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 f459e34684ad36d43364f3a2e6c6eca73cedf0b6..22f5d9421f6a611818f99e04f79cd88886a9dcbd 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;
                        }