netdev - [PATCH] sch/netem: fix use after free in netem

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Message-ID: <20240901182438.4992-1-stephen@networkplumber.org>
Date: Sun,  1 Sep 2024 11:16:07 -0700
From: Stephen Hemminger <stephen@...workplumber.org>
To: netdev@...r.kernel.org
Cc: edumazet@...gle.com,
	Stephen Hemminger <stephen@...workplumber.org>,
	Budimir Markovic <markovicbudimir@...il.com>
Subject: [PATCH] sch/netem: fix use after free in netem_dequeue

If netem_dequeue() enqueues packet to inner qdisc and that qdisc
returns __NET_XMIT_STOLEN. The packet is dropped but
qdisc_tree_reduce_backlog() is not called to update the parent's
q.qlen, leading to the similar use-after-free as Commit
e04991a48dbaf382 ("netem: fix return value if duplicate enqueue
fails")

Commands to trigger KASAN UaF:

ip link add type dummy
ip link set lo up
ip link set dummy0 up
tc qdisc add dev lo parent root handle 1: drr
tc filter add dev lo parent 1: basic classid 1:1
tc class add dev lo classid 1:1 drr
tc qdisc add dev lo parent 1:1 handle 2: netem
tc qdisc add dev lo parent 2: handle 3: drr
tc filter add dev lo parent 3: basic classid 3:1 action mirred egress
redirect dev dummy0
tc class add dev lo classid 3:1 drr
ping -c1 -W0.01 localhost # Trigger bug
tc class del dev lo classid 1:1
tc class add dev lo classid 1:1 drr
ping -c1 -W0.01 localhost # UaF

Fixes: 50612537e9ab ("netem: fix classful handling")
Reported-by: Budimir Markovic <markovicbudimir@...il.com>
Signed-off-by: Stephen Hemminger <stephen@...workplumber.org>
---
 net/sched/sch_netem.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index 0f8d581438c3..39382ee1e331 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -742,11 +742,10 @@ static struct sk_buff *netem_dequeue(struct Qdisc *sch)
 
 				err = qdisc_enqueue(skb, q->qdisc, &to_free);
 				kfree_skb_list(to_free);
-				if (err != NET_XMIT_SUCCESS &&
-				    net_xmit_drop_count(err)) {
-					qdisc_qstats_drop(sch);
-					qdisc_tree_reduce_backlog(sch, 1,
-								  pkt_len);
+				if (err != NET_XMIT_SUCCESS) {
+					if (net_xmit_drop_count(err))
+						qdisc_qstats_drop(sch);
+					qdisc_tree_reduce_backlog(sch, 1, pkt_len);
 				}
 				goto tfifo_dequeue;
 			}
-- 
2.45.2