af_packet: Don't use skb after dev_queue_xmit()

tpacket_snd() can change and kfree an skb after dev_queue_xmit(),
which is illegal.

With debugging by: Stephen Hemminger <shemminger@vyatta.com>

Reported-by: Michael Breuer <mbreuer@majjas.com>
With help from: David S. Miller <davem@davemloft.net>
Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Tested-by: Michael Breuer<mbreuer@majjas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/packet/af_packet.c

@@ -1021,8 +1021,20 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
 
 		status = TP_STATUS_SEND_REQUEST;
 		err = dev_queue_xmit(skb);
-		if (unlikely(err > 0 && (err = net_xmit_errno(err)) != 0))
-			goto out_xmit;
+		if (unlikely(err > 0)) {
+			err = net_xmit_errno(err);
+			if (err && __packet_get_status(po, ph) ==
+				   TP_STATUS_AVAILABLE) {
+				/* skb was destructed already */
+				skb = NULL;
+				goto out_status;
+			}
+			/*
+			 * skb was dropped but not destructed yet;
+			 * let's treat it like congestion or err < 0
+			 */
+			err = 0;
+		}
 		packet_increment_head(&po->tx_ring);
 		len_sum += tp_len;
 	} while (likely((ph != NULL) ||
@@ -1033,9 +1045,6 @@ static int tpacket_snd(struct packet_sock *po, struct msghdr *msg)
 	err = len_sum;
 	goto out_put;
 
-out_xmit:
-	skb->destructor = sock_wfree;
-	atomic_dec(&po->tx_ring.pending);
 out_status:
 	__packet_set_status(po, ph, status);
 	kfree_skb(skb);