vxlan: Pull inner IP header in vxlan_xmit_one().

Ensure the inner IP header is part of the skb's linear data before
setting old_iph. Otherwise, on a non-linear skb, old_iph could point
outside of the packet data.

Unlike classical VXLAN, which always encapsulates Ethernet packets,
VXLAN-GPE can transport IP packets directly. In that case, we need to
look at skb->protocol to figure out if an Ethernet header is present.

Fixes: d342894 ("vxlan: virtual extensible lan")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Link: https://patch.msgid.link/2aa75f6fa62ac9dbe4f16ad5ba75dd04a51d4b99.1718804000.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Guillaume Nault

drivers/net/vxlan/vxlan_core.c

@@ -2339,7 +2339,7 @@ void vxlan_xmit_one(struct sk_buff *skb, struct net_device *dev,
 	struct ip_tunnel_key *pkey;
 	struct ip_tunnel_key key;
 	struct vxlan_dev *vxlan = netdev_priv(dev);
-	const struct iphdr *old_iph = ip_hdr(skb);
+	const struct iphdr *old_iph;
 	struct vxlan_metadata _md;
 	struct vxlan_metadata *md = &_md;
 	unsigned int pkt_len = skb->len;
@@ -2353,8 +2353,15 @@ void vxlan_xmit_one(struct sk_buff *skb, struct net_device *dev,
 	bool use_cache;
 	bool udp_sum = false;
 	bool xnet = !net_eq(vxlan->net, dev_net(vxlan->dev));
+	bool no_eth_encap;
 	__be32 vni = 0;
 
+	no_eth_encap = flags & VXLAN_F_GPE && skb->protocol != htons(ETH_P_TEB);
+	if (!skb_vlan_inet_prepare(skb, no_eth_encap))
+		goto drop;
+
+	old_iph = ip_hdr(skb);
+
 	info = skb_tunnel_info(skb);
 	use_cache = ip_tunnel_dst_cache_usable(skb, info);