tuntap: Fix for a race in accessing numqueues

A patch for fixing a race between queue selection and changing queues
was introduced in commit 92bb73ea("tuntap: fix a possible race between
queue selection and changing queues").

The fix was to prevent the driver from re-reading the tun->numqueues
more than once within tun_select_queue() using ACCESS_ONCE().

We have been experiancing 'Divide-by-zero' errors in tun_net_xmit()
since we moved from 3.6 to 3.10, and believe that they come from a
simular source where the value of tun->numqueues changes to zero
between the first and a subsequent read of tun->numqueues.

The fix is a simular use of ACCESS_ONCE(), as well as a multiply
instead of a divide in the if statement.
Signed-off-by: Dominic Curran <dominic.curran@citrix.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Maxim Krasnyansky <maxk@qti.qualcomm.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Acked-by: Max Krasnyansky <maxk@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -738,15 +738,17 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
 	struct tun_struct *tun = netdev_priv(dev);
 	int txq = skb->queue_mapping;
 	struct tun_file *tfile;
+	u32 numqueues = 0;
 
 	rcu_read_lock();
 	tfile = rcu_dereference(tun->tfiles[txq]);
+	numqueues = ACCESS_ONCE(tun->numqueues);
 
 	/* Drop packet if interface is not attached */
-	if (txq >= tun->numqueues)
+	if (txq >= numqueues)
 		goto drop;
 
-	if (tun->numqueues == 1) {
+	if (numqueues == 1) {
 		/* Select queue was not called for the skbuff, so we extract the
 		 * RPS hash and save it into the flow_table here.
 		 */
@@ -779,8 +781,8 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
 	/* Limit the number of packets queued by dividing txq length with the
 	 * number of queues.
 	 */
-	if (skb_queue_len(&tfile->socket.sk->sk_receive_queue)
-			  >= dev->tx_queue_len / tun->numqueues)
+	if (skb_queue_len(&tfile->socket.sk->sk_receive_queue) * numqueues
+			  >= dev->tx_queue_len)
 		goto drop;
 
 	if (unlikely(skb_orphan_frags(skb, GFP_ATOMIC)))