xfrm6: Fix the nexthdr offset in _decode_session6.

xfrm_decode_session() was originally designed for the
usage in the receive path where the correct nexthdr offset
is stored in IP6CB(skb)->nhoff. Over time this function
spread to code that is used in the output path (netfilter,
vti) where IP6CB(skb)->nhoff is not set. As a result, we
get a wrong nexthdr and the upper layer flow informations
are wrong. This can leed to incorrect policy lookups.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

1 files changed, 7 insertions, 1 deletions

diff --git a/net/ipv6/xfrm6_policy.c b/net/ipv6/xfrm6_policy.c
index aa48302f00a1..48bf5a06847b 100644
--- a/net/ipv6/xfrm6_policy.c
+++ b/net/ipv6/xfrm6_policy.c
@@ -134,8 +134,14 @@ _decode_session6(struct sk_buff *skb, struct flowi *fl, int reverse)
         u16 offset = sizeof(*hdr);
         struct ipv6_opt_hdr *exthdr;
         const unsigned char *nh = skb_network_header(skb);
-        u8 nexthdr = nh[IP6CB(skb)->nhoff];
+        u16 nhoff = IP6CB(skb)->nhoff;
         int oif = 0;
+        u8 nexthdr;
+
+        if (!nhoff)
+                nhoff = offsetof(struct ipv6hdr, nexthdr);
+
+        nexthdr = nh[nhoff];
 
         if (skb_dst(skb))
                 oif = skb_dst(skb)->dev->ifindex;