linux-kernel - Re: [PATCH] virtio_net: fix use after free

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Date:	Thu, 30 Oct 2014 20:36:49 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Rusty Russell <rusty@...tcorp.com.au>,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	Jason Wang <jasowang@...hat.com>
Subject: Re: [PATCH] virtio_net: fix use after free

On Wed, 2014-10-15 at 16:23 +0300, Michael S. Tsirkin wrote:
> commit 0b725a2ca61bedc33a2a63d0451d528b268cf975
>     net: Remove ndo_xmit_flush netdev operation, use signalling instead.
> 
> added code that looks at skb->xmit_more after the skb has
> been put in TX VQ. Since some paths process the ring and free the skb
> immediately, this can cause use after free.
> 
> Fix by storing xmit_more in a local variable.
> 
> Cc: David S. Miller <davem@...emloft.net>
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> ---
> 
> David, am I using the API correctly?
> Seems to work for me.
> You used __netif_subqueue_stopped but that seems to use
> a slightly more expensive test_bit internally.
> The reason I added a variable for the txq here is because it's handy for
> BQL patch later on.
> 
> 
>  drivers/net/virtio_net.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 3d0ce44..13d0a8b 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -920,6 +920,8 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
>  	int qnum = skb_get_queue_mapping(skb);
>  	struct send_queue *sq = &vi->sq[qnum];
>  	int err;
> +	struct netdev_queue *txq = netdev_get_tx_queue(dev, qnum);
> +	bool kick = !skb->xmit_more;
>  
>  	/* Free up any pending old buffers before queueing new ones. */
>  	free_old_xmit_skbs(sq);
> @@ -956,7 +958,7 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
>  		}
>  	}
>  
> -	if (__netif_subqueue_stopped(dev, qnum) || !skb->xmit_more)
> +	if (kick || netif_xmit_stopped(txq))
>  		virtqueue_kick(sq->vq);
>  
>  	return NETDEV_TX_OK;

I must say I am kind of confused by this patch.

Why the skb_orphan(skb) & nf_reset(skb) do not have the same issue ?


It looks like following patch is needed ?

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index ec2a8b41ed41..17cc42c6a559 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -927,6 +927,10 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 	/* Free up any pending old buffers before queueing new ones. */
 	free_old_xmit_skbs(sq);
 
+	/* Don't wait up for transmitted skbs to be freed. */
+	skb_orphan(skb);
+	nf_reset(skb);
+
 	/* Try to transmit */
 	err = xmit_skb(sq, skb);
 
@@ -941,10 +945,6 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 		return NETDEV_TX_OK;
 	}
 
-	/* Don't wait up for transmitted skbs to be freed. */
-	skb_orphan(skb);
-	nf_reset(skb);
-
 	/* Apparently nice girls don't return TX_BUSY; stop the queue
 	 * before it gets out of hand.  Naturally, this wastes entries. */
 	if (sq->vq->num_free < 2+MAX_SKB_FRAGS) {


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