net: sched: fix uses after free

syzbot reported one use-after-free in pfifo_fast_enqueue() [1]

Issue here is that we can not reuse skb after a successful skb_array_produce()
since another cpu might have consumed it already.

I believe a similar problem exists in try_bulk_dequeue_skb_slow()
in case we put an skb into qdisc_enqueue_skb_bad_txq() for lockless qdisc.

[1]
BUG: KASAN: use-after-free in qdisc_pkt_len include/net/sch_generic.h:610 [inline]
BUG: KASAN: use-after-free in qdisc_qstats_cpu_backlog_inc include/net/sch_generic.h:712 [inline]
BUG: KASAN: use-after-free in pfifo_fast_enqueue+0x4bc/0x5e0 net/sched/sch_generic.c:639
Read of size 4 at addr ffff8801cede37e8 by task syzkaller717588/5543

CPU: 1 PID: 5543 Comm: syzkaller717588 Not tainted 4.16.0-rc4+ #265
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:17 [inline]
 dump_stack+0x194/0x24d lib/dump_stack.c:53
 print_address_description+0x73/0x250 mm/kasan/report.c:256
 kasan_report_error mm/kasan/report.c:354 [inline]
 kasan_report+0x23c/0x360 mm/kasan/report.c:412
 __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:432
 qdisc_pkt_len include/net/sch_generic.h:610 [inline]
 qdisc_qstats_cpu_backlog_inc include/net/sch_generic.h:712 [inline]
 pfifo_fast_enqueue+0x4bc/0x5e0 net/sched/sch_generic.c:639
 __dev_xmit_skb net/core/dev.c:3216 [inline]

Fixes: c5ad119f ("net: sched: pfifo_fast use skb_array")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot+ed43b6903ab968b16f54@syzkaller.appspotmail.com
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc:	Cong Wang <xiyou.wangcong@gmail.com>
Cc:	Jiri Pirko <jiri@resnulli.us>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_generic.c

@@ -106,6 +106,14 @@ static inline void qdisc_enqueue_skb_bad_txq(struct Qdisc *q,
 
 	__skb_queue_tail(&q->skb_bad_txq, skb);
 
+	if (qdisc_is_percpu_stats(q)) {
+		qdisc_qstats_cpu_backlog_inc(q, skb);
+		qdisc_qstats_cpu_qlen_inc(q);
+	} else {
+		qdisc_qstats_backlog_inc(q, skb);
+		q->q.qlen++;
+	}
+
 	if (lock)
 		spin_unlock(lock);
 }
@@ -196,14 +204,6 @@ static void try_bulk_dequeue_skb_slow(struct Qdisc *q,
 			break;
 		if (unlikely(skb_get_queue_mapping(nskb) != mapping)) {
 			qdisc_enqueue_skb_bad_txq(q, nskb);
-
-			if (qdisc_is_percpu_stats(q)) {
-				qdisc_qstats_cpu_backlog_inc(q, nskb);
-				qdisc_qstats_cpu_qlen_inc(q);
-			} else {
-				qdisc_qstats_backlog_inc(q, nskb);
-				q->q.qlen++;
-			}
 			break;
 		}
 		skb->next = nskb;
@@ -628,6 +628,7 @@ static int pfifo_fast_enqueue(struct sk_buff *skb, struct Qdisc *qdisc,
 	int band = prio2band[skb->priority & TC_PRIO_MAX];
 	struct pfifo_fast_priv *priv = qdisc_priv(qdisc);
 	struct skb_array *q = band2list(priv, band);
+	unsigned int pkt_len = qdisc_pkt_len(skb);
 	int err;
 
 	err = skb_array_produce(q, skb);
@@ -636,7 +637,10 @@ static int pfifo_fast_enqueue(struct sk_buff *skb, struct Qdisc *qdisc,
 		return qdisc_drop_cpu(skb, qdisc, to_free);
 
 	qdisc_qstats_cpu_qlen_inc(qdisc);
-	qdisc_qstats_cpu_backlog_inc(qdisc, skb);
+	/* Note: skb can not be used after skb_array_produce(),
+	 * so we better not use qdisc_qstats_cpu_backlog_inc()
+	 */
+	this_cpu_add(qdisc->cpu_qstats->backlog, pkt_len);
 	return NET_XMIT_SUCCESS;
 }