sit: update dev->needed_headroom in ipip6_tunnel_bind_dev()

When a tunnel device is bound with the underlying device, its
dev->needed_headroom needs to be updated properly. IPv4 tunnels
already do the same in ip_tunnel_bind_dev(). Otherwise we may
not have enough header room for skb, especially after commit
b17f709 ("gue: TX support for using remote checksum offload option").

Fixes: 32b8a8e ("sit: add IPv4 over IPv4 support")
Reported-by: Palash Oswal <[email protected]>
Link: https://lore.kernel.org/netdev/CAGyP=7fDcSPKu6nttbGwt7RXzE3uyYxLjCSE97J64pRxJP8jPA@mail.gmail.com/
Cc: Kuniyuki Iwashima <[email protected]>
Cc: Eric Dumazet <[email protected]>
Signed-off-by: Cong Wang <[email protected]>
Reviewed-by: Eric Dumazet <[email protected]>
Reviewed-by: Kuniyuki Iwashima <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

Author: Cong Wang

net/ipv6/sit.c

@@ -1095,12 +1095,13 @@ static netdev_tx_t sit_tunnel_xmit(struct sk_buff *skb,
 
 static void ipip6_tunnel_bind_dev(struct net_device *dev)
 {
+	struct ip_tunnel *tunnel = netdev_priv(dev);
+	int t_hlen = tunnel->hlen + sizeof(struct iphdr);
 	struct net_device *tdev = NULL;
-	struct ip_tunnel *tunnel;
+	int hlen = LL_MAX_HEADER;
 	const struct iphdr *iph;
 	struct flowi4 fl4;
 
-	tunnel = netdev_priv(dev);
 	iph = &tunnel->parms.iph;
 
 	if (iph->daddr) {
@@ -1123,14 +1124,15 @@ static void ipip6_tunnel_bind_dev(struct net_device *dev)
 		tdev = __dev_get_by_index(tunnel->net, tunnel->parms.link);
 
 	if (tdev && !netif_is_l3_master(tdev)) {
-		int t_hlen = tunnel->hlen + sizeof(struct iphdr);
 		int mtu;
 
 		mtu = tdev->mtu - t_hlen;
 		if (mtu < IPV6_MIN_MTU)
 			mtu = IPV6_MIN_MTU;
 		WRITE_ONCE(dev->mtu, mtu);
+		hlen = tdev->hard_header_len + tdev->needed_headroom;
 	}
+	dev->needed_headroom = t_hlen + hlen;
 }
 
 static void ipip6_tunnel_update(struct ip_tunnel *t, struct ip_tunnel_parm *p,