tuntap: Fix for a race in accessing numqueues

A patch for fixing a race between queue selection and changing queues
was introduced in commit 92bb73ea2("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>

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 34cca74c99ed4f36060f1de55af2ffb523b67b0b..bcf01af4b879252dbe5d6339dd7134cb848280f4 100644 (file)
--- a/drivers/net/tun.c
+++ b/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)))