xen-netback: make sure that hashes are not send to unaware frontends

In the case when a frontend only negotiates a single queue with xen-
netback it is possible for a skbuff with a s/w hash to result in a
hash extra_info segment being sent to the frontend even when no hash
algorithm has been configured. (The ndo_select_queue() entry point makes
sure the hash is not set if no algorithm is configured, but this entry
point is not called when there is only a single queue). This can result
in a frontend that is unable to handle extra_info segments being given
such a segment, causing it to crash.

This patch fixes the problem by clearing the hash in ndo_start_xmit()
instead, which is clearly guaranteed to be called irrespective of the
number of queues.
Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netback/interface.c

@@ -149,17 +149,8 @@ static u16 xenvif_select_queue(struct net_device *dev, struct sk_buff *skb,
 	struct xenvif *vif = netdev_priv(dev);
 	unsigned int size = vif->hash.size;
 
-	if (vif->hash.alg == XEN_NETIF_CTRL_HASH_ALGORITHM_NONE) {
-		u16 index = fallback(dev, skb) % dev->real_num_tx_queues;
-
-		/* Make sure there is no hash information in the socket
-		 * buffer otherwise it would be incorrectly forwarded
-		 * to the frontend.
-		 */
-		skb_clear_hash(skb);
-
-		return index;
-	}
+	if (vif->hash.alg == XEN_NETIF_CTRL_HASH_ALGORITHM_NONE)
+		return fallback(dev, skb) % dev->real_num_tx_queues;
 
 	xenvif_set_skb_hash(vif, skb);
 
@@ -208,6 +199,13 @@ static int xenvif_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	cb = XENVIF_RX_CB(skb);
 	cb->expires = jiffies + vif->drain_timeout;
 
+	/* If there is no hash algorithm configured then make sure there
+	 * is no hash information in the socket buffer otherwise it
+	 * would be incorrectly forwarded to the frontend.
+	 */
+	if (vif->hash.alg == XEN_NETIF_CTRL_HASH_ALGORITHM_NONE)
+		skb_clear_hash(skb);
+
 	xenvif_rx_queue_tail(queue, skb);
 	xenvif_kick_thread(queue);