net: Fix GRE RX to use skb_transport_header for GRE header offset

GRE assumes that the GRE header is at skb_network_header +
ip_hrdlen(skb). It is more general to use skb_transport_header
and this allows the possbility of inserting additional header
between IP and GRE (which is what we will done in Generic UDP
Encapsulation for GRE).

Signed-off-by: Tom Herbert <therbert@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 3 deletions

diff --git a/net/ipv4/gre_demux.c b/net/ipv4/gre_demux.c
index 7e0756da8737..4a7b5b2a1ce3 100644
--- a/net/ipv4/gre_demux.c
+++ b/net/ipv4/gre_demux.c
@@ -98,7 +98,6 @@ EXPORT_SYMBOL_GPL(gre_build_header);
 static int parse_gre_header(struct sk_buff *skb, struct tnl_ptk_info *tpi,
                             bool *csum_err)
 {
-        unsigned int ip_hlen = ip_hdrlen(skb);
         const struct gre_base_hdr *greh;
         __be32 *options;
         int hdr_len;
@@ -106,7 +105,7 @@ static int parse_gre_header(struct sk_buff *skb, struct tnl_ptk_info *tpi,
         if (unlikely(!pskb_may_pull(skb, sizeof(struct gre_base_hdr))))
                 return -EINVAL;
 
-        greh = (struct gre_base_hdr *)(skb_network_header(skb) + ip_hlen);
+        greh = (struct gre_base_hdr *)skb_transport_header(skb);
         if (unlikely(greh->flags & (GRE_VERSION | GRE_ROUTING)))
                 return -EINVAL;
 
@@ -116,7 +115,7 @@ static int parse_gre_header(struct sk_buff *skb, struct tnl_ptk_info *tpi,
         if (!pskb_may_pull(skb, hdr_len))
                 return -EINVAL;
 
-        greh = (struct gre_base_hdr *)(skb_network_header(skb) + ip_hlen);
+        greh = (struct gre_base_hdr *)skb_transport_header(skb);
         tpi->proto = greh->protocol;
 
         options = (__be32 *)(greh + 1);