netdev - [PATCH net] netem: fix a use after free

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Message-ID: <1466460043.6850.21.camel@edumazet-glaptop3.roam.corp.google.com>
Date:	Mon, 20 Jun 2016 15:00:43 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev <netdev@...r.kernel.org>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	WANG Cong <xiyou.wangcong@...il.com>,
	Stephen Hemminger <stephen@...workplumber.org>
Subject: [PATCH net] netem: fix a use after free

From: Eric Dumazet <edumazet@...gle.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 <edumazet@...gle.com>
Cc: WANG Cong <xiyou.wangcong@...il.com>
Cc: Jamal Hadi Salim <jhs@...atatu.com>
Cc: Stephen Hemminger <stephen@...workplumber.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;
 			}