Re: [Xen-devel] [PATCH 2/4] xen-netfront: drop skb when skb->len > 65535

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Mon, 2013-03-18 at 14:00 +0000, David Vrabel wrote:
> On 18/03/13 13:48, Ian Campbell wrote:
> > On Mon, 2013-03-18 at 13:46 +0000, David Vrabel wrote:
> >> On 18/03/13 10:35, Wei Liu wrote:
> >>> The `size' field of Xen network wire format is uint16_t, anything bigger 
> >>> than
> >>> 65535 will cause overflow.
> >>
> >> The backend needs to be able to handle these bad packets without
> >> disconnecting the VIF -- we can't fix all the frontend drivers.
> > 
> > Agreed, although that doesn't imply that we shouldn't fix the frontend
> > where we can -- such as upstream as Wei does here.
> 
> Yes, frontends should be fixed where possible.
> 
> This is what I came up with for the backend.  I don't have time to look
> into it further but, Wei, feel free to use it as a starting point.
> 

Thanks for this patch.

I haven't gone through XSA-39 discussion, this is why I didn't come up
with a fix for backend -- I need to make sure dropping packet like this
won't re-exhibit the security hole.


Wei.

> David
> 
> diff --git a/drivers/net/xen-netback/netback.c
> b/drivers/net/xen-netback/netback.c
> index cd49ba9..18e2671 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -899,10 +899,11 @@ static void netbk_fatal_tx_err(struct xenvif *vif)
>  static int netbk_count_requests(struct xenvif *vif,
>                               struct xen_netif_tx_request *first,
>                               struct xen_netif_tx_request *txp,
> -                             int work_to_do)
> +                             int work_to_do, int idx)
>  {
>       RING_IDX cons = vif->tx.req_cons;
>       int frags = 0;
> +     bool drop = false;
> 
>       if (!(first->flags & XEN_NETTXF_more_data))
>               return 0;
> @@ -922,10 +923,20 @@ static int netbk_count_requests(struct xenvif *vif,
> 
>               memcpy(txp, RING_GET_REQUEST(&vif->tx, cons + frags),
>                      sizeof(*txp));
> -             if (txp->size > first->size) {
> -                     netdev_err(vif->dev, "Frag is bigger than frame.\n");
> -                     netbk_fatal_tx_err(vif);
> -                     return -EIO;
> +
> +             /*
> +              * If the guest submitted a frame >= 64 KiB then
> +              * first->size overflowed and following frags will
> +              * appear to be larger than the frame.
> +              *
> +              * This cannot be a fatal error as there are buggy
> +              * frontends that do this.
> +              *
> +              * Consume all the frags and drop the packet.
> +              */
> +             if (!drop && txp->size > first->size) {
> +                     netdev_dbg(vif->dev, "Frag is bigger than frame.\n");
> +                     drop = true;
>               }
> 
>               first->size -= txp->size;
> @@ -938,6 +949,12 @@ static int netbk_count_requests(struct xenvif *vif,
>                       return -EINVAL;
>               }
>       } while ((txp++)->flags & XEN_NETTXF_more_data);
> +
> +     if (drop) {
> +             netbk_tx_err(vif, txp, idx + frags);
> +             return -EIO;
> +     }
> +
>       return frags;
>  }
> 
> @@ -1327,7 +1344,7 @@ static unsigned xen_netbk_tx_build_gops(struct
> xen_netbk *netbk)
>                               continue;
>               }
> 
> -             ret = netbk_count_requests(vif, &txreq, txfrags, work_to_do);
> +             ret = netbk_count_requests(vif, &txreq, txfrags, work_to_do, 
> idx);
>               if (unlikely(ret < 0))
>                       continue;
> 



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