gre: fix improper error handling

Fix improper protocol err_handler, current implementation is fully
unapplicable and may cause kernel crash due to double kfree_skb.
Signed-off-by: Dmitry Kozlov <xeb@mail.ru>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/gre.c

@@ -15,6 +15,7 @@
 #include <linux/kmod.h>
 #include <linux/skbuff.h>
 #include <linux/in.h>
+#include <linux/ip.h>
 #include <linux/netdevice.h>
 #include <linux/spinlock.h>
 #include <net/protocol.h>
@@ -96,27 +97,17 @@ static int gre_rcv(struct sk_buff *skb)
 static void gre_err(struct sk_buff *skb, u32 info)
 {
 	const struct gre_protocol *proto;
-	u8 ver;
-
-	if (!pskb_may_pull(skb, 12))
-		goto drop;
+	const struct iphdr *iph = (const struct iphdr *)skb->data;
+	u8 ver = skb->data[(iph->ihl<<2) + 1]&0x7f;
 
-	ver = skb->data[1]&0x7f;
 	if (ver >= GREPROTO_MAX)
-		goto drop;
+		return;
 
 	rcu_read_lock();
 	proto = rcu_dereference(gre_proto[ver]);
-	if (!proto || !proto->err_handler)
-		goto drop_unlock;
-	proto->err_handler(skb, info);
-	rcu_read_unlock();
-	return;
-
-drop_unlock:
+	if (proto && proto->err_handler)
+		proto->err_handler(skb, info);
 	rcu_read_unlock();
-drop:
-	kfree_skb(skb);
 }
 
 static const struct net_protocol net_gre_protocol = {