vxlan: test dev->flags & IFF_UP before calling gro_cells_receive()

Same reasons than the ones explained in commit 4179cb5a
("vxlan: test dev->flags & IFF_UP before calling netif_rx()")

netif_rx() or gro_cells_receive() must be called under a strict contract.

At device dismantle phase, core networking clears IFF_UP
and flush_all_backlogs() is called after rcu grace period
to make sure no incoming packet might be in a cpu backlog
and still referencing the device.

A similar protocol is used for gro_cells infrastructure, as
gro_cells_destroy() will be called only after a full rcu
grace period is observed after IFF_UP has been cleared.

Most drivers call netif_rx() from their interrupt handler,
and since the interrupts are disabled at device dismantle,
netif_rx() does not have to check dev->flags & IFF_UP

Virtual drivers do not have this guarantee, and must
therefore make the check themselves.

Otherwise we risk use-after-free and/or crashes.

Fixes: d342894c ("vxlan: virtual extensible lan")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/vxlan.c

@@ -1731,6 +1731,14 @@ static int vxlan_rcv(struct sock *sk, struct sk_buff *skb)
 		goto drop;
 	}
 
+	rcu_read_lock();
+
+	if (unlikely(!(vxlan->dev->flags & IFF_UP))) {
+		rcu_read_unlock();
+		atomic_long_inc(&vxlan->dev->rx_dropped);
+		goto drop;
+	}
+
 	stats = this_cpu_ptr(vxlan->dev->tstats);
 	u64_stats_update_begin(&stats->syncp);
 	stats->rx_packets++;
@@ -1738,6 +1746,9 @@ static int vxlan_rcv(struct sock *sk, struct sk_buff *skb)
 	u64_stats_update_end(&stats->syncp);
 
 	gro_cells_receive(&vxlan->gro_cells, skb);
+
+	rcu_read_unlock();
+
 	return 0;
 
 drop: