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@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Link: https://patch.msgid.link/20240901182438.4992-1-stephen@networkplumber.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index 0f8d581438c392aaed3d3b0fb63f3b33e2c4d09e..39382ee1e33108fb0f495c4c858e28e4f06759a0 100644 (file)
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -742,11 +742,10 @@ deliver:
 
                                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;
                        }