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Filename : context_config.py
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# -*- coding: utf-8 -*-
# Copyright 2020 Google Inc. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""Manages device context mTLS certificates."""

from __future__ import absolute_import
from __future__ import print_function
from __future__ import division
from __future__ import unicode_literals

import atexit
import json
import os

from boto import config

import gslib
from gslib import exception
from gslib.utils import boto_util
from gslib.utils import execution_util

# Maintain a single context configuration.
_singleton_config = None

# Metadata JSON that stores information about the default certificate provider.
_DEFAULT_METADATA_PATH = os.path.expanduser(
    os.path.join('~', '.secureConnect', 'context_aware_metadata.json'))
_CERT_PROVIDER_COMMAND = "cert_provider_command"
_CERT_PROVIDER_COMMAND_PASSPHRASE_OPTION = "--with_passphrase"


class CertProvisionError(Exception):
  """Represents errors when provisioning a client certificate."""
  pass


class ContextConfigSingletonAlreadyExistsError(Exception):
  """Error for when create_context_config is called multiple times."""
  pass


def _is_pem_section_marker(line):
  """Returns (begin:bool, end:bool, name:str)."""
  if line.startswith('-----BEGIN ') and line.endswith('-----'):
    return True, False, line[11:-5]
  elif line.startswith('-----END ') and line.endswith('-----'):
    return False, True, line[9:-5]
  else:
    return False, False, ''


def _split_pem_into_sections(contents, logger):
  """Returns dict with {name: section} by parsing contents in PEM format.

  A simple parser for PEM file. Please see RFC 7468 for the format of PEM
  file. Not using regex to improve performance catching nested matches.
  Note: This parser requires the post-encapsulation label of a section to
  match its pre-encapsulation label. It ignores a section without a
  matching label.

  Args:
    contents (str): Contents of a PEM file.
    logger (logging.logger): gsutil logger.

  Returns:
    A dict of the PEM file sections.
  """
  result = {}
  pem_lines = []
  pem_section_name = None

  for line in contents.splitlines():
    line = line.strip()
    if not line:
      continue

    begin, end, name = _is_pem_section_marker(line)
    if begin:
      if pem_section_name:
        logger.warning('Section %s missing end line and will be ignored.' %
                       pem_section_name)
      if name in result.keys():
        logger.warning('Section %s already exists, and the older section will '
                       'be ignored.' % name)
      pem_section_name = name
      pem_lines = []
    elif end:
      if not pem_section_name:
        logger.warning(
            'Section %s missing a beginning line and will be ignored.' % name)
      elif pem_section_name != name:
        logger.warning('Section %s missing a matching end line. Found: %s' %
                       (pem_section_name, name))
        pem_section_name = None

    if pem_section_name:
      pem_lines.append(line)
      if end:
        result[name] = '\n'.join(pem_lines) + '\n'
        pem_section_name = None

  if pem_section_name:
    logger.warning('Section %s missing an end line.' % pem_section_name)

  return result


def _check_path():
  """Checks for content aware metadata.

  If content aware metadata exists, return its absolute path;
  otherwise, returns None.

  Returns:
    str: Absolute path if exists. Otherwise, None.
  """
  metadata_path = os.path.expanduser(_DEFAULT_METADATA_PATH)
  if not os.path.exists(metadata_path):
    return None
  return metadata_path


def _read_metadata_file(metadata_path):
  """Loads context aware metadata from the given path.

  Returns:
      dict: The metadata JSON.

  Raises:
      CertProvisionError: If failed to parse metadata as JSON.
  """
  try:
    with open(metadata_path) as f:
      return json.load(f)
  except ValueError as e:
    raise CertProvisionError(e)


def _default_command():
  """Loads default cert provider command.

  Returns:
      str: The default command.

  Raises:
      CertProvisionError: If command cannot be found.
  """
  metadata_path = _check_path()

  if not metadata_path:
    raise CertProvisionError("Client certificate provider file not found.")

  metadata_json = _read_metadata_file(metadata_path)

  if _CERT_PROVIDER_COMMAND not in metadata_json:
    raise CertProvisionError("Client certificate provider command not found.")

  command = metadata_json[_CERT_PROVIDER_COMMAND]
  if (_CERT_PROVIDER_COMMAND_PASSPHRASE_OPTION not in command):
    command.append(_CERT_PROVIDER_COMMAND_PASSPHRASE_OPTION)
  return command


class _ContextConfig(object):
  """Represents the configurations associated with context aware access.

  Only one instance of Config can be created for the program.
  """

  def __init__(self, logger):
    """Initializes config.

    Args:
      logger (logging.logger): gsutil logger.
    """
    self.logger = logger

    self.use_client_certificate = config.getbool('Credentials',
                                                 'use_client_certificate')
    self.client_cert_path = None
    if not self.use_client_certificate:
      # Don't spend time generating values gsutil won't need.
      return

    # Generates certificate and deletes it afterwards.
    atexit.register(self._unprovision_client_cert)
    self.client_cert_path = os.path.join(boto_util.GetGsutilStateDir(),
                                         'caa_cert.pem')
    try:
      # Certs provisioned using endpoint verification are stored as a
      # single file holding both the public certificate and the private key.
      self._provision_client_cert(self.client_cert_path)
    except CertProvisionError as e:
      self.logger.error('Failed to provision client certificate: %s' % e)

  def _provision_client_cert(self, cert_path):
    """Executes certificate provider to obtain client certificate and keys."""
    cert_command_string = config.get('Credentials', 'cert_provider_command',
                                     None)
    if cert_command_string:
      cert_command = cert_command_string.split(' ')
    else:
      # Load the default certificate provider if sit is not provided by user.
      cert_command = _default_command()

    try:
      command_stdout_string, _ = execution_util.ExecuteExternalCommand(
          cert_command)

      sections = _split_pem_into_sections(command_stdout_string, self.logger)
      with open(cert_path, 'w+') as f:
        f.write(sections['CERTIFICATE'])
        if 'ENCRYPTED PRIVATE KEY' in sections:
          f.write(sections['ENCRYPTED PRIVATE KEY'])
          self.client_cert_password = sections['PASSPHRASE'].splitlines()[1]
        else:
          f.write(sections['PRIVATE KEY'])
          self.client_cert_password = None
    except (exception.ExternalBinaryError, OSError) as e:
      raise CertProvisionError(e)
    except KeyError as e:
      raise CertProvisionError(
          'Invalid output format from certificate provider, no %s' % e)

  def _unprovision_client_cert(self):
    """Cleans up any files or resources provisioned during config init."""
    if self.client_cert_path is not None:
      try:
        os.remove(self.client_cert_path)
        self.logger.debug('Unprovisioned client cert: %s' %
                          self.client_cert_path)
      except OSError as e:
        self.logger.error('Failed to remove client certificate: %s' % e)


def create_context_config(logger):
  """Should be run once at gsutil startup. Creates global singleton.

  Args:
    logger (logging.logger): For logging during config functions.

  Returns:
    New ContextConfig singleton.

  Raises:
    Exception if singleton already exists.
  """
  global _singleton_config
  if not _singleton_config:
    _singleton_config = _ContextConfig(logger)
    return _singleton_config
  raise ContextConfigSingletonAlreadyExistsError


def get_context_config():
  """Retrieves ContextConfig global singleton.

  Returns:
    ContextConfig or None if global singleton doesn't exist.
  """
  return _singleton_config