netdev - [PATCH net v3 4/5] net_sched: qfq: Fix double list add in class with netem as child qdisc

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Message-ID: <20250425220710.3964791-5-victor@mojatatu.com>
Date: Fri, 25 Apr 2025 19:07:08 -0300
From: Victor Nogueira <victor@...atatu.com>
To: netdev@...r.kernel.org
Cc: jhs@...atatu.com,
	xiyou.wangcong@...il.com,
	jiri@...nulli.us,
	davem@...emloft.net,
	edumazet@...gle.com,
	kuba@...nel.org,
	pabeni@...hat.com,
	toke@...hat.com,
	gerrard.tai@...rlabs.sg,
	pctammela@...atatu.com,
	stephen@...workplumber.org
Subject: [PATCH net v3 4/5] net_sched: qfq: 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 qfq, there won't be a UAF, but the code will add the same
classifier to the list twice, which will cause memory corruption.

This patch checks whether the class was already added to the agg->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: 37d9cf1a3ce3 ("sched: Fix detection of empty queues in child qdiscs")
Acked-by: Jamal Hadi Salim <jhs@...atatu.com>
Signed-off-by: Victor Nogueira <victor@...atatu.com>
---
 net/sched/sch_qfq.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/net/sched/sch_qfq.c b/net/sched/sch_qfq.c
index 687a932eb9b2..bf1282cb22eb 100644
--- a/net/sched/sch_qfq.c
+++ b/net/sched/sch_qfq.c
@@ -202,6 +202,11 @@ struct qfq_sched {
  */
 enum update_reason {enqueue, requeue};
 
+static bool cl_is_active(struct qfq_class *cl)
+{
+	return !list_empty(&cl->alist);
+}
+
 static struct qfq_class *qfq_find_class(struct Qdisc *sch, u32 classid)
 {
 	struct qfq_sched *q = qdisc_priv(sch);
@@ -1215,7 +1220,6 @@ static int qfq_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 	struct qfq_class *cl;
 	struct qfq_aggregate *agg;
 	int err = 0;
-	bool first;
 
 	cl = qfq_classify(skb, sch, &err);
 	if (cl == NULL) {
@@ -1237,7 +1241,6 @@ static int qfq_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 	}
 
 	gso_segs = skb_is_gso(skb) ? skb_shinfo(skb)->gso_segs : 1;
-	first = !cl->qdisc->q.qlen;
 	err = qdisc_enqueue(skb, cl->qdisc, to_free);
 	if (unlikely(err != NET_XMIT_SUCCESS)) {
 		pr_debug("qfq_enqueue: enqueue failed %d\n", err);
@@ -1253,8 +1256,8 @@ static int qfq_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 	++sch->q.qlen;
 
 	agg = cl->agg;
-	/* if the queue was not empty, then done here */
-	if (!first) {
+	/* if the class is active, then done here */
+	if (cl_is_active(cl)) {
 		if (unlikely(skb == cl->qdisc->ops->peek(cl->qdisc)) &&
 		    list_first_entry(&agg->active, struct qfq_class, alist)
 		    == cl && cl->deficit < len)
-- 
2.34.1