virtio: fix virtio_net xmit of freed skb bug

On Mon, 2008-05-26 at 17:42 +1000, Rusty Russell wrote:
> 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.
...
> diff -r 564237b31993 drivers/net/virtio_net.c
> --- a/drivers/net/virtio_net.c	Mon May 19 12:22:00 2008 +1000
> +++ b/drivers/net/virtio_net.c	Mon May 19 12:24:58 2008 +1000
> @@ -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;

With this, may drop an skb and then later in the function discover that
we could have sent it after all. Poor wee skb :)

How about the incremental patch below?

Cheers,
Mark.

Subject: [PATCH] virtio_net: Delay dropping tx skbs

Currently we drop the skb in start_xmit() if we have a
queued buffer and fail to transmit it.

However, if we delay dropping it until we've stopped the
queue and enabled the tx notification callback, then there
is a chance space might become available for it.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>