selinux: Fix packet forwarding checks on postrouting

The IPSKB_FORWARDED and IP6SKB_FORWARDED flags are used only in the
multicast forwarding case to indicate that a packet looped back after
forward. So these flags are not a good indicator for packet forwarding.
A better indicator is the incoming interface. If we have no socket context,
but an incoming interface and we see the packet in the ip postroute hook,
the packet is going to be forwarded.

With this patch we use the incoming interface as an indicator on packet
forwarding.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Acked-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: Eric Paris <eparis@redhat.com>

security/selinux/hooks.c

@@ -4475,27 +4475,14 @@ static unsigned int selinux_ip_postroute(struct sk_buff *skb, int ifindex,
 	 * from the sending socket, otherwise use the kernel's sid */
 	sk = skb->sk;
 	if (sk == NULL) {
-		switch (family) {
-		case PF_INET:
-			if (IPCB(skb)->flags & IPSKB_FORWARDED)
-				secmark_perm = PACKET__FORWARD_OUT;
-			else
-				secmark_perm = PACKET__SEND;
-			break;
-		case PF_INET6:
-			if (IP6CB(skb)->flags & IP6SKB_FORWARDED)
-				secmark_perm = PACKET__FORWARD_OUT;
-			else
-				secmark_perm = PACKET__SEND;
-			break;
-		default:
-			return NF_DROP_ERR(-ECONNREFUSED);
-		}
-		if (secmark_perm == PACKET__FORWARD_OUT) {
+		if (skb->skb_iif) {
+			secmark_perm = PACKET__FORWARD_OUT;
 			if (selinux_skb_peerlbl_sid(skb, family, &peer_sid))
 				return NF_DROP;
-		} else
+		} else {
+			secmark_perm = PACKET__SEND;
 			peer_sid = SECINITSID_KERNEL;
+		}
 	} else {
 		struct sk_security_struct *sksec = sk->sk_security;
 		peer_sid = sksec->sid;