[NETFILTER]: ip_nat_proto_gre: do not modify/corrupt GREv0 packets through NAT

While porting some changes of the 2.6.21-rc7 pptp/proto_gre conntrack
and nat modules to a 2.4.32 kernel I noticed that the gre_key function
returns a wrong pointer to the GRE key of a version 0 packet thus
corrupting the packet payload.

The intended behaviour for GREv0 packets is to act like
ip_conntrack_proto_generic/ip_nat_proto_unknown so I have ripped the
offending functions (not used anymore) and modified the
ip_nat_proto_gre modules to not touch version 0 (non PPTP) packets.
Signed-off-by: Jorge Boncompte <jorge@dti2.net>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Adrian Bunk <bunk@stusta.de>

include/linux/netfilter_ipv4/ip_conntrack_proto_gre.h

@@ -90,25 +90,6 @@ int ip_ct_gre_keymap_add(struct ip_conntrack *ct,
 /* delete keymap entries */
 void ip_ct_gre_keymap_destroy(struct ip_conntrack *ct);
 
-
-/* get pointer to gre key, if present */
-static inline u_int32_t *gre_key(struct gre_hdr *greh)
-{
-	if (!greh->key)
-		return NULL;
-	if (greh->csum || greh->routing)
-		return (u_int32_t *) (greh+sizeof(*greh)+4);
-	return (u_int32_t *) (greh+sizeof(*greh));
-}
-
-/* get pointer ot gre csum, if present */
-static inline u_int16_t *gre_csum(struct gre_hdr *greh)
-{
-	if (!greh->csum)
-		return NULL;
-	return (u_int16_t *) (greh+sizeof(*greh));
-}
-
 #endif /* __KERNEL__ */
 
 #endif /* _CONNTRACK_PROTO_GRE_H */

net/ipv4/netfilter/ip_nat_proto_gre.c

@@ -71,6 +71,11 @@ gre_unique_tuple(struct ip_conntrack_tuple *tuple,
 	u_int16_t *keyptr;
 	unsigned int min, i, range_size;
 
+	/* If there is no master conntrack we are not PPTP,
+	   do not change tuples */
+	if (!conntrack->master)
+		return 0;
+		
 	if (maniptype == IP_NAT_MANIP_SRC)
 		keyptr = &tuple->src.u.gre.key;
 	else
@@ -123,19 +128,9 @@ gre_manip_pkt(struct sk_buff **pskb,
 	if (maniptype == IP_NAT_MANIP_DST) {
 		/* key manipulation is always dest */
 		switch (greh->version) {
-		case 0:
-			if (!greh->key) {
-				DEBUGP("can't nat GRE w/o key\n");
-				break;
-			}
-			if (greh->csum) {
-				/* FIXME: Never tested this code... */
-				*(gre_csum(greh)) = 
-					ip_nat_cheat_check(~*(gre_key(greh)),
-							tuple->dst.u.gre.key,
-							*(gre_csum(greh)));
-			}
-			*(gre_key(greh)) = tuple->dst.u.gre.key;
+		case GRE_VERSION_1701:
+			/* We do not currently NAT any GREv0 packets.
+			 * Try to behave like "ip_nat_proto_unknown" */
 			break;
 		case GRE_VERSION_PPTP:
 			DEBUGP("call_id -> 0x%04x\n",