apparmor: mediate the implicit connect of TCP fast open sendmsg

sendmsg()/sendto() with MSG_FASTOPEN is a combination of connect(2) and
write(2): it opens the connection in the SYN. apparmor_socket_sendmsg()
only checks AA_MAY_SEND, so a profile that grants send but denies connect
lets a confined task open an outbound TCP/MPTCP connection that connect(2)
would have refused, bypassing connect mediation.

Mediate the implicit connect when MSG_FASTOPEN is set and a destination
is supplied. Add it to apparmor_socket_sendmsg() (not the shared
aa_sock_msg_perm() helper, which recvmsg also uses) and call aa_sk_perm()
directly, mirroring the selinux and tomoyo fixes. sk_is_tcp() does not
cover MPTCP fast open, so the SOCK_STREAM/IPPROTO_MPTCP arm is explicit.

Fixes: cf60af0 ("net-tcp: Fast Open client - sendmsg(MSG_FASTOPEN)")
Cc: stable@vger.kernel.org
Signed-off-by: Bryam Vargas <hexlabsecurity@proton.me>
Signed-off-by: John Johansen <john.johansen@canonical.com>

Author: bryamzxz

security/apparmor/lsm.c

@@ -1422,7 +1422,21 @@ static int aa_sock_msg_perm(const char *op, u32 request, struct socket *sock,
 static int apparmor_socket_sendmsg(struct socket *sock,
 				   struct msghdr *msg, int size)
 {
-	return aa_sock_msg_perm(OP_SENDMSG, AA_MAY_SEND, sock, msg, size);
+	int error = aa_sock_msg_perm(OP_SENDMSG, AA_MAY_SEND, sock, msg, size);
+
+	if (error)
+		return error;
+
+	/* TCP fast open carries connect() semantics in sendmsg(); mediate
+	 * the implicit connect so it cannot bypass the connect permission.
+	 */
+	if ((msg->msg_flags & MSG_FASTOPEN) && msg->msg_name &&
+	    (sk_is_tcp(sock->sk) ||
+	     (sk_is_inet(sock->sk) && sock->sk->sk_type == SOCK_STREAM &&
+	      sock->sk->sk_protocol == IPPROTO_MPTCP)))
+		error = aa_sk_perm(OP_CONNECT, AA_MAY_CONNECT, sock->sk);
+
+	return error;
 }
 
 static int apparmor_socket_recvmsg(struct socket *sock,