bpo-36037: Fix test_ssl for strict OpenSSL policy (GH-11940)

Fix test_ssl for strict OpenSSL configuration like RHEL8 strict crypto policy.
Use older TLS version for minimum TLS version of the server SSL context if
needed, to test TLS version older than default minimum TLS version.

Author: vstinner

Lib/test/test_ssl.py

@@ -33,6 +33,19 @@
 IS_OPENSSL_1_1_1 = not IS_LIBRESSL and ssl.OPENSSL_VERSION_INFO >= (1, 1, 1)
 PY_SSL_DEFAULT_CIPHERS = sysconfig.get_config_var('PY_SSL_DEFAULT_CIPHERS')
 
+PROTOCOL_TO_TLS_VERSION = {}
+for proto, ver in (
+    ("PROTOCOL_SSLv23", "SSLv3"),
+    ("PROTOCOL_TLSv1", "TLSv1"),
+    ("PROTOCOL_TLSv1_1", "TLSv1_1"),
+):
+    try:
+        proto = getattr(ssl, proto)
+        ver = getattr(ssl.TLSVersion, ver)
+    except AttributeError:
+        continue
+    PROTOCOL_TO_TLS_VERSION[proto] = ver
+
 def data_file(*name):
     return os.path.join(os.path.dirname(__file__), *name)
 
@@ -1092,7 +1105,11 @@ def test_min_max_version(self):
         # Fedora override the setting to TLS 1.0.
         self.assertIn(
             ctx.minimum_version,
-            {ssl.TLSVersion.MINIMUM_SUPPORTED, ssl.TLSVersion.TLSv1}
+            {ssl.TLSVersion.MINIMUM_SUPPORTED,
+             # Fedora 29 uses TLS 1.0 by default
+             ssl.TLSVersion.TLSv1,
+             # RHEL 8 uses TLS 1.2 by default
+             ssl.TLSVersion.TLSv1_2}
         )
         self.assertEqual(
             ctx.maximum_version, ssl.TLSVersion.MAXIMUM_SUPPORTED
@@ -2609,6 +2626,17 @@ def try_protocol_combo(server_protocol, client_protocol, expect_success,
     server_context = ssl.SSLContext(server_protocol)
     server_context.options |= server_options
 
+    min_version = PROTOCOL_TO_TLS_VERSION.get(client_protocol, None)
+    if (min_version is not None
+    # SSLContext.minimum_version is only available on recent OpenSSL
+    # (setter added in OpenSSL 1.1.0, getter added in OpenSSL 1.1.1)
+    and hasattr(server_context, 'minimum_version')
+    and server_protocol == ssl.PROTOCOL_TLS
+    and server_context.minimum_version > min_version):
+        # If OpenSSL configuration is strict and requires more recent TLS
+        # version, we have to change the minimum to test old TLS versions.
+        server_context.minimum_version = min_version
+
     # NOTE: we must enable "ALL" ciphers on the client, otherwise an
     # SSLv23 client will send an SSLv3 hello (rather than SSLv2)
     # starting from OpenSSL 1.0.0 (see issue #8322).

Misc/NEWS.d/next/Tests/2019-02-19-15-21-14.bpo-36037.75wG9_.rst

@@ -0,0 +1,3 @@
+Fix test_ssl for strict OpenSSL configuration like RHEL8 strict crypto policy.
+Use older TLS version for minimum TLS version of the server SSL context if
+needed, to test TLS version older than default minimum TLS version.