netem: Update sch->q.qlen before qdisc_tree_reduce_backlog()

[ Upstream commit 638ba5089324796c2ee49af10427459c2de35f71 ]

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@fau.de>
Reported-by: Mingi Cho <mincho@theori.io>
Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Link: https://patch.msgid.link/20250204005841.223511-4-xiyou.wangcong@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index f47ab622399f31338947bc6cb8e0e611671ee053..cb38e58ee771d8148960a898a3d1649ad971dc3a 100644 (file)
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -739,9 +739,9 @@ deliver:
                                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;
                        }