sched/rt: pick_next_rt_entity(): check list_entry

From: Pietro Borrello <[email protected]>

commit 7c4a5b89a0b5a57a64b601775b296abf77a9fe97 upstream.

Commit 326587b84078 ("sched: fix goto retry in pick_next_task_rt()") removed any path which could make pick_next_rt_entity() return NULL. However, BUG_ON(!rt_se) in _pick_next_task_rt() (the only caller of pick_next_rt_entity()) still checks the error condition, which can never happen, since list_entry() never returns NULL. Remove the BUG_ON check, and instead emit a warning in the only possible error condition here: the queue being empty which should never happen.

[Srish: Fixes CVE-2023-1077: sched/rt: pick_next_rt_entity(): check list_entry
An insufficient list empty checking in pick_next_rt_entity(). The _pick_next_task_rt() checks pick_next_rt_entity() returns NULL or not but pick_next_rt_entity() never returns NULL. So, even if the list is empty, _pick_next_task_rt() continues its process.]

---
 kernel/sched/rt.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

index 9c6c3572b..394c66442 100644
@@ -1522,6 +1522,8 @@ static struct sched_rt_entity *pick_next_rt_entity(struct rq *rq,
 	BUG_ON(idx >= MAX_RT_PRIO);
 
 	queue = array->queue + idx;
+	if (SCHED_WARN_ON(list_empty(queue)))
+		return NULL;
 	next = list_entry(queue->next, struct sched_rt_entity, run_list);
 
 	return next;

@@ -1535,7 +1537,8 @@ static struct task_struct *_pick_next_task_rt(struct rq *rq)
 
 	do {
 		rt_se = pick_next_rt_entity(rq, rt_rq);
-		BUG_ON(!rt_se);
+		if (unlikely(!rt_se))
+			return NULL;
 		rt_rq = group_rt_rq(rt_se);
 	} while (rt_rq);
 

--
2.35.6

Now queued up, thanks.

greg k-h