neighbour: fix a race in neigh_destroy()

There is a race in neighbour code, because neigh_destroy() uses
skb_queue_purge(&neigh->arp_queue) without holding neighbour lock,
while other parts of the code assume neighbour rwlock is what
protects arp_queue

Convert all skb_queue_purge() calls to the __skb_queue_purge() variant

Use __skb_queue_head_init() instead of skb_queue_head_init()
to make clear we do not use arp_queue.lock

And hold neigh->lock in neigh_destroy() to close the race.
Reported-by: Joe Jin <joe.jin@oracle.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/neighbour.c

@@ -231,7 +231,7 @@ static void neigh_flush_dev(struct neigh_table *tbl, struct net_device *dev)
 				   we must kill timers etc. and move
 				   it to safe state.
 				 */
-				skb_queue_purge(&n->arp_queue);
+				__skb_queue_purge(&n->arp_queue);
 				n->arp_queue_len_bytes = 0;
 				n->output = neigh_blackhole;
 				if (n->nud_state & NUD_VALID)
@@ -286,7 +286,7 @@ static struct neighbour *neigh_alloc(struct neigh_table *tbl, struct net_device
 	if (!n)
 		goto out_entries;
 
-	skb_queue_head_init(&n->arp_queue);
+	__skb_queue_head_init(&n->arp_queue);
 	rwlock_init(&n->lock);
 	seqlock_init(&n->ha_lock);
 	n->updated	  = n->used = now;
@@ -708,7 +708,9 @@ void neigh_destroy(struct neighbour *neigh)
 	if (neigh_del_timer(neigh))
 		pr_warn("Impossible event\n");
 
-	skb_queue_purge(&neigh->arp_queue);
+	write_lock_bh(&neigh->lock);
+	__skb_queue_purge(&neigh->arp_queue);
+	write_unlock_bh(&neigh->lock);
 	neigh->arp_queue_len_bytes = 0;
 
 	if (dev->netdev_ops->ndo_neigh_destroy)
@@ -858,7 +860,7 @@ static void neigh_invalidate(struct neighbour *neigh)
 		neigh->ops->error_report(neigh, skb);
 		write_lock(&neigh->lock);
 	}
-	skb_queue_purge(&neigh->arp_queue);
+	__skb_queue_purge(&neigh->arp_queue);
 	neigh->arp_queue_len_bytes = 0;
 }
 
@@ -1210,7 +1212,7 @@ int neigh_update(struct neighbour *neigh, const u8 *lladdr, u8 new,
 
 			write_lock_bh(&neigh->lock);
 		}
-		skb_queue_purge(&neigh->arp_queue);
+		__skb_queue_purge(&neigh->arp_queue);
 		neigh->arp_queue_len_bytes = 0;
 	}
 out: