net_sched: hfsc: Fix a potential UAF in hfsc_dequeue() too

Similarly to the previous patch, we need to safe guard hfsc_dequeue()
too. But for this one, we don't have a reliable reproducer.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Reported-by: Gerrard Tai <gerrard.tai@starlabs.sg>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/20250417184732.943057-3-xiyou.wangcong@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: congwang

net/sched/sch_hfsc.c

@@ -1641,10 +1641,16 @@ hfsc_dequeue(struct Qdisc *sch)
 		if (cl->qdisc->q.qlen != 0) {
 			/* update ed */
 			next_len = qdisc_peek_len(cl->qdisc);
-			if (realtime)
-				update_ed(cl, next_len);
-			else
-				update_d(cl, next_len);
+			/* Check queue length again since some qdisc implementations
+			 * (e.g., netem/codel) might empty the queue during the peek
+			 * operation.
+			 */
+			if (cl->qdisc->q.qlen != 0) {
+				if (realtime)
+					update_ed(cl, next_len);
+				else
+					update_d(cl, next_len);
+			}
 		} else {
 			/* the class becomes passive */
 			eltree_remove(cl);