[PATCH] xen-netfront: Avoid deref'ing skbafter it is potentially freed.

[PATCH] xen-netfront: Avoid deref'ing skbafter it is potentially freed.

[Jeremy Fitzhardinge]

xennet_tx_bug_gc can free the skb before we use it, so make sure we don't.

Jeff, this is -rc material.

Signed-off-by: Keir Fraser <keir@xensource.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: Jeff Garzik <jeff@garzik.org>

diff -r 8bfc43f6d1b0 drivers/net/xen-netfront.c
--- a/drivers/net/xen-netfront.c	Tue Aug 07 14:26:30 2007 -0700
+++ b/drivers/net/xen-netfront.c	Mon Aug 13 09:39:15 2007 -0700
@@ -566,15 +566,16 @@ static int xennet_start_xmit(struct sk_b
 	if (notify)
 		notify_remote_via_irq(np->netdev->irq);
 
+	np->stats.tx_bytes += skb->len;
+	np->stats.tx_packets++;
+
+	/* Note: It is not safe to access skb after xennet_tx_buf_gc()! */
 	xennet_tx_buf_gc(dev);
 
 	if (!netfront_tx_slot_available(np))
 		netif_stop_queue(dev);
 
 	spin_unlock_irq(&np->tx_lock);
-
-	np->stats.tx_bytes += skb->len;
-	np->stats.tx_packets++;
 
 	return 0;
 

Re: [PATCH] xen-netfront: Avoid deref'ing skbafter it is potentially freed.

[Jesper Juhl]

On Monday 13 August 2007 21:54:37 Jeremy Fitzhardinge wrote:
> xennet_tx_bug_gc can free the skb before we use it, so make sure we don't.
> 
> Jeff, this is -rc material.
> 
> Signed-off-by: Keir Fraser <keir@xensource.com>
> Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
> Cc: Jeff Garzik <jeff@garzik.org>
> 
> diff -r 8bfc43f6d1b0 drivers/net/xen-netfront.c
> --- a/drivers/net/xen-netfront.c	Tue Aug 07 14:26:30 2007 -0700
> +++ b/drivers/net/xen-netfront.c	Mon Aug 13 09:39:15 2007 -0700
> @@ -566,15 +566,16 @@ static int xennet_start_xmit(struct sk_b
>  	if (notify)
>  		notify_remote_via_irq(np->netdev->irq);
>  
> +	np->stats.tx_bytes += skb->len;
> +	np->stats.tx_packets++;
> +
> +	/* Note: It is not safe to access skb after xennet_tx_buf_gc()! */
>  	xennet_tx_buf_gc(dev);
>  
>  	if (!netfront_tx_slot_available(np))
>  		netif_stop_queue(dev);
>  
>  	spin_unlock_irq(&np->tx_lock);
> -
> -	np->stats.tx_bytes += skb->len;
> -	np->stats.tx_packets++;
>  
>  	return 0;
>  
This moves the updating of both tx_bytes and tx_packets inside the 
spinlock, but as far as I can see we only _really_ need to move the 
tx_bytes update. Considering that we generally want to do as little 
work as possible while holding a lock, wouldn't the following be 
slightly better?


Signed-off-by: Keir Fraser <keir@xensource.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
---

 drivers/net/xen-netfront.c |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index 489f69c..640e270 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -566,6 +566,9 @@ static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	if (notify)
 		notify_remote_via_irq(np->netdev->irq);
 
+	np->stats.tx_bytes += skb->len;
+
+	/* Note: It is not safe to access skb after xennet_tx_buf_gc()! */
 	xennet_tx_buf_gc(dev);
 
 	if (!netfront_tx_slot_available(np))
@@ -573,7 +576,6 @@ static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
 
 	spin_unlock_irq(&np->tx_lock);
 
-	np->stats.tx_bytes += skb->len;
 	np->stats.tx_packets++;
 
 	return 0;

Re: [PATCH] xen-netfront: Avoid deref'ing skbafter it is potentially freed.

[Jeremy Fitzhardinge]

Jesper Juhl wrote:
> This moves the updating of both tx_bytes and tx_packets inside the 
> spinlock, but as far as I can see we only _really_ need to move the 
> tx_bytes update. Considering that we generally want to do as little 
> work as possible while holding a lock, wouldn't the following be 
> slightly better?
>   

Hm, I think it would be better to keep them together.  The second add is
going to be pretty much free, particularly since the tx_bytes add will
probably pull tx_packets into cache.

I have a followup patch to convert it to using the netdevice stats
structure, which will definitely put them in the same cacheline (though
perhaps the stats structure should group tx and rx members together?).

    J

    J

Re: [PATCH] xen-netfront: Avoid deref'ing skbafter it is potentially freed.

[Jeff Garzik]

Jeremy Fitzhardinge wrote:
> xennet_tx_bug_gc can free the skb before we use it, so make sure we don't.
> 
> Jeff, this is -rc material.
> 
> Signed-off-by: Keir Fraser <keir@xensource.com>
> Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
> Cc: Jeff Garzik <jeff@garzik.org>

applied