macvlan: Fix potential use-after free for broadcasts

BugLink: https://bugs.launchpad.net/bugs/1878232

commit 260916df upstream.

When we postpone a broadcast packet we save the source port in
the skb if it is local.  However, the source port can disappear
before we get a chance to process the packet.

This patch fixes this by holding a ref count on the netdev.

It also delays the skb->cb modification until after we allocate
the new skb as you should not modify shared skbs.

Fixes: 412ca155 ("macvlan: Move broadcasts into a work queue")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ian May <ian.may@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/net/macvlan.c

@@ -305,6 +305,8 @@ static void macvlan_process_broadcast(struct work_struct *w)
 
 		rcu_read_unlock();
 
+		if (src)
+			dev_put(src->dev);
 		kfree_skb(skb);
 
 		cond_resched();
@@ -312,6 +314,7 @@ static void macvlan_process_broadcast(struct work_struct *w)
 }
 
 static void macvlan_broadcast_enqueue(struct macvlan_port *port,
+				      const struct macvlan_dev *src,
 				      struct sk_buff *skb)
 {
 	struct sk_buff *nskb;
@@ -321,8 +324,12 @@ static void macvlan_broadcast_enqueue(struct macvlan_port *port,
 	if (!nskb)
 		goto err;
 
+	MACVLAN_SKB_CB(nskb)->src = src;
+
 	spin_lock(&port->bc_queue.lock);
 	if (skb_queue_len(&port->bc_queue) < MACVLAN_BC_QUEUE_LEN) {
+		if (src)
+			dev_hold(src->dev);
 		__skb_queue_tail(&port->bc_queue, nskb);
 		err = 0;
 	}
@@ -432,8 +439,7 @@ static rx_handler_result_t macvlan_handle_frame(struct sk_buff **pskb)
 			goto out;
 		}
 
-		MACVLAN_SKB_CB(skb)->src = src;
-		macvlan_broadcast_enqueue(port, skb);
+		macvlan_broadcast_enqueue(port, src, skb);
 
 		return RX_HANDLER_PASS;
 	}