net: neigh: don't call kfree_skb() under spin_lock_irqsave()

It is not allowed to call kfree_skb() from hardware interrupt
context or with interrupts being disabled. So add all skb to
a tmp list, then free them after spin_unlock_irqrestore() at
once.

Fixes: 66ba215c ("neigh: fix possible DoS due to net iface start/stop loop")
Suggested-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/neighbour.c

@@ -309,14 +309,17 @@ static int neigh_del_timer(struct neighbour *n)
 
 static void pneigh_queue_purge(struct sk_buff_head *list, struct net *net)
 {
+	struct sk_buff_head tmp;
 	unsigned long flags;
 	struct sk_buff *skb;
 
+	skb_queue_head_init(&tmp);
 	spin_lock_irqsave(&list->lock, flags);
 	skb = skb_peek(list);
 	while (skb != NULL) {
 		struct sk_buff *skb_next = skb_peek_next(skb, list);
 		struct net_device *dev = skb->dev;
+
 		if (net == NULL || net_eq(dev_net(dev), net)) {
 			struct in_device *in_dev;
 
@@ -326,13 +329,16 @@ static void pneigh_queue_purge(struct sk_buff_head *list, struct net *net)
 				in_dev->arp_parms->qlen--;
 			rcu_read_unlock();
 			__skb_unlink(skb, list);
-
-			dev_put(dev);
-			kfree_skb(skb);
+			__skb_queue_tail(&tmp, skb);
 		}
 		skb = skb_next;
 	}
 	spin_unlock_irqrestore(&list->lock, flags);
+
+	while ((skb = __skb_dequeue(&tmp))) {
+		dev_put(skb->dev);
+		kfree_skb(skb);
+	}
 }
 
 static void neigh_flush_dev(struct neigh_table *tbl, struct net_device *dev,