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

From: David Vrabel <[email-masked]>
Date: Mon, 18 Mar 2013 14:00:00 +0000

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.

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;