sctp: do not allow the v4 socket to bind a v4mapped v6 address

The check in sctp_sockaddr_af is not robust enough to forbid binding a
v4mapped v6 addr on a v4 socket.

The worse thing is that v4 socket's bind_verify would not convert this
v4mapped v6 addr to a v4 addr. syzbot even reported a crash as the v4
socket bound a v6 addr.

This patch is to fix it by doing the common sa.sa_family check first,
then AF_INET check for v4mapped v6 addrs.

Fixes: 7dab83de ("sctp: Support ipv6only AF_INET6 sockets.")
Reported-by: syzbot+7b7b518b1228d2743963@syzkaller.appspotmail.com
Acked-by: NNeil Horman <nhorman@tuxdriver.com>
Signed-off-by: NXin Long <lucien.xin@gmail.com>
Acked-by: NMarcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sctp/socket.c

@@ -335,16 +335,14 @@ static struct sctp_af *sctp_sockaddr_af(struct sctp_sock *opt,
 	if (len < sizeof (struct sockaddr))
 		return NULL;
 
+	if (!opt->pf->af_supported(addr->sa.sa_family, opt))
+		return NULL;
+
 	/* V4 mapped address are really of AF_INET family */
 	if (addr->sa.sa_family == AF_INET6 &&
-	    ipv6_addr_v4mapped(&addr->v6.sin6_addr)) {
-		if (!opt->pf->af_supported(AF_INET, opt))
-			return NULL;
-	} else {
-		/* Does this PF support this AF? */
-		if (!opt->pf->af_supported(addr->sa.sa_family, opt))
-			return NULL;
-	}
+	    ipv6_addr_v4mapped(&addr->v6.sin6_addr) &&
+	    !opt->pf->af_supported(AF_INET, opt))
+		return NULL;
 
 	/* If we get this far, af is valid. */
 	af = sctp_get_af_specific(addr->sa.sa_family);