address open redirect with 3 forward slashes

Ie36401c782f023d1d5f2623732619105dc2cfa24 was intended
to address OSSA-2021-002 (CVE-2021-3654) however after its
release it was discovered that the fix only worked
for urls with 2 leading slashes or more then 4.

This change adresses the missing edgecase for 3 leading slashes
and also maintian support for rejecting 2+.

Conflicts:
  nova/console/websocketproxy.py
  nova/tests/unit/console/test_websocketproxy.py

NOTE(melwitt): The conflict and difference in websocketproxy.py from
the cherry picked change: HTTPStatus.BAD_REQUEST => 400 is due to the
fact that HTTPStatus does not exist in Python 2.7. The conflict in
test_websocketproxy.py is because change
I23ac1cc79482d0fabb359486a4b934463854cae5 (Allow TLS ciphers/protocols
to be configurable for console proxies) is not in Train. The difference
in test_websocketproxy.py from the cherry picked change is due to a
difference in internal implementation [1] in Python < 3.6. See change
I546d376869a992601b443fb95acf1034da2a8f36 for reference.

[1] 34eeed4290

Change-Id: I95f68be76330ff09e5eabb5ef8dd9a18f5547866
co-authored-by: Matteo Pozza
Closes-Bug: #1927677
(cherry picked from commit 6fbd0b758dcac71323f3be179b1a9d1c17a4acc5)
(cherry picked from commit 47dad4836a26292e9d34e516e1525ecf00be127c)
(cherry picked from commit 9588cdbfd4649ea53d60303f2d10c5d62a070a07)
(cherry picked from commit 0997043f459ac616b594363b5b253bd0ae6ed9eb)

nova/console/websocketproxy.py

@@ -313,14 +313,9 @@ class NovaProxyRequestHandler(NovaProxyRequestHandlerBase,
         if os.path.isdir(path):
             parts = urlparse.urlsplit(self.path)
             if not parts.path.endswith('/'):
-                # redirect browser - doing basically what apache does
-                new_parts = (parts[0], parts[1], parts[2] + '/',
-                              parts[3], parts[4])
-                new_url = urlparse.urlunsplit(new_parts)
-
                 # Browsers interpret "Location: //uri" as an absolute URI
                 # like "http://URI"
-                if new_url.startswith('//'):
+                if self.path.startswith('//'):
                     self.send_error(400,
                                     "URI must not start with //")
                     return None

nova/tests/unit/console/test_websocketproxy.py

@@ -659,6 +659,38 @@ class NovaProxyRequestHandlerBaseTestCase(test.NoDBTestCase):
         # Verify no redirect happens and instead a 400 Bad Request is returned.
         self.assertIn('400 URI must not start with //', result[0].decode())
 
+    def test_reject_open_redirect_3_slashes(self):
+        # This will test the behavior when an attempt is made to cause an open
+        # redirect. It should be rejected.
+        mock_req = mock.MagicMock()
+        mock_req.makefile().readline.side_effect = [
+            b'GET ///example.com/%2F.. HTTP/1.1\r\n',
+            b''
+        ]
+
+        client_addr = ('8.8.8.8', 54321)
+        mock_server = mock.MagicMock()
+        # This specifies that the server will be able to handle requests other
+        # than only websockets.
+        mock_server.only_upgrade = False
+
+        # Constructing a handler will process the mock_req request passed in.
+        handler = websocketproxy.NovaProxyRequestHandler(
+            mock_req, client_addr, mock_server)
+
+        # Collect the response data to verify at the end. The
+        # SimpleHTTPRequestHandler writes the response data to a 'wfile'
+        # attribute.
+        output = io.BytesIO()
+        handler.wfile = output
+        # Process the mock_req again to do the capture.
+        handler.do_GET()
+        output.seek(0)
+        result = output.readlines()
+
+        # Verify no redirect happens and instead a 400 Bad Request is returned.
+        self.assertIn('400 URI must not start with //', result[0].decode())
+
 
 class NovaWebsocketSecurityProxyTestCase(test.NoDBTestCase):