bpo-31453: Add setter for min/max protocol version (#5259)

OpenSSL 1.1 has introduced a new API to set the minimum and maximum
supported protocol version. The API is easier to use than the old
OP_NO_TLS1 option flags, too.

Since OpenSSL has no call to set minimum version to highest supported,
the implementation emulate maximum_version = MINIMUM_SUPPORTED and
minimum_version = MAXIMUM_SUPPORTED by figuring out the minumum and
maximum supported version at compile time.

Signed-off-by: Christian Heimes <christian@python.org>

2 files changed, 151 insertions, 3 deletions

diff --git a/Lib/ssl.py b/Lib/ssl.py
index 75ebcc1..2db8873 100644
--- a/Lib/ssl.py
+++ b/Lib/ssl.py
@@ -112,9 +112,11 @@ except ImportError:
     pass
 
 
-from _ssl import HAS_SNI, HAS_ECDH, HAS_NPN, HAS_ALPN, HAS_TLSv1_3
-from _ssl import _DEFAULT_CIPHERS
-from _ssl import _OPENSSL_API_VERSION
+from _ssl import (
+    HAS_SNI, HAS_ECDH, HAS_NPN, HAS_ALPN, HAS_SSLv2, HAS_SSLv3, HAS_TLSv1,
+    HAS_TLSv1_1, HAS_TLSv1_2, HAS_TLSv1_3
+)
+from _ssl import _DEFAULT_CIPHERS, _OPENSSL_API_VERSION
 
 
 _IntEnum._convert(
@@ -153,6 +155,16 @@ _PROTOCOL_NAMES = {value: name for name, value in _SSLMethod.__members__.items()
 _SSLv2_IF_EXISTS = getattr(_SSLMethod, 'PROTOCOL_SSLv2', None)
 
 
+class TLSVersion(_IntEnum):
+    MINIMUM_SUPPORTED = _ssl.PROTO_MINIMUM_SUPPORTED
+    SSLv3 = _ssl.PROTO_SSLv3
+    TLSv1 = _ssl.PROTO_TLSv1
+    TLSv1_1 = _ssl.PROTO_TLSv1_1
+    TLSv1_2 = _ssl.PROTO_TLSv1_2
+    TLSv1_3 = _ssl.PROTO_TLSv1_3
+    MAXIMUM_SUPPORTED = _ssl.PROTO_MAXIMUM_SUPPORTED
+
+
 if sys.platform == "win32":
     from _ssl import enum_certificates, enum_crls
 
@@ -467,6 +479,25 @@ class SSLContext(_SSLContext):
                 self._load_windows_store_certs(storename, purpose)
         self.set_default_verify_paths()
 
+    if hasattr(_SSLContext, 'minimum_version'):
+        @property
+        def minimum_version(self):
+            return TLSVersion(super().minimum_version)
+
+        @minimum_version.setter
+        def minimum_version(self, value):
+            if value == TLSVersion.SSLv3:
+                self.options &= ~Options.OP_NO_SSLv3
+            super(SSLContext, SSLContext).minimum_version.__set__(self, value)
+
+        @property
+        def maximum_version(self):
+            return TLSVersion(super().maximum_version)
+
+        @maximum_version.setter
+        def maximum_version(self, value):
+            super(SSLContext, SSLContext).maximum_version.__set__(self, value)
+
     @property
     def options(self):
         return Options(super().options)
diff --git a/Lib/test/test_ssl.py b/Lib/test/test_ssl.py
index ca2357e..8d98b80 100644
--- a/Lib/test/test_ssl.py
+++ b/Lib/test/test_ssl.py
@@ -1077,6 +1077,69 @@ class ContextTests(unittest.TestCase):
             with self.assertRaises(AttributeError):
                 ctx.hostname_checks_common_name = True
 
+    @unittest.skipUnless(hasattr(ssl.SSLContext, 'minimum_version'),
+                         "required OpenSSL 1.1.0g")
+    def test_min_max_version(self):
+        ctx = ssl.SSLContext(ssl.PROTOCOL_TLS_SERVER)
+        self.assertEqual(
+            ctx.minimum_version, ssl.TLSVersion.MINIMUM_SUPPORTED
+        )
+        self.assertEqual(
+            ctx.maximum_version, ssl.TLSVersion.MAXIMUM_SUPPORTED
+        )
+
+        ctx.minimum_version = ssl.TLSVersion.TLSv1_1
+        ctx.maximum_version = ssl.TLSVersion.TLSv1_2
+        self.assertEqual(
+            ctx.minimum_version, ssl.TLSVersion.TLSv1_1
+        )
+        self.assertEqual(
+            ctx.maximum_version, ssl.TLSVersion.TLSv1_2
+        )
+
+        ctx.minimum_version = ssl.TLSVersion.MINIMUM_SUPPORTED
+        ctx.maximum_version = ssl.TLSVersion.TLSv1
+        self.assertEqual(
+            ctx.minimum_version, ssl.TLSVersion.MINIMUM_SUPPORTED
+        )
+        self.assertEqual(
+            ctx.maximum_version, ssl.TLSVersion.TLSv1
+        )
+
+        ctx.maximum_version = ssl.TLSVersion.MAXIMUM_SUPPORTED
+        self.assertEqual(
+            ctx.maximum_version, ssl.TLSVersion.MAXIMUM_SUPPORTED
+        )
+
+        ctx.maximum_version = ssl.TLSVersion.MINIMUM_SUPPORTED
+        self.assertIn(
+            ctx.maximum_version,
+            {ssl.TLSVersion.TLSv1, ssl.TLSVersion.SSLv3}
+        )
+
+        ctx.minimum_version = ssl.TLSVersion.MAXIMUM_SUPPORTED
+        self.assertIn(
+            ctx.minimum_version,
+            {ssl.TLSVersion.TLSv1_2, ssl.TLSVersion.TLSv1_3}
+        )
+
+        with self.assertRaises(ValueError):
+            ctx.minimum_version = 42
+
+        ctx = ssl.SSLContext(ssl.PROTOCOL_TLSv1_1)
+
+        self.assertEqual(
+            ctx.minimum_version, ssl.TLSVersion.MINIMUM_SUPPORTED
+        )
+        self.assertEqual(
+            ctx.maximum_version, ssl.TLSVersion.MAXIMUM_SUPPORTED
+        )
+        with self.assertRaises(ValueError):
+            ctx.minimum_version = ssl.TLSVersion.MINIMUM_SUPPORTED
+        with self.assertRaises(ValueError):
+            ctx.maximum_version = ssl.TLSVersion.TLSv1
+
+
     @unittest.skipUnless(have_verify_flags(),
                          "verify_flags need OpenSSL > 0.9.8")
     def test_verify_flags(self):
@@ -3457,6 +3520,60 @@ class ThreadedTests(unittest.TestCase):
                 })
                 self.assertEqual(s.version(), 'TLSv1.3')
 
+    @unittest.skipUnless(hasattr(ssl.SSLContext, 'minimum_version'),
+                         "required OpenSSL 1.1.0g")
+    def test_min_max_version(self):
+        client_context, server_context, hostname = testing_context()
+        # client TLSv1.0 to 1.2
+        client_context.minimum_version = ssl.TLSVersion.TLSv1
+        client_context.maximum_version = ssl.TLSVersion.TLSv1_2
+        # server only TLSv1.2
+        server_context.minimum_version = ssl.TLSVersion.TLSv1_2
+        server_context.maximum_version = ssl.TLSVersion.TLSv1_2
+
+        with ThreadedEchoServer(context=server_context) as server:
+            with client_context.wrap_socket(socket.socket(),
+                                            server_hostname=hostname) as s:
+                s.connect((HOST, server.port))
+                self.assertEqual(s.version(), 'TLSv1.2')
+
+        # client 1.0 to 1.2, server 1.0 to 1.1
+        server_context.minimum_version = ssl.TLSVersion.TLSv1
+        server_context.maximum_version = ssl.TLSVersion.TLSv1_1
+
+        with ThreadedEchoServer(context=server_context) as server:
+            with client_context.wrap_socket(socket.socket(),
+                                            server_hostname=hostname) as s:
+                s.connect((HOST, server.port))
+                self.assertEqual(s.version(), 'TLSv1.1')
+
+        # client 1.0, server 1.2 (mismatch)
+        server_context.minimum_version = ssl.TLSVersion.TLSv1_2
+        server_context.maximum_version = ssl.TLSVersion.TLSv1_2
+        client_context.minimum_version = ssl.TLSVersion.TLSv1
+        client_context.maximum_version = ssl.TLSVersion.TLSv1
+        with ThreadedEchoServer(context=server_context) as server:
+            with client_context.wrap_socket(socket.socket(),
+                                            server_hostname=hostname) as s:
+                with self.assertRaises(ssl.SSLError) as e:
+                    s.connect((HOST, server.port))
+                self.assertIn("alert", str(e.exception))
+
+
+    @unittest.skipUnless(hasattr(ssl.SSLContext, 'minimum_version'),
+                         "required OpenSSL 1.1.0g")
+    @unittest.skipUnless(ssl.HAS_SSLv3, "requires SSLv3 support")
+    def test_min_max_version_sslv3(self):
+        client_context, server_context, hostname = testing_context()
+        server_context.minimum_version = ssl.TLSVersion.SSLv3
+        client_context.minimum_version = ssl.TLSVersion.SSLv3
+        client_context.maximum_version = ssl.TLSVersion.SSLv3
+        with ThreadedEchoServer(context=server_context) as server:
+            with client_context.wrap_socket(socket.socket(),
+                                            server_hostname=hostname) as s:
+                s.connect((HOST, server.port))
+                self.assertEqual(s.version(), 'SSLv3')
+
     @unittest.skipUnless(ssl.HAS_ECDH, "test requires ECDH-enabled OpenSSL")
     def test_default_ecdh_curve(self):
         # Issue #21015: elliptic curve-based Diffie Hellman key exchange