[PATCH 4 of 5] sslutil: more robustly detect protocol support

[Gregory Szorc]

# HG changeset patch
# User Gregory Szorc <gregory.szorc at gmail.com>
# Date 1468779391 25200
#      Sun Jul 17 11:16:31 2016 -0700
# Node ID 306645544688957bf8729e1b03301e5240b0b8ed
# Parent  418f9f9b31c31e53bd233047be49e3993ceccfc1
sslutil: more robustly detect protocol support

The Python ssl module conditionally sets the TLS 1.1 and TLS 1.2
constants depending on whether HAVE_TLSv1_2 is defined. Yes, these
are both tied to the same constant (I would think there would be
separate constants for each version). Perhaps support for TLS 1.1
and 1.2 were added at the same time and the assumption is that
OpenSSL either has neither or both. I don't know.

Strictly speaking, we should introduce hghave tests for TLS 1.2 and
modify tests accordingly. However, I'm inclined to leave things as
they are because I'm curious what environments don't have TLS 1.2
in 2016 and want someone to report a bug so we can find and
erradicate installs/distributions that are shipping an insecure
Python 2.7.

diff --git a/mercurial/sslutil.py b/mercurial/sslutil.py
--- a/mercurial/sslutil.py
+++ b/mercurial/sslutil.py
@@ -32,16 +32,24 @@ from . import (
 configprotocols = set([
     'tls1.0',
     'tls1.1',
     'tls1.2',
 ])
 
 hassni = getattr(ssl, 'HAS_SNI', False)
 
+# TLS 1.1 and 1.2 may not be supported if the OpenSSL Python is compiled
+# against don't support them.
+supportedprotocols = set(['tls1.0'])
+if getattr(ssl, 'PROTOCOL_TLSv1_1', 0):
+    supportedprotocols.add('tls1.1')
+if getattr(ssl, 'PROTOCOL_TLSv1_2', 0):
+    supportedprotocols.add('tls1.2')
+
 try:
     # ssl.SSLContext was added in 2.7.9 and presence indicates modern
     # SSL/TLS features are available.
     SSLContext = ssl.SSLContext
     modernssl = True
     _canloaddefaultcerts = util.safehasattr(SSLContext, 'load_default_certs')
 except AttributeError:
     modernssl = False
@@ -143,25 +151,23 @@ def _hostsettings(ui, hostname):
     def validateprotocol(protocol, key):
         if protocol not in configprotocols:
             raise error.Abort(
                 _('unsupported protocol from hostsecurity.%s: %s') %
                 (key, protocol),
                 hint=_('valid protocols: %s') %
                      ' '.join(sorted(configprotocols)))
 
-    # Legacy Python can only do TLS 1.0. We default to TLS 1.1+ where we
-    # can because TLS 1.0 has known vulnerabilities (like BEAST and POODLE).
-    # We allow users to downgrade to TLS 1.0+ via config options in case a
-    # legacy server is encountered.
-    if modernssl:
+    # We default to TLS 1.1+ where we can because TLS 1.0 has known
+    # vulnerabilities (like BEAST and POODLE). We allow users to downgrade to
+    # TLS 1.0+ via config options in case a legacy server is encountered.
+    if 'tls1.1' in supportedprotocols:
         defaultprotocol = 'tls1.1'
     else:
-        # Let people on legacy Python versions know they are borderline
-        # secure.
+        # Let people know they are borderline secure.
         # We don't document this config option because we want people to see
         # the bold warnings on the web site.
         # internal config: hostsecurity.disabletls10warning
         if not ui.configbool('hostsecurity', 'disabletls10warning'):
             ui.warn(_('warning: connecting to %s using legacy security '
                       'technology (TLS 1.0); see '
                       'https://mercurial-scm.org/wiki/SecureConnections for '
                       'more info\n') % hostname)
@@ -283,17 +289,17 @@ def protocolsettings(protocol):
     # support TLS 1.2.
     #
     # The PROTOCOL_TLSv* constants select a specific TLS version
     # only (as opposed to multiple versions). So the method for
     # supporting multiple TLS versions is to use PROTOCOL_SSLv23 and
     # disable protocols via SSLContext.options and OP_NO_* constants.
     # However, SSLContext.options doesn't work unless we have the
     # full/real SSLContext available to us.
-    if not modernssl:
+    if supportedprotocols == set(['tls1.0']):
         if protocol != 'tls1.0':
             raise error.Abort(_('current Python does not support protocol '
                                 'setting %s') % protocol,
                               hint=_('upgrade Python or disable setting since '
                                      'only TLS 1.0 is supported'))
 
         return ssl.PROTOCOL_TLSv1, 0