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

[Eric Dumazet <eric.dumazet@xxxxxxxxx>]

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@xxxxxxxxxx>
> Reported-by: Sowmini Varadhan <sowmini.varadhan@xxxxxxxxxx>
> Tested-by: Sowmini Varadhan <sowmini.varadhan@xxxxxxxxxx>
> ---
> Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> Cc: Juergen Gross <jgross@xxxxxxxx>
> ---
>  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)) {



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