Perform manual validity checking of the X.509 certificate for constraints.

Given that we're getting a constraint so that we can validate time, if our
own time is out we can fail the automatic validity checking since it is
based on the wallclock. Instead, disable the automatic validity checking
and perform manual checks based on the time reported from the server via
the HTTP header.
Discussed at length with and ok deraadt@

src/usr.sbin/ntpd/constraint.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: constraint.c,v 1.39 2019/01/20 16:40:42 otto Exp $	*/
+/*	$OpenBSD: constraint.c,v 1.40 2019/01/21 08:38:22 jsing Exp $	*/
 
 /*
  * Copyright (c) 2015 Reyk Floeter <reyk@openbsd.org>
@@ -874,6 +874,13 @@ httpsdate_init(const char *addr, const char *port, const char *hostname,
 	if (tls_config_set_ca_mem(httpsdate->tls_config, ca, ca_len) == -1)
 		goto fail;
 
+	/*
+	 * Due to the fact that we're trying to determine a constraint for time
+	 * we do our own certificate validity checking, since the automatic
+	 * version is based on our wallclock, which may well be inaccurate...
+	 */
+	tls_config_insecure_noverifytime(httpsdate->tls_config);
+
 	return (httpsdate);
 
  fail:
@@ -904,6 +911,7 @@ httpsdate_request(struct httpsdate *httpsdate, struct timeval *when)
 {
 	size_t	 outlen = 0, maxlength = CONSTRAINT_MAXHEADERLENGTH, len;
 	char	*line, *p, *buf;
+	time_t	 httptime;
 	ssize_t	 ret;
 
 	if ((httpsdate->tls_ctx = tls_client()) == NULL)
@@ -972,6 +980,19 @@ httpsdate_request(struct httpsdate *httpsdate, struct timeval *when)
 		free(line);
 	}
 
+	/*
+	 * Now manually check the validity of the certificate presented in the
+	 * TLS handshake, based on the time specified by the server's HTTP Date:
+	 * header.
+	 */
+	httptime = timegm(&httpsdate->tls_tm);
+	if (httptime <= tls_peer_cert_notbefore(httpsdate->tls_ctx) ||
+	    httptime >= tls_peer_cert_notafter(httpsdate->tls_ctx)) {
+		log_warnx("tls certificate invalid: %s (%s):",
+		    httpsdate->tls_addr, httpsdate->tls_hostname);
+		goto fail;
+	}
+
 	return (0);
 
  fail: