netfilter: ipv6: fix crash caused by ipv6_find_hdr()

When calling:
    ipv6_find_hdr(skb, &thoff, -1, NULL)
on a fragmented packet, thoff would be left with a random
value causing callers to read random memory offsets with:
    skb_header_pointer(skb, thoff, ...)

Now we force ipv6_find_hdr() to return a failure in this case.
Calling:
  ipv6_find_hdr(skb, &thoff, -1, &fragoff)
will set fragoff as expected, and not return a failure.

Change-Id: Ib474e8a4267dd2b300feca325811330329684a88
Signed-off-by: JP Abgrall <jpa@google.com>

net/ipv6/exthdrs_core.c

@@ -166,15 +166,15 @@ EXPORT_SYMBOL_GPL(ipv6_find_tlv);
  * to explore inner IPv6 header, eg. ICMPv6 error messages.
  *
  * If target header is found, its offset is set in *offset and return protocol
- * number. Otherwise, return -1.
+ * number. Otherwise, return -ENOENT or -EBADMSG.
  *
  * If the first fragment doesn't contain the final protocol header or
  * NEXTHDR_NONE it is considered invalid.
  *
  * Note that non-1st fragment is special case that "the protocol number
  * of last header" is "next header" field in Fragment header. In this case,
- * *offset is meaningless and fragment offset is stored in *fragoff if fragoff
+ * *offset is meaningless. If fragoff is not NULL, the fragment offset is
- * isn't NULL.
+ * stored in *fragoff; if it is NULL, return -EINVAL.
  *
  * if flags is not NULL and it's a fragment, then the frag flag
  * IP6_FH_F_FRAG will be set. If it's an AH header, the
@@ -253,9 +253,12 @@ int ipv6_find_hdr(const struct sk_buff *skb, unsigned int *offset,
 				if (target < 0 &&
 				    ((!ipv6_ext_hdr(hp->nexthdr)) ||
 				     hp->nexthdr == NEXTHDR_NONE)) {
-					if (fragoff)
+					if (fragoff) {
 						*fragoff = _frag_off;
 						return hp->nexthdr;
+					} else {
+						return -EINVAL;
+					}
 				}
 				return -ENOENT;
 			}