/fair: Fix null pointer dereference of empty queues

[Mohammed Naser <mnaser@vexxhost.com>]

Calling put_prev_task() can result in nr_running being updated
to zero, which would then crash the system when the kernel
attempts to pick_next_entity() with an empty queue.

Signed-off-by: Mohammed Naser <mnaser@vexxhost.com>
---
 kernel/sched/fair.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0d4632f..dd0a74a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5374,12 +5374,11 @@ again:
 simple:
 	cfs_rq = &rq->cfs;
 #endif
+	put_prev_task(rq, prev);
 
 	if (!cfs_rq->nr_running)
 		goto idle;
 
-	put_prev_task(rq, prev);
-
 	do {
 		se = pick_next_entity(cfs_rq, NULL);
 		set_next_entity(cfs_rq, se);
@@ -5415,7 +5414,10 @@ idle:
 static void put_prev_task_fair(struct rq *rq, struct task_struct *prev)
 {
 	struct sched_entity *se = &prev->se;
-	struct cfs_rq *cfs_rq;
+	struct cfs_rq *cfs_rq = &rq->cfs;
+
+	if (!cfs_rq->nr_running)
+		return;
 
 	for_each_sched_entity(se) {
 		cfs_rq = cfs_rq_of(se);
-- 
2.3.2 (Apple Git-55)