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

From: David Vrabel <[email-masked]>
Date: Wed, 20 Mar 2013 20:02:16 +0000

On 18/03/13 14:00, 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.

Got some time to test this (or more correctly, something similar with
XCP's kernel) and some fixes to the suggested patch are needed.  See below.

> 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)

idx should be of RING_IDX type.

>  {
>  	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);

This needs to be netbk_tx_err(vif, first, idx + frags) or the guest will
crash as we push a bunch of invalid responses.

David

> +		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;