net: erspan: reload pointer after pskb_may_pull

pskb_may_pull() can change skb->data, so we need to re-load pkt_md
and ershdr at the right place.

Fixes: 94d7d8f29287 ("ip6_gre: add erspan v2 support")
Fixes: f551c91de262 ("net: erspan: introduce erspan v2 for ip_gre")
Signed-off-by: William Tu <u9012063@gmail.com>
Cc: Haishuang Yan <yanhaishuang@cmss.chinamobile.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_gre.c

@@ -279,7 +279,6 @@ static int erspan_rcv(struct sk_buff *skb, struct tnl_ptk_info *tpi,
 	 * Use ERSPAN 10-bit session ID as key.
 	 */
 	tpi->key = cpu_to_be32(ntohs(ershdr->session_id) & ID_MASK);
-	pkt_md = (struct erspan_metadata *)(ershdr + 1);
 	tunnel = ip_tunnel_lookup(itn, skb->dev->ifindex,
 				  tpi->flags | TUNNEL_KEY,
 				  iph->saddr, iph->daddr, tpi->key);
@@ -289,6 +288,9 @@ static int erspan_rcv(struct sk_buff *skb, struct tnl_ptk_info *tpi,
 		if (unlikely(!pskb_may_pull(skb, len)))
 			return PACKET_REJECT;
 
+		ershdr = (struct erspan_base_hdr *)(skb->data + gre_hdr_len);
+		pkt_md = (struct erspan_metadata *)(ershdr + 1);
+
 		if (__iptunnel_pull_header(skb,
 					   len,
 					   htons(ETH_P_TEB),

net/ipv6/ip6_gre.c

@@ -515,7 +515,6 @@ static int ip6erspan_rcv(struct sk_buff *skb, int gre_hdr_len,
 
 	ver = (ntohs(ershdr->ver_vlan) & VER_MASK) >> VER_OFFSET;
 	tpi->key = cpu_to_be32(ntohs(ershdr->session_id) & ID_MASK);
-	pkt_md = (struct erspan_metadata *)(ershdr + 1);
 
 	tunnel = ip6gre_tunnel_lookup(skb->dev,
 				      &ipv6h->saddr, &ipv6h->daddr, tpi->key,
@@ -526,6 +525,9 @@ static int ip6erspan_rcv(struct sk_buff *skb, int gre_hdr_len,
 		if (unlikely(!pskb_may_pull(skb, len)))
 			return PACKET_REJECT;
 
+		ershdr = (struct erspan_base_hdr *)skb->data;
+		pkt_md = (struct erspan_metadata *)(ershdr + 1);
+
 		if (__iptunnel_pull_header(skb, len,
 					   htons(ETH_P_TEB),
 					   false, false) < 0)