wireproto: check permissions when executing "batch" command (BC) (SEC)
For as long as the "batch" command has existed (introduced by
bd88561afb4b and first released as part of Mercurial 1.9), that command
(like most wire commands introduced after 2008) lacked an entry in
the hgweb permissions table. And since we don't verify permissions if
an entry is missing from the permissions table, this meant that
executing a command via "batch" would bypass all permissions
checks.
The security implications are significant: a Mercurial HTTP server
would allow writes via "batch" wire protocol commands as long as
the HTTP request were processed by Mercurial and the process running
the Mercurial HTTP server had write access to the repository. The
Mercurial defaults of servers being read-only and the various web.*
config options to define access control were bypassed.
In addition, "batch" could be used to exfiltrate data from servers
that were configured to not allow read access.
Both forms of permissions bypass could be mitigated to some extent
by using HTTP authentication. This would prevent HTTP requests from
hitting Mercurial's server logic. However, any authenticated request
would still be able to bypass permissions checks via "batch" commands.
The easiest exploit was to send "pushkey" commands via "batch" and
modify the state of bookmarks, phases, and obsolescence markers.
However, I suspect a well-crafted HTTP request could trick the server
into running the "unbundle" wire protocol command, effectively
performing a full `hg push` to create new changesets on the remote.
This commit plugs this gaping security hole by having the "batch"
command perform permissions checking on each sub-command that is
being batched. We do this by threading a permissions checking
callable all the way to the protocol handler. The threading is a
bit hacky from a code perspective. But it preserves API compatibility,
which is the proper thing to do on the stable branch.
One of the subtle things we do is assume that a command with an
undefined permission is a "push" command. This is the safest thing to
do from a security perspective: we don't want to take chances that
a command could perform a write even though the server is configured
to not allow writes.
As the test changes demonstrate, it is no longer possible to bypass
permissions via the "batch" wire protocol command.
.. bc::
The "batch" wire protocol command now enforces permissions of
each invoked sub-command. Wire protocol commands must define
their operation type or the "batch" command will assume they
can write data and will prevent their execution on HTTP servers
unless the HTTP request method is POST, the server is configured
to allow pushes, and the (possibly authenticated) HTTP user is
authorized to perform a push.
# demandimportpy3 - global demand-loading of modules for Mercurial
#
# Copyright 2017 Facebook Inc.
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.
"""Lazy loading for Python 3.6 and above.
This uses the new importlib finder/loader functionality available in Python 3.5
and up. The code reuses most of the mechanics implemented inside importlib.util,
but with a few additions:
* Allow excluding certain modules from lazy imports.
* Expose an interface that's substantially the same as demandimport for
Python 2.
This also has some limitations compared to the Python 2 implementation:
* Much of the logic is per-package, not per-module, so any packages loaded
before demandimport is enabled will not be lazily imported in the future. In
practice, we only expect builtins to be loaded before demandimport is
enabled.
"""
# This line is unnecessary, but it satisfies test-check-py3-compat.t.
from __future__ import absolute_import
import contextlib
import importlib.abc
import importlib.machinery
import importlib.util
import sys
_deactivated = False
class _lazyloaderex(importlib.util.LazyLoader):
"""This is a LazyLoader except it also follows the _deactivated global and
the ignore list.
"""
def exec_module(self, module):
"""Make the module load lazily."""
if _deactivated or module.__name__ in ignore:
self.loader.exec_module(module)
else:
super().exec_module(module)
# This is 3.6+ because with Python 3.5 it isn't possible to lazily load
# extensions. See the discussion in https://bugs.python.org/issue26186 for more.
_extensions_loader = _lazyloaderex.factory(
importlib.machinery.ExtensionFileLoader)
_bytecode_loader = _lazyloaderex.factory(
importlib.machinery.SourcelessFileLoader)
_source_loader = _lazyloaderex.factory(importlib.machinery.SourceFileLoader)
def _makefinder(path):
return importlib.machinery.FileFinder(
path,
# This is the order in which loaders are passed in in core Python.
(_extensions_loader, importlib.machinery.EXTENSION_SUFFIXES),
(_source_loader, importlib.machinery.SOURCE_SUFFIXES),
(_bytecode_loader, importlib.machinery.BYTECODE_SUFFIXES),
)
ignore = []
def init(ignorelist):
global ignore
ignore = ignorelist
def isenabled():
return _makefinder in sys.path_hooks and not _deactivated
def disable():
try:
while True:
sys.path_hooks.remove(_makefinder)
except ValueError:
pass
def enable():
sys.path_hooks.insert(0, _makefinder)
@contextlib.contextmanager
def deactivated():
# This implementation is a bit different from Python 2's. Python 3
# maintains a per-package finder cache in sys.path_importer_cache (see
# PEP 302). This means that we can't just call disable + enable.
# If we do that, in situations like:
#
# demandimport.enable()
# ...
# from foo.bar import mod1
# with demandimport.deactivated():
# from foo.bar import mod2
#
# mod2 will be imported lazily. (The converse also holds -- whatever finder
# first gets cached will be used.)
#
# Instead, have a global flag the LazyLoader can use.
global _deactivated
demandenabled = isenabled()
if demandenabled:
_deactivated = True
try:
yield
finally:
if demandenabled:
_deactivated = False