Smack: type confusion in smak sendmsg() handler

Smack security handler for sendmsg() syscall
is vulnerable to type confusion issue what
can allow to privilege escalation into root
or cause denial of service.

A malicious attacker can create socket of one
type for example AF_UNIX and pass is into
sendmsg() function ensuring that this is
AF_INET socket.

Remedy
Do not trust user supplied data.
Proposed fix below.
Signed-off-by: Roman Kubiak <r.kubiak@samsung.com>
Signed-off-by: Mateusz Fruba <m.fruba@samsung.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>

security/smack/smack_lsm.c

@@ -3780,7 +3780,7 @@ static int smack_socket_sendmsg(struct socket *sock, struct msghdr *msg,
 	if (sip == NULL)
 		return 0;
 
-	switch (sip->sin_family) {
+	switch (sock->sk->sk_family) {
 	case AF_INET:
 		rc = smack_netlabel_send(sock->sk, sip);
 		break;