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

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>

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index 71ec9986ed37f4f6d36d3b5e0c19fc3338948aa5..fdd79d3ccd8ce76fbc7dc279b21898377a3bf8b8 100644 (file)
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -749,9 +749,9 @@ 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;
                        }