net_sched: ets: Fix double list add in class with netem as child qdisc

As described in Gerrard's report [1], there are use cases where a netem
child qdisc will make the parent qdisc's enqueue callback reentrant.
In the case of ets, there won't be a UAF, but the code will add the same
classifier to the list twice, which will cause memory corruption.

In addition to checking for qlen being zero, this patch checks whether
the class was already added to the active_list (cl_is_active) before
doing the addition to cater for the reentrant case.

[1] https://lore.kernel.org/netdev/CAHcdcOm+03OD2j6R0=YHKqmy=VgJ8xEOKuP6c7mSgnp-TEJJbw@mail.gmail.com/

Fixes: 37d9cf1 ("sched: Fix detection of empty queues in child qdiscs")
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Victor Nogueira <victor@mojatatu.com>
Link: https://patch.msgid.link/20250425220710.3964791-4-victor@mojatatu.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: vbnogueira

net/sched/sch_ets.c

@@ -74,6 +74,11 @@ static const struct nla_policy ets_class_policy[TCA_ETS_MAX + 1] = {
 	[TCA_ETS_QUANTA_BAND] = { .type = NLA_U32 },
 };
 
+static bool cl_is_active(struct ets_class *cl)
+{
+	return !list_empty(&cl->alist);
+}
+
 static int ets_quantum_parse(struct Qdisc *sch, const struct nlattr *attr,
 			     unsigned int *quantum,
 			     struct netlink_ext_ack *extack)
@@ -416,7 +421,6 @@ static int ets_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 	struct ets_sched *q = qdisc_priv(sch);
 	struct ets_class *cl;
 	int err = 0;
-	bool first;
 
 	cl = ets_classify(skb, sch, &err);
 	if (!cl) {
@@ -426,7 +430,6 @@ static int ets_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 		return err;
 	}
 
-	first = !cl->qdisc->q.qlen;
 	err = qdisc_enqueue(skb, cl->qdisc, to_free);
 	if (unlikely(err != NET_XMIT_SUCCESS)) {
 		if (net_xmit_drop_count(err)) {
@@ -436,7 +439,7 @@ static int ets_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 		return err;
 	}
 
-	if (first && !ets_class_is_strict(q, cl)) {
+	if (!cl_is_active(cl) && !ets_class_is_strict(q, cl)) {
 		list_add_tail(&cl->alist, &q->active);
 		cl->deficit = cl->quantum;
 	}