[PATCH net-next] xen-netfront: reject short packets and handle non-linear packets

[PATCH net-next] xen-netfront: reject short packets and handle non-linear packets

[Paul Durrant]

Sowmini points out two vulnerabilities in xen-netfront:

a) The code assumes that skb->len is at least ETH_HLEN.
b) The code assumes that at least ETH_HLEN octets are in the linear
   port of the socket buffer.

This patch adds tests for both of these, and in the case of the latter
pulls sufficient bytes into the linear area.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reported-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Tested-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
---
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Juergen Gross <jgross@suse.com>
---
 drivers/net/xen-netfront.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index 40f26b6..0478809 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -567,6 +567,10 @@ static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	u16 queue_index;
 	struct sk_buff *nskb;
 
+	/* Basic sanity check */
+	if (unlikely(skb->len < ETH_HLEN))
+		goto drop;
+
 	/* Drop the packet if no queues are set up */
 	if (num_queues < 1)
 		goto drop;
@@ -609,6 +613,11 @@ static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	}
 
 	len = skb_headlen(skb);
+	if (unlikely(len < ETH_HLEN)) {
+		if (!__pskb_pull_tail(skb, ETH_HLEN - len))
+			goto drop;
+		len = ETH_HLEN;
+	}
 
 	spin_lock_irqsave(&queue->tx_lock, flags);
 
-- 
2.1.4

Re: [PATCH net-next] xen-netfront: reject short packets and handle non-linear packets

[Eric Dumazet]

On Wed, 2017-01-25 at 16:26 +0000, Paul Durrant wrote:
> Sowmini points out two vulnerabilities in xen-netfront:
> 
> a) The code assumes that skb->len is at least ETH_HLEN.
> b) The code assumes that at least ETH_HLEN octets are in the linear
>    port of the socket buffer.
> 
> This patch adds tests for both of these, and in the case of the latter
> pulls sufficient bytes into the linear area.
> 
> Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
> Reported-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
> Tested-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
> ---
> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> Cc: Juergen Gross <jgross@suse.com>
> ---
>  drivers/net/xen-netfront.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
> index 40f26b6..0478809 100644
> --- a/drivers/net/xen-netfront.c
> +++ b/drivers/net/xen-netfront.c
> @@ -567,6 +567,10 @@ static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
>  	u16 queue_index;
>  	struct sk_buff *nskb;
>  
> +	/* Basic sanity check */
> +	if (unlikely(skb->len < ETH_HLEN))
> +		goto drop;
> +
>  	/* Drop the packet if no queues are set up */
>  	if (num_queues < 1)
>  		goto drop;
> @@ -609,6 +613,11 @@ static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
>  	}
>  
>  	len = skb_headlen(skb);
> +	if (unlikely(len < ETH_HLEN)) {
> +		if (!__pskb_pull_tail(skb, ETH_HLEN - len))
> +			goto drop;
> +		len = ETH_HLEN;
> +	}

Looks like duplicated code, and buggy, considering the code above

page = virt_to_page(skb->data);
offset = offset_in_page(skb->data);

Your patch might end up with skb->data/head being reallocated, and use
after free would happen.

What about something like that ?

diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index 40f26b69beb11459f0566fc1d1d739aa75e643bf..99a67fe4de86d3141169143b0820d00968cb09f2 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -583,6 +583,8 @@ static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
 			skb->len);
 		goto drop;
 	}
+	if (!pskb_may_pull(skb, ETH_HLEN))
+		goto drop;
 
 	slots = xennet_count_skb_slots(skb);
 	if (unlikely(slots > MAX_XEN_SKB_FRAGS + 1)) {