xen-netfront: reduce gso_max_size to account for max TCP header

The maximum packet including header that can be handled by netfront / netback
wire format is 65535. Reduce gso_max_size accordingly.

Drop skb and print warning when skb->len > 65535. This can 1) save the effort
to send malformed packet to netback, 2) help spotting misconfiguration of
netfront in the future.

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-netfront.c

@@ -36,7 +36,7 @@
 #include <linux/skbuff.h>
 #include <linux/ethtool.h>
 #include <linux/if_ether.h>
-#include <linux/tcp.h>
+#include <net/tcp.h>
 #include <linux/udp.h>
 #include <linux/moduleparam.h>
 #include <linux/mm.h>
@@ -547,6 +547,16 @@ static int xennet_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	unsigned int len = skb_headlen(skb);
 	unsigned long flags;
 
+	/* If skb->len is too big for wire format, drop skb and alert
+	 * user about misconfiguration.
+	 */
+	if (unlikely(skb->len > XEN_NETIF_MAX_TX_SIZE)) {
+		net_alert_ratelimited(
+			"xennet: skb->len = %u, too big for wire format\n",
+			skb->len);
+		goto drop;
+	}
+
 	slots = DIV_ROUND_UP(offset + len, PAGE_SIZE) +
 		xennet_count_skb_frag_slots(skb);
 	if (unlikely(slots > MAX_SKB_FRAGS + 1)) {
@@ -1058,7 +1068,8 @@ err:
 
 static int xennet_change_mtu(struct net_device *dev, int mtu)
 {
-	int max = xennet_can_sg(dev) ? 65535 - ETH_HLEN : ETH_DATA_LEN;
+	int max = xennet_can_sg(dev) ?
+		XEN_NETIF_MAX_TX_SIZE - MAX_TCP_HEADER : ETH_DATA_LEN;
 
 	if (mtu > max)
 		return -EINVAL;
@@ -1362,6 +1373,8 @@ static struct net_device *xennet_create_dev(struct xenbus_device *dev)
 	SET_ETHTOOL_OPS(netdev, &xennet_ethtool_ops);
 	SET_NETDEV_DEV(netdev, &dev->dev);
 
+	netif_set_gso_max_size(netdev, XEN_NETIF_MAX_TX_SIZE - MAX_TCP_HEADER);
+
 	np->netdev = netdev;
 
 	netif_carrier_off(netdev);

include/xen/interface/io/netif.h

@@ -47,6 +47,7 @@
 #define _XEN_NETTXF_extra_info		(3)
 #define  XEN_NETTXF_extra_info		(1U<<_XEN_NETTXF_extra_info)
 
+#define XEN_NETIF_MAX_TX_SIZE 0xFFFF
 struct xen_netif_tx_request {
     grant_ref_t gref;      /* Reference to buffer page */
     uint16_t offset;       /* Offset within buffer page */