Improve TLSv1.3 server handling of no shared groups.

While we currently correctly handle the no-shared-group case, it currently
fails late when we try to create the key share. Improve detection and
handling so that we fail sooner and send an alert to the client when
processing client key shares.

While here rename preferred_group_found to shared_group_found - we look for
the client preferred group, but any group that we select will always be in
the client list (even if it's the last one).

Reported by the tlspuffin team.

ok tb@

lib/libssl/ssl_tlsext.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: ssl_tlsext.c,v 1.162 2026/06/08 12:05:25 tb Exp $ */
+/* $OpenBSD: ssl_tlsext.c,v 1.163 2026/06/14 15:47:49 jsing Exp $ */
 /*
  * Copyright (c) 2016, 2017, 2019 Joel Sing <jsing@openbsd.org>
  * Copyright (c) 2017 Doug Hogan <doug@openbsd.org>
@@ -1501,7 +1501,7 @@ tlsext_keyshare_server_process(SSL *s, uint16_t msg_type, CBS *cbs, int *alert)
 	const uint16_t *client_groups = NULL, *server_groups = NULL;
 	size_t client_groups_len = 0, server_groups_len = 0;
 	size_t i, j, client_groups_index;
-	int preferred_group_found = 0;
+	int shared_group_found = 0;
 	int decode_error;
 	uint16_t client_preferred_group = 0;
 	uint16_t group;
@@ -1577,21 +1577,32 @@ tlsext_keyshare_server_process(SSL *s, uint16_t msg_type, CBS *cbs, int *alert)
 
 	/*
 	 * Find the group that is most preferred by the client that
-	 * we also support.
+	 * is also supported by the server.
 	 */
-	for (i = 0; i < client_groups_len && !preferred_group_found; i++) {
+	for (i = 0; i < client_groups_len && !shared_group_found; i++) {
 		if (!ssl_security_supported_group(s, client_groups[i]))
 			continue;
 		for (j = 0; j < server_groups_len; j++) {
 			if (server_groups[j] == client_groups[i]) {
+				/* XXX - this should be equivalent to tls1_check_group() */
 				client_preferred_group = client_groups[i];
 				s->s3->hs.tls13.server_group = client_preferred_group;
-				preferred_group_found = 1;
+				shared_group_found = 1;
 				break;
 			}
 		}
 	}
 
+	if (!shared_group_found) {
+		/*
+		 * There are no supported groups that are shared between the
+		 * client and server - this is treated as a handshake failure
+		 * or as insufficient security - see RFC 8446 section 4.1.1.
+		 */
+		*alert = TLS13_ALERT_HANDSHAKE_FAILURE;
+		return 0;
+	}
+
 	if (!CBS_get_u16_length_prefixed(cbs, &client_shares))
 		return 0;
 
@@ -1637,7 +1648,7 @@ tlsext_keyshare_server_process(SSL *s, uint16_t msg_type, CBS *cbs, int *alert)
 		 * less preferred, and we choose to to use it instead of
 		 * requesting the more preferred group.
 		 */
-		if (!preferred_group_found || group != client_preferred_group)
+		if (group != client_preferred_group)
 			continue;
 
 		/* Decode and store the selected key share. */