mercurial/sslutil.py

sslutil: use CA loaded state to drive validation logic

Until now, sslkwargs may set web.cacerts=! to indicate
that system certs could not be found. This is really
obtuse because sslkwargs effectively sets state on a global
object which bypasses wrapsocket() and is later consulted
by validator.__call__. This is madness.

This patch introduces an attribute on the wrapped socket
instance indicating whether system CAs were loaded. We
can set this directly inside wrapsocket() because that
function knows everything that sslkwargs() does - and more.

With this attribute set on the socket, we refactor
validator.__call__ to use it.

Since we no longer have a need for setting web.cacerts=!
in sslkwargs, we remove that.

I think the new logic is much easier to understand and will
enable behavior to be changed more easily.

# sslutil.py - SSL handling for mercurial
#
# Copyright 2005, 2006, 2007, 2008 Matt Mackall <[e-mail]>
# Copyright 2006, 2007 Alexis S. L. Carvalho <[e-mail]>
# Copyright 2006 Vadim Gelfer <[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 ssl
import sys

from .i18n import _
from . import (
    error,
    util,
)

# Python 2.7.9+ overhauled the built-in SSL/TLS features of Python. It added
# support for TLS 1.1, TLS 1.2, SNI, system CA stores, etc. These features are
# all exposed via the "ssl" module.
#
# Depending on the version of Python being used, SSL/TLS support is either
# modern/secure or legacy/insecure. Many operations in this module have
# separate code paths depending on support in Python.

hassni = getattr(ssl, 'HAS_SNI', False)

try:
    OP_NO_SSLv2 = ssl.OP_NO_SSLv2
    OP_NO_SSLv3 = ssl.OP_NO_SSLv3
except AttributeError:
    OP_NO_SSLv2 = 0x1000000
    OP_NO_SSLv3 = 0x2000000

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
    _canloaddefaultcerts = False

    # We implement SSLContext using the interface from the standard library.
    class SSLContext(object):
        # ssl.wrap_socket gained the "ciphers" named argument in 2.7.
        _supportsciphers = sys.version_info >= (2, 7)

        def __init__(self, protocol):
            # From the public interface of SSLContext
            self.protocol = protocol
            self.check_hostname = False
            self.options = 0
            self.verify_mode = ssl.CERT_NONE

            # Used by our implementation.
            self._certfile = None
            self._keyfile = None
            self._certpassword = None
            self._cacerts = None
            self._ciphers = None

        def load_cert_chain(self, certfile, keyfile=None, password=None):
            self._certfile = certfile
            self._keyfile = keyfile
            self._certpassword = password

        def load_default_certs(self, purpose=None):
            pass

        def load_verify_locations(self, cafile=None, capath=None, cadata=None):
            if capath:
                raise error.Abort('capath not supported')
            if cadata:
                raise error.Abort('cadata not supported')

            self._cacerts = cafile

        def set_ciphers(self, ciphers):
            if not self._supportsciphers:
                raise error.Abort('setting ciphers not supported')

            self._ciphers = ciphers

        def wrap_socket(self, socket, server_hostname=None, server_side=False):
            # server_hostname is unique to SSLContext.wrap_socket and is used
            # for SNI in that context. So there's nothing for us to do with it
            # in this legacy code since we don't support SNI.

            args = {
                'keyfile': self._keyfile,
                'certfile': self._certfile,
                'server_side': server_side,
                'cert_reqs': self.verify_mode,
                'ssl_version': self.protocol,
                'ca_certs': self._cacerts,
            }

            if self._supportsciphers:
                args['ciphers'] = self._ciphers

            return ssl.wrap_socket(socket, **args)

def wrapsocket(sock, keyfile, certfile, ui, cert_reqs=ssl.CERT_NONE,
               ca_certs=None, serverhostname=None):
    """Add SSL/TLS to a socket.

    This is a glorified wrapper for ``ssl.wrap_socket()``. It makes sane
    choices based on what security options are available.

    In addition to the arguments supported by ``ssl.wrap_socket``, we allow
    the following additional arguments:

    * serverhostname - The expected hostname of the remote server. If the
      server (and client) support SNI, this tells the server which certificate
      to use.
    """
    # Despite its name, PROTOCOL_SSLv23 selects the highest protocol
    # that both ends support, including TLS protocols. On legacy stacks,
    # the highest it likely goes in TLS 1.0. On modern stacks, it can
    # 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.
    #
    # SSLv2 and SSLv3 are broken. We ban them outright.
    if modernssl:
        protocol = ssl.PROTOCOL_SSLv23
    else:
        protocol = ssl.PROTOCOL_TLSv1

    # TODO use ssl.create_default_context() on modernssl.
    sslcontext = SSLContext(protocol)

    # This is a no-op on old Python.
    sslcontext.options |= OP_NO_SSLv2 | OP_NO_SSLv3

    # This still works on our fake SSLContext.
    sslcontext.verify_mode = cert_reqs

    if certfile is not None:
        def password():
            f = keyfile or certfile
            return ui.getpass(_('passphrase for %s: ') % f, '')
        sslcontext.load_cert_chain(certfile, keyfile, password)

    if ca_certs is not None:
        sslcontext.load_verify_locations(cafile=ca_certs)
        caloaded = True
    else:
        # This is a no-op on old Python.
        sslcontext.load_default_certs()
        caloaded = _canloaddefaultcerts

    sslsocket = sslcontext.wrap_socket(sock, server_hostname=serverhostname)
    # check if wrap_socket failed silently because socket had been
    # closed
    # - see http://bugs.python.org/issue13721
    if not sslsocket.cipher():
        raise error.Abort(_('ssl connection failed'))

    sslsocket._hgcaloaded = caloaded

    return sslsocket

def _verifycert(cert, hostname):
    '''Verify that cert (in socket.getpeercert() format) matches hostname.
    CRLs is not handled.

    Returns error message if any problems are found and None on success.
    '''
    if not cert:
        return _('no certificate received')
    dnsname = hostname.lower()
    def matchdnsname(certname):
        return (certname == dnsname or
                '.' in dnsname and certname == '*.' + dnsname.split('.', 1)[1])

    san = cert.get('subjectAltName', [])
    if san:
        certnames = [value.lower() for key, value in san if key == 'DNS']
        for name in certnames:
            if matchdnsname(name):
                return None
        if certnames:
            return _('certificate is for %s') % ', '.join(certnames)

    # subject is only checked when subjectAltName is empty
    for s in cert.get('subject', []):
        key, value = s[0]
        if key == 'commonName':
            try:
                # 'subject' entries are unicode
                certname = value.lower().encode('ascii')
            except UnicodeEncodeError:
                return _('IDN in certificate not supported')
            if matchdnsname(certname):
                return None
            return _('certificate is for %s') % certname
    return _('no commonName or subjectAltName found in certificate')


# CERT_REQUIRED means fetch the cert from the server all the time AND
# validate it against the CA store provided in web.cacerts.

def _plainapplepython():
    """return true if this seems to be a pure Apple Python that
    * is unfrozen and presumably has the whole mercurial module in the file
      system
    * presumably is an Apple Python that uses Apple OpenSSL which has patches
      for using system certificate store CAs in addition to the provided
      cacerts file
    """
    if sys.platform != 'darwin' or util.mainfrozen() or not sys.executable:
        return False
    exe = os.path.realpath(sys.executable).lower()
    return (exe.startswith('/usr/bin/python') or
            exe.startswith('/system/library/frameworks/python.framework/'))

def _defaultcacerts():
    """return path to default CA certificates or None."""
    if _plainapplepython():
        dummycert = os.path.join(os.path.dirname(__file__), 'dummycert.pem')
        if os.path.exists(dummycert):
            return dummycert

    return None

def sslkwargs(ui, host):
    """Determine arguments to pass to wrapsocket().

    ``host`` is the hostname being connected to.
    """
    kws = {'ui': ui}

    # If a host key fingerprint is on file, it is the only thing that matters
    # and CA certs don't come into play.
    hostfingerprint = ui.config('hostfingerprints', host)
    if hostfingerprint:
        return kws

    # The code below sets up CA verification arguments. If --insecure is
    # used, we don't take CAs into consideration, so return early.
    if ui.insecureconnections:
        return kws

    cacerts = ui.config('web', 'cacerts')
    # TODO remove check when we stop setting this config.
    if cacerts == '!':
        return kws

    # If a value is set in the config, validate against a path and load
    # and require those certs.
    if cacerts:
        cacerts = util.expandpath(cacerts)
        if not os.path.exists(cacerts):
            raise error.Abort(_('could not find web.cacerts: %s') % cacerts)

        kws.update({'ca_certs': cacerts,
                    'cert_reqs': ssl.CERT_REQUIRED})
        return kws

    # No CAs in config. See if we can load defaults.
    cacerts = _defaultcacerts()

    # We found an alternate CA bundle to use. Load it.
    if cacerts:
        ui.debug('using %s to enable OS X system CA\n' % cacerts)
        ui.setconfig('web', 'cacerts', cacerts, 'defaultcacerts')
        kws.update({'ca_certs': cacerts,
                    'cert_reqs': ssl.CERT_REQUIRED})
        return kws

    # FUTURE this can disappear once wrapsocket() is secure by default.
    if _canloaddefaultcerts:
        kws['cert_reqs'] = ssl.CERT_REQUIRED
        return kws

    return kws

class validator(object):
    def __init__(self, ui, host):
        self.ui = ui
        self.host = host

    def __call__(self, sock, strict=False):
        host = self.host

        if not sock.cipher(): # work around http://bugs.python.org/issue13721
            raise error.Abort(_('%s ssl connection error') % host)
        try:
            peercert = sock.getpeercert(True)
            peercert2 = sock.getpeercert()
        except AttributeError:
            raise error.Abort(_('%s ssl connection error') % host)

        if not peercert:
            raise error.Abort(_('%s certificate error: '
                               'no certificate received') % host)

        # If a certificate fingerprint is pinned, use it and only it to
        # validate the remote cert.
        hostfingerprints = self.ui.configlist('hostfingerprints', host)
        peerfingerprint = util.sha1(peercert).hexdigest()
        nicefingerprint = ":".join([peerfingerprint[x:x + 2]
            for x in xrange(0, len(peerfingerprint), 2)])
        if hostfingerprints:
            fingerprintmatch = False
            for hostfingerprint in hostfingerprints:
                if peerfingerprint.lower() == \
                        hostfingerprint.replace(':', '').lower():
                    fingerprintmatch = True
                    break
            if not fingerprintmatch:
                raise error.Abort(_('certificate for %s has unexpected '
                                   'fingerprint %s') % (host, nicefingerprint),
                                 hint=_('check hostfingerprint configuration'))
            self.ui.debug('%s certificate matched fingerprint %s\n' %
                          (host, nicefingerprint))
            return

        # If insecure connections were explicitly requested via --insecure,
        # print a warning and do no verification.
        #
        # It may seem odd that this is checked *after* host fingerprint pinning.
        # This is for backwards compatibility (for now). The message is also
        # the same as below for BC.
        if self.ui.insecureconnections:
            self.ui.warn(_('warning: %s certificate with fingerprint %s not '
                           'verified (check hostfingerprints or web.cacerts '
                           'config setting)\n') %
                         (host, nicefingerprint))
            return

        if not sock._hgcaloaded:
            if strict:
                raise error.Abort(_('%s certificate with fingerprint %s not '
                                    'verified') % (host, nicefingerprint),
                                  hint=_('check hostfingerprints or '
                                         'web.cacerts config setting'))
            else:
                self.ui.warn(_('warning: %s certificate with fingerprint %s '
                               'not verified (check hostfingerprints or '
                               'web.cacerts config setting)\n') %
                             (host, nicefingerprint))

            return

        msg = _verifycert(peercert2, host)
        if msg:
            raise error.Abort(_('%s certificate error: %s') % (host, msg),
                             hint=_('configure hostfingerprint %s or use '
                                    '--insecure to connect insecurely') %
                                  nicefingerprint)