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>

1 files changed, 9 insertions, 5 deletions

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index fe7cdf2a2a23..d50f4fe352b3 100644
--- 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);