Only perform the downgrade check if our max version is less than TLSv1.3.

Issue noticed by kn@ when talking to a TLSv1.3 capable mail server, but
with smtpd capping max version to TLSv1.2.

ok beck@

1 files changed, 17 insertions, 15 deletions

diff --git a/src/lib/libssl/tls13_client.c b/src/lib/libssl/tls13_client.c
index 737a1015a5..fb21b54621 100644
--- a/src/lib/libssl/tls13_client.c
+++ b/src/lib/libssl/tls13_client.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: tls13_client.c,v 1.33 2020/01/25 09:20:56 jsing Exp $ */
+/* $OpenBSD: tls13_client.c,v 1.34 2020/01/25 14:23:27 jsing Exp $ */
 /*
  * Copyright (c) 2018, 2019 Joel Sing <jsing@openbsd.org>
  *
@@ -288,20 +288,22 @@ tls13_server_hello_process(struct tls13_ctx *ctx, CBS *cbs)
                 goto err;
 
         if (tls13_server_hello_is_legacy(cbs)) {
-                /*
-                 * RFC 8446 section 4.1.3, We must not downgrade if
-                 * the server random value contains the TLS 1.2 or 1.1
-                 * magical value.
-                 */
-                if (!CBS_skip(&server_random, CBS_len(&server_random) -
-                    sizeof(tls13_downgrade_12)))
-                        goto err;
-                if (CBS_mem_equal(&server_random, tls13_downgrade_12,
-                    sizeof(tls13_downgrade_12)) ||
-                    CBS_mem_equal(&server_random, tls13_downgrade_11,
-                    sizeof(tls13_downgrade_11))) {
-                        ctx->alert = SSL_AD_ILLEGAL_PARAMETER;
-                        goto err;
+                if (ctx->hs->max_version >= TLS1_3_VERSION) {
+                        /*
+                         * RFC 8446 section 4.1.3, We must not downgrade if
+                         * the server random value contains the TLS 1.2 or 1.1
+                         * magical value.
+                         */
+                        if (!CBS_skip(&server_random, CBS_len(&server_random) -
+                            sizeof(tls13_downgrade_12)))
+                                goto err;
+                        if (CBS_mem_equal(&server_random, tls13_downgrade_12,
+                            sizeof(tls13_downgrade_12)) ||
+                            CBS_mem_equal(&server_random, tls13_downgrade_11,
+                            sizeof(tls13_downgrade_11))) {
+                                ctx->alert = SSL_AD_ILLEGAL_PARAMETER;
+                                goto err;
+                        }
                 }
 
                 if (!CBS_skip(cbs, CBS_len(cbs)))