netdev - [Patch net v3 3/4] netem: Update sch->q.qlen before qdisc_tree_reduce

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Message-Id: <20250204005841.223511-4-xiyou.wangcong@gmail.com>
Date: Mon,  3 Feb 2025 16:58:40 -0800
From: Cong Wang <xiyou.wangcong@...il.com>
To: netdev@...r.kernel.org
Cc: jhs@...atatu.com,
	jiri@...nulli.us,
	pctammela@...atatu.com,
	mincho@...ori.io,
	quanglex97@...il.com,
	Cong Wang <cong.wang@...edance.com>,
	Martin Ottens <martin.ottens@....de>
Subject: [Patch net v3 3/4] netem: Update sch->q.qlen before qdisc_tree_reduce_backlog()

From: Cong Wang <cong.wang@...edance.com>

qdisc_tree_reduce_backlog() notifies parent qdisc only if child
qdisc becomes empty, therefore we need to reduce the backlog of the
child qdisc before calling it. Otherwise it would miss the opportunity
to call cops->qlen_notify(), in the case of DRR, it resulted in UAF
since DRR uses ->qlen_notify() to maintain its active list.

Fixes: f8d4bc455047 ("net/sched: netem: account for backlog updates from child qdisc")
Cc: Martin Ottens <martin.ottens@....de>
Reported-by: Mingi Cho <mincho@...ori.io>
Signed-off-by: Cong Wang <cong.wang@...edance.com>
---
 net/sched/sch_netem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index 71ec9986ed37..fdd79d3ccd8c 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -749,9 +749,9 @@ static struct sk_buff *netem_dequeue(struct Qdisc *sch)
 				if (err != NET_XMIT_SUCCESS) {
 					if (net_xmit_drop_count(err))
 						qdisc_qstats_drop(sch);
-					qdisc_tree_reduce_backlog(sch, 1, pkt_len);
 					sch->qstats.backlog -= pkt_len;
 					sch->q.qlen--;
+					qdisc_tree_reduce_backlog(sch, 1, pkt_len);
 				}
 				goto tfifo_dequeue;
 			}
-- 
2.34.1