virtio: fix virtio_net xmit of freed skb bug

If we fail to transmit a packet, we assume the queue is full and put
the skb into last_xmit_skb.  However, if more space frees up before we
xmit it, we loop, and the result can be transmitting the same skb twice.

Fix is simple: set skb to NULL if we've used it in some way, and check
before sending.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index fe7cdf2a2a236bc7257e1e5e3e2a5d2ef1086ad3..d50f4fe352b321aa7d16c17f1676099eefe04f52 100644 (file)
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -287,21 +287,25 @@ again:
        free_old_xmit_skbs(vi);
 
        /* If we has a buffer left over from last time, send it now. */
-       if (vi->last_xmit_skb) {
+       if (unlikely(vi->last_xmit_skb)) {
                if (xmit_skb(vi, vi->last_xmit_skb) != 0) {
                        /* Drop this skb: we only queue one. */
                        vi->dev->stats.tx_dropped++;
                        kfree_skb(skb);
+                       skb = NULL;
                        goto stop_queue;
                }
                vi->last_xmit_skb = NULL;
        }
 
        /* Put new one in send queue and do transmit */
-       __skb_queue_head(&vi->send, skb);
-       if (xmit_skb(vi, skb) != 0) {
-               vi->last_xmit_skb = skb;
-               goto stop_queue;
+       if (likely(skb)) {
+               __skb_queue_head(&vi->send, skb);
+               if (xmit_skb(vi, skb) != 0) {
+                       vi->last_xmit_skb = skb;
+                       skb = NULL;
+                       goto stop_queue;
+               }
        }
 done:
        vi->svq->vq_ops->kick(vi->svq);