netdev: fix mtu check when TSO is enabled

In case the device where is coming from the packet has TSO enabled,
we should not check the mtu size value as this one could be bigger
than the expected value.

This is the case for the macvlan driver when the lower device has
TSO enabled. The macvlan inherit this feature and forward the packets
without fragmenting them. Then the packets go through dev_forward_skb
and are dropped. This patch fix this by checking TSO is not enabled
when we want to check the mtu size.
Signed-off-by: Daniel Lezcano <daniel.lezcano@free.fr>
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -1454,6 +1454,27 @@ static inline void net_timestamp_check(struct sk_buff *skb)
 		__net_timestamp(skb);
 }
 
+static inline bool is_skb_forwardable(struct net_device *dev,
+				      struct sk_buff *skb)
+{
+	unsigned int len;
+
+	if (!(dev->flags & IFF_UP))
+		return false;
+
+	len = dev->mtu + dev->hard_header_len + VLAN_HLEN;
+	if (skb->len <= len)
+		return true;
+
+	/* if TSO is enabled, we don't care about the length as the packet
+	 * could be forwarded without being segmented before
+	 */
+	if (skb_is_gso(skb))
+		return true;
+
+	return false;
+}
+
 /**
  * dev_forward_skb - loopback an skb to another netif
  *
@@ -1477,8 +1498,7 @@ int dev_forward_skb(struct net_device *dev, struct sk_buff *skb)
 	skb_orphan(skb);
 	nf_reset(skb);
 
-	if (unlikely(!(dev->flags & IFF_UP) ||
-		     (skb->len > (dev->mtu + dev->hard_header_len + VLAN_HLEN)))) {
+	if (unlikely(!is_skb_forwardable(dev, skb))) {
 		atomic_long_inc(&dev->rx_dropped);
 		kfree_skb(skb);
 		return NET_RX_DROP;