virtio-net: fix data corruption with OOM

virtio net used to unlink skbs from send queues on error,
but ever since 48925e372f
we do not do this. This causes guest data corruption and crashes
with vhost since net core can requeue the skb or free it without
it being taken off the list.

This patch fixes this by queueing the skb after successful
transmit.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/virtio_net.c

@@ -517,8 +517,7 @@ again:
 	/* Free up any pending old buffers before queueing new ones. */
 	free_old_xmit_skbs(vi);
 
-	/* Put new one in send queue and do transmit */
-	__skb_queue_head(&vi->send, skb);
+	/* Try to transmit */
 	capacity = xmit_skb(vi, skb);
 
 	/* This can happen with OOM and indirect buffers. */
@@ -532,8 +531,17 @@ again:
 		}
 		return NETDEV_TX_BUSY;
 	}
-
 	vi->svq->vq_ops->kick(vi->svq);
+
+	/*
+	 * Put new one in send queue.  You'd expect we'd need this before
+	 * xmit_skb calls add_buf(), since the callback can be triggered
+	 * immediately after that.  But since the callback just triggers
+	 * another call back here, normal network xmit locking prevents the
+	 * race.
+	 */
+	__skb_queue_head(&vi->send, skb);
+
 	/* Don't wait up for transmitted skbs to be freed. */
 	skb_orphan(skb);
 	nf_reset(skb);