[PATCH net] netem: fix a use after free

[Eric Dumazet]

From: Eric Dumazet <eduma...@google.com>

If the packet was dropped by lower qdisc, then we must not
access it later.

Save qdisc_pkt_len(skb) in a temp variable.

Fixes: 2ccccf5fb43f ("net_sched: update hierarchical backlog too")
Signed-off-by: Eric Dumazet <eduma...@google.com>
Cc: WANG Cong <xiyou.wangc...@gmail.com>
Cc: Jamal Hadi Salim <j...@mojatatu.com>
Cc: Stephen Hemminger <step...@networkplumber.org>
---
 net/sched/sch_netem.c |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index 205bed00dd34..178f1630a036 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -650,14 +650,14 @@ deliver:
 #endif
 
                        if (q->qdisc) {
+                               unsigned int pkt_len = qdisc_pkt_len(skb);
                                int err = qdisc_enqueue(skb, q->qdisc);
 
-                               if (unlikely(err != NET_XMIT_SUCCESS)) {
-                                       if (net_xmit_drop_count(err)) {
-                                               qdisc_qstats_drop(sch);
-                                               qdisc_tree_reduce_backlog(sch, 
1,
-                                                                         
qdisc_pkt_len(skb));
-                                       }
+                               if (err != NET_XMIT_SUCCESS &&
+                                   net_xmit_drop_count(err)) {
+                                       qdisc_qstats_drop(sch);
+                                       qdisc_tree_reduce_backlog(sch, 1,
+                                                                 pkt_len);
                                }
                                goto tfifo_dequeue;
                        }