Smack: Fix IPv6 handling of 0 secmark

Handle the case where the skb for an IPv6 packet contains
a 0 in the secmark for a packet generated locally. This
can only happen for system packets, so allow the access.
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>

security/smack/smack_lsm.c

@@ -3907,6 +3907,8 @@ static int smack_socket_sock_rcv_skb(struct sock *sk, struct sk_buff *skb)
 #ifdef SMACK_IPV6_SECMARK_LABELING
 		if (skb && skb->secmark != 0)
 			skp = smack_from_secid(skb->secmark);
+		else if (smk_ipv6_localhost(&sadd))
+			break;
 		else
 			skp = smack_ipv6host_label(&sadd);
 		if (skp == NULL)