mercurial/policy.py

sslutil: abort when unable to verify peer connection (BC)

Previously, when we connected to a server and were unable to verify
its certificate against a trusted certificate authority we would
issue a warning and continue to connect. This is obviously not
great behavior because the x509 certificate model is based upon
trust of specific CAs. Failure to enforce that trust erodes security.
This behavior was defined several years ago when Python did not
support loading the system trusted CA store (Python 2.7.9's
backports of Python 3's improvements to the "ssl" module enabled
this).

This commit changes behavior when connecting to abort if the peer
certificate can't be validated. With an empty/default Mercurial
configuration, the peer certificate can be validated if Python is
able to load the system trusted CA store. Environments able to load
the system trusted CA store include:

* Python 2.7.9+ on most platforms and installations
* Python 2.7 distributions with a modern ssl module (e.g. RHEL7's
  patched 2.7.5 package)
* Python shipped on OS X

Environments unable to load the system trusted CA store include:

* Python 2.6
* Python 2.7 on many existing Linux installs (because they don't
  ship 2.7.9+ or haven't backported modern ssl module)
* Python 2.7.9+ on some installs where Python is unable to locate
  the system CA store (this is hopefully rare)

Users of these Pythongs will need to configure Mercurial to load the
system CA store using web.cacerts. This should ideally be performed
by packagers (by setting web.cacerts in the global/system hgrc file).
Where Mercurial packagers aren't setting this, the linked URL in the
new abort message can contain instructions for users.

In the future, we may want to add more code for finding the system
CA store. For example, many Linux distributions have the CA store
at well-known locations (such as /etc/ssl/certs/ca-certificates.crt
in the case of Ubuntu). This will enable CA loading to "just work"
on more Python configurations and will be best for our users since
they won't have to change anything after upgrading to a Mercurial
with this patch.

We may also want to consider distributing a trusted CA store with
Mercurial. Although we should think long and hard about that because
most systems have a global CA store and Mercurial should almost
certainly use the same store used by everything else on the system.

# policy.py - module policy logic for Mercurial.
#
# Copyright 2015 Gregory Szorc <[e-mail]>
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.

from __future__ import absolute_import

import os
import sys

# Rules for how modules can be loaded. Values are:
#
#    c - require C extensions
#    allow - allow pure Python implementation when C loading fails
#    py - only load pure Python modules
#
# By default, require the C extensions for performance reasons.
policy = 'c'
try:
    from . import __modulepolicy__
    policy = __modulepolicy__.modulepolicy
except ImportError:
    pass

# PyPy doesn't load C extensions.
#
# The canonical way to do this is to test platform.python_implementation().
# But we don't import platform and don't bloat for it here.
if '__pypy__' in sys.builtin_module_names:
    policy = 'py'

# Our C extensions aren't yet compatible with Python 3. So use pure Python
# on Python 3 for now.
if sys.version_info[0] >= 3:
    policy = 'py'

# Environment variable can always force settings.
policy = os.environ.get('HGMODULEPOLICY', policy)