net/sched: sch_taprio: reset child qdiscs before freeing them

syzkaller shows that packets can still be dequeued while taprio_destroy()
is running. Let sch_taprio use the reset() function to cancel the advance
timer and drop all skbs from the child qdiscs.

Fixes: 5a781cc ("tc: Add support for configuring the taprio scheduler")
Link: https://syzkaller.appspot.com/bug?id=f362872379bf8f0017fb667c1ab158f2d1e764ae
Reported-by: syzbot+8971da381fb5a31f542d@syzkaller.appspotmail.com
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Acked-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Link: https://lore.kernel.org/r/63b6d79b0e830ebb0283e020db4df3cdfdfb2b94.1608142843.git.dcaratti@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: dcaratti

net/sched/sch_taprio.c

@@ -1597,6 +1597,21 @@ static int taprio_change(struct Qdisc *sch, struct nlattr *opt,
 	return err;
 }
 
+static void taprio_reset(struct Qdisc *sch)
+{
+	struct taprio_sched *q = qdisc_priv(sch);
+	struct net_device *dev = qdisc_dev(sch);
+	int i;
+
+	hrtimer_cancel(&q->advance_timer);
+	if (q->qdiscs) {
+		for (i = 0; i < dev->num_tx_queues && q->qdiscs[i]; i++)
+			qdisc_reset(q->qdiscs[i]);
+	}
+	sch->qstats.backlog = 0;
+	sch->q.qlen = 0;
+}
+
 static void taprio_destroy(struct Qdisc *sch)
 {
 	struct taprio_sched *q = qdisc_priv(sch);
@@ -1607,7 +1622,6 @@ static void taprio_destroy(struct Qdisc *sch)
 	list_del(&q->taprio_list);
 	spin_unlock(&taprio_list_lock);
 
-	hrtimer_cancel(&q->advance_timer);
 
 	taprio_disable_offload(dev, q, NULL);
 
@@ -1954,6 +1968,7 @@ static struct Qdisc_ops taprio_qdisc_ops __read_mostly = {
 	.init		= taprio_init,
 	.change		= taprio_change,
 	.destroy	= taprio_destroy,
+	.reset		= taprio_reset,
 	.peek		= taprio_peek,
 	.dequeue	= taprio_dequeue,
 	.enqueue	= taprio_enqueue,