net: ip_gre: fix possible use-after-free in erspan_rcv

erspan tunnels run __iptunnel_pull_header on received skbs to remove
gre and erspan headers. This can determine a possible use-after-free
accessing pkt_md pointer in erspan_rcv since the packet will be 'uncloned'
running pskb_expand_head if it is a cloned gso skb (e.g if the packet has
been sent though a veth device). Fix it resetting pkt_md pointer after
__iptunnel_pull_header

Fixes: 1d7e2ed ("net: erspan: refactor existing erspan code")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_gre.c

@@ -259,7 +259,6 @@ static int erspan_rcv(struct sk_buff *skb, struct tnl_ptk_info *tpi,
 	struct net *net = dev_net(skb->dev);
 	struct metadata_dst *tun_dst = NULL;
 	struct erspan_base_hdr *ershdr;
-	struct erspan_metadata *pkt_md;
 	struct ip_tunnel_net *itn;
 	struct ip_tunnel *tunnel;
 	const struct iphdr *iph;
@@ -282,18 +281,16 @@ 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),
 					   false, false) < 0)
 			goto drop;
 
 		if (tunnel->collect_md) {
+			struct erspan_metadata *pkt_md, *md;
 			struct ip_tunnel_info *info;
-			struct erspan_metadata *md;
+			unsigned char *gh;
 			__be64 tun_id;
 			__be16 flags;
 
@@ -306,6 +303,14 @@ static int erspan_rcv(struct sk_buff *skb, struct tnl_ptk_info *tpi,
 			if (!tun_dst)
 				return PACKET_REJECT;
 
+			/* skb can be uncloned in __iptunnel_pull_header, so
+			 * old pkt_md is no longer valid and we need to reset
+			 * it
+			 */
+			gh = skb_network_header(skb) +
+			     skb_network_header_len(skb);
+			pkt_md = (struct erspan_metadata *)(gh + gre_hdr_len +
+							    sizeof(*ershdr));
 			md = ip_tunnel_info_opts(&tun_dst->u.tun_info);
 			md->version = ver;
 			md2 = &md->u.md2;