xen-netback: don't disconnect frontend when seeing oversize packet

Some frontend drivers are sending packets > 64 KiB in length. This length
overflows the length field in the first slot making the following slots have
an invalid length.

Turn this error back into a non-fatal error by dropping the packet. To avoid
having the following slots having fatal errors, consume all slots in the
packet.

This does not reopen the security hole in XSA-39 as if the packet as an
invalid number of slots it will still hit fatal error case.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netback/netback.c

@@ -975,12 +975,22 @@ static int netbk_count_requests(struct xenvif *vif,
 
 		memcpy(txp, RING_GET_REQUEST(&vif->tx, cons + slots),
 		       sizeof(*txp));
-		if (txp->size > first->size) {
-			netdev_err(vif->dev,
-				   "Invalid tx request, slot size %u > remaining size %u\n",
-				   txp->size, first->size);
-			netbk_fatal_tx_err(vif);
-			return -EIO;
+
+		/* If the guest submitted a frame >= 64 KiB then
+		 * first->size overflowed and following slots will
+		 * appear to be larger than the frame.
+		 *
+		 * This cannot be fatal error as there are buggy
+		 * frontends that do this.
+		 *
+		 * Consume all slots and drop the packet.
+		 */
+		if (!drop_err && txp->size > first->size) {
+			if (net_ratelimit())
+				netdev_dbg(vif->dev,
+					   "Invalid tx request, slot size %u > remaining size %u\n",
+					   txp->size, first->size);
+			drop_err = -EIO;
 		}
 
 		first->size -= txp->size;