Don't detect a downgrade where the server has a protocol version hole

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7013)

ssl/statem/statem_lib.c

@@ -1665,9 +1665,16 @@ static void check_for_downgrade(SSL *s, int vers, DOWNGRADE *dgrd)
     if (vers == TLS1_2_VERSION
             && ssl_version_supported(s, TLS1_3_VERSION, NULL)) {
         *dgrd = DOWNGRADE_TO_1_2;
-    } else if (!SSL_IS_DTLS(s) && vers < TLS1_2_VERSION
-            && (ssl_version_supported(s, TLS1_2_VERSION, NULL)
-                || ssl_version_supported(s, TLS1_3_VERSION, NULL))) {
+    } else if (!SSL_IS_DTLS(s)
+            && vers < TLS1_2_VERSION
+               /*
+                * We need to ensure that a server that disables TLSv1.2
+                * (creating a hole between TLSv1.3 and TLSv1.1) can still
+                * complete handshakes with clients that support TLSv1.2 and
+                * below. Therefore we do not enable the sentinel if TLSv1.3 is
+                * enabled and TLSv1.2 is not.
+                */
+            && ssl_version_supported(s, TLS1_2_VERSION, NULL)) {
         *dgrd = DOWNGRADE_TO_1_1;
     } else {
         *dgrd = DOWNGRADE_NONE;

test/recipes/70-test_tls13downgrade.t

@@ -45,7 +45,7 @@ use constant {
 $proxy->filter(\&downgrade_filter);
 my $testtype = DOWNGRADE_TO_TLS_1_2;
 $proxy->start() or plan skip_all => "Unable to start up Proxy for tests";
-plan tests => 5;
+plan tests => 6;
 ok(TLSProxy::Message->fail(), "Downgrade TLSv1.3 to TLSv1.2");
 
 #Test 2: Downgrade from TLSv1.3 to TLSv1.1
@@ -74,13 +74,20 @@ ok(TLSProxy::Message->fail()
    "Fallback from TLSv1.3");
 
 SKIP: {
-    skip "TLSv1.1 disabled", 1 if disabled("tls1_1");
-    #Test 5: A protocol "hole" should not be detected as a downgrade
+    skip "TLSv1.1 disabled", 2 if disabled("tls1_1");
+    #Test 5: A client side protocol "hole" should not be detected as a downgrade
     $proxy->clear();
     $proxy->filter(undef);
     $proxy->clientflags("-no_tls1_2");
     $proxy->start();
-    ok(TLSProxy::Message->success(), "TLSv1.2 protocol hole");
+    ok(TLSProxy::Message->success(), "TLSv1.2 client-side protocol hole");
+
+    #Test 6: A server side protocol "hole" should not be detected as a downgrade
+    $proxy->clear();
+    $proxy->filter(undef);
+    $proxy->serverflags("-no_tls1_2");
+    $proxy->start();
+    ok(TLSProxy::Message->success(), "TLSv1.2 server-side protocol hole");
 }
 
 sub downgrade_filter