gre: fix error handler

1) gre_parse_header() can be called from gre_err()

   At this point transport header points to ICMP header, not the inner
header.

2) We can not really change transport header as ipgre_err() will later
assume transport header still points to ICMP header (using icmp_hdr())

3) pskb_may_pull() logic in gre_parse_header() really works
  if we are interested at zone pointed by skb->data

4) As Jiri explained in commit b7f8fe251e46 ("gre: do not pull header in
ICMP error processing") we should not pull headers in error handler.

So this fix :

A) changes gre_parse_header() to use skb->data instead of
skb_transport_header()

B) Adds a nhs parameter to gre_parse_header() so that we can skip the
not pulled IP header from error path.
  This offset is 0 for normal receive path.

C) remove obsolete IPV6 includes

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Tom Herbert <tom@herbertland.com>
Cc: Maciej Żenczykowski <maze@google.com>
Cc: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/gre.h

@@ -26,7 +26,7 @@ int gre_del_protocol(const struct gre_protocol *proto, u8 version);
 struct net_device *gretap_fb_dev_create(struct net *net, const char *name,
 				       u8 name_assign_type);
 int gre_parse_header(struct sk_buff *skb, struct tnl_ptk_info *tpi,
-		     bool *csum_err, __be16 proto);
+		     bool *csum_err, __be16 proto, int nhs);
 
 static inline int gre_calc_hlen(__be16 o_flags)
 {

net/ipv4/gre_demux.c

@@ -62,26 +62,26 @@ EXPORT_SYMBOL_GPL(gre_del_protocol);
 
 /* Fills in tpi and returns header length to be pulled. */
 int gre_parse_header(struct sk_buff *skb, struct tnl_ptk_info *tpi,
-		     bool *csum_err, __be16 proto)
+		     bool *csum_err, __be16 proto, int nhs)
 {
 	const struct gre_base_hdr *greh;
 	__be32 *options;
 	int hdr_len;
 
-	if (unlikely(!pskb_may_pull(skb, sizeof(struct gre_base_hdr))))
+	if (unlikely(!pskb_may_pull(skb, nhs + sizeof(struct gre_base_hdr))))
 		return -EINVAL;
 
-	greh = (struct gre_base_hdr *)skb_transport_header(skb);
+	greh = (struct gre_base_hdr *)(skb->data + nhs);
 	if (unlikely(greh->flags & (GRE_VERSION | GRE_ROUTING)))
 		return -EINVAL;
 
 	tpi->flags = gre_flags_to_tnl_flags(greh->flags);
 	hdr_len = gre_calc_hlen(tpi->flags);
 
-	if (!pskb_may_pull(skb, hdr_len))
+	if (!pskb_may_pull(skb, nhs + hdr_len))
 		return -EINVAL;
 
-	greh = (struct gre_base_hdr *)skb_transport_header(skb);
+	greh = (struct gre_base_hdr *)(skb->data + nhs);
 	tpi->proto = greh->protocol;
 
 	options = (__be32 *)(greh + 1);

net/ipv4/ip_gre.c

@@ -49,12 +49,6 @@
 #include <net/gre.h>
 #include <net/dst_metadata.h>
 
-#if IS_ENABLED(CONFIG_IPV6)
-#include <net/ipv6.h>
-#include <net/ip6_fib.h>
-#include <net/ip6_route.h>
-#endif
-
 /*
    Problems & solutions
    --------------------
@@ -217,12 +211,14 @@ static void gre_err(struct sk_buff *skb, u32 info)
 	 * by themselves???
 	 */
 
+	const struct iphdr *iph = (struct iphdr *)skb->data;
 	const int type = icmp_hdr(skb)->type;
 	const int code = icmp_hdr(skb)->code;
 	struct tnl_ptk_info tpi;
 	bool csum_err = false;
 
-	if (gre_parse_header(skb, &tpi, &csum_err, htons(ETH_P_IP)) < 0) {
+	if (gre_parse_header(skb, &tpi, &csum_err, htons(ETH_P_IP),
+			     iph->ihl * 4) < 0) {
 		if (!csum_err)		/* ignore csum errors. */
 			return;
 	}
@@ -338,7 +334,7 @@ static int gre_rcv(struct sk_buff *skb)
 	}
 #endif
 
-	hdr_len = gre_parse_header(skb, &tpi, &csum_err, htons(ETH_P_IP));
+	hdr_len = gre_parse_header(skb, &tpi, &csum_err, htons(ETH_P_IP), 0);
 	if (hdr_len < 0)
 		goto drop;
 

net/ipv6/ip6_gre.c

@@ -468,7 +468,7 @@ static int gre_rcv(struct sk_buff *skb)
 	bool csum_err = false;
 	int hdr_len;
 
-	hdr_len = gre_parse_header(skb, &tpi, &csum_err, htons(ETH_P_IPV6));
+	hdr_len = gre_parse_header(skb, &tpi, &csum_err, htons(ETH_P_IPV6), 0);
 	if (hdr_len < 0)
 		goto drop;