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safe mode to disable executing any external programs except git #2029

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119 changes: 111 additions & 8 deletions git/cmd.py
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Original file line number Diff line number Diff line change
Expand Up @@ -26,6 +26,7 @@
CommandError,
GitCommandError,
GitCommandNotFound,
UnsafeExecutionError,
UnsafeOptionError,
UnsafeProtocolError,
)
Expand Down Expand Up @@ -627,6 +628,7 @@ class Git(metaclass=_GitMeta):

__slots__ = (
"_working_dir",
"_safe",
"cat_file_all",
"cat_file_header",
"_version_info",
Expand Down Expand Up @@ -961,17 +963,56 @@ def check_unsafe_options(cls, options: List[str], unsafe_options: List[str]) ->

CatFileContentStream: TypeAlias = _CatFileContentStream

def __init__(self, working_dir: Union[None, PathLike] = None) -> None:
def __init__(self, working_dir: Union[None, PathLike] = None, safe: bool = False) -> None:
"""Initialize this instance with:

:param working_dir:
Git directory we should work in. If ``None``, we always work in the current
directory as returned by :func:`os.getcwd`.
This is meant to be the working tree directory if available, or the
``.git`` directory in case of bare repositories.

:param safe:
Lock down the configuration to make it as safe as possible
when working with publicly accessible, untrusted
repositories. This disables all known options that can run
external programs and limits networking to the HTTP protocol
via ``https://`` URLs. This might not cover Git config
options that were added since this was implemented, or
options that have unknown exploit vectors. It is a best
effort defense rather than an exhaustive protection measure.

In order to make this more likely to work with submodules,
some attempts are made to rewrite remote URLs to ``https://``
using `insteadOf` in the config. This might not work on all
projects, so submodules should always use ``https://`` URLs.

:envvar:`GIT_TERMINAL_PROMPT` is set to `false` and these
environment variables are forced to `/bin/true`:
:envvar:`GIT_ASKPASS`, :envvar:`GIT_EDITOR`,
:envvar:`GIT_PAGER`, :envvar:`GIT_SSH`,
:envvar:`GIT_SSH_COMMAND`, and :envvar:`SSH_ASKPASS`.

Git config options are supplied via the command line to set
up key parts of safe mode.

- Direct options for executing external commands are set to ``/bin/true``:
``core.askpass``, ``core.sshCommand`` and ``credential.helper``.

- External password prompts are disabled by skipping authentication using
``http.emptyAuth=true``.

- Any use of an fsmonitor daemon is disabled using ``core.fsmonitor=false``.

- Hook scripts are disabled using ``core.hooksPath=/dev/null``.

It was not possible to cover all config items that might execute an external
command, for example, ``receive.procReceiveRefs``,
``uploadpack.packObjectsHook`` and ``remote.<name>.vcs``.
"""
super().__init__()
self._working_dir = expand_path(working_dir)
self._safe = safe
self._git_options: Union[List[str], Tuple[str, ...]] = ()
self._persistent_git_options: List[str] = []

Expand Down Expand Up @@ -1218,6 +1259,8 @@ def execute(

:raise git.exc.GitCommandError:

:raise git.exc.UnsafeExecutionError:

:note:
If you add additional keyword arguments to the signature of this method, you
must update the ``execute_kwargs`` variable housed in this module.
Expand All @@ -1227,6 +1270,64 @@ def execute(
if self.GIT_PYTHON_TRACE and (self.GIT_PYTHON_TRACE != "full" or as_process):
_logger.info(" ".join(redacted_command))

if shell is None:
# Get the value of USE_SHELL with no deprecation warning. Do this without
# warnings.catch_warnings, to avoid a race condition with application code
# configuring warnings. The value could be looked up in type(self).__dict__
# or Git.__dict__, but those can break under some circumstances. This works
# the same as self.USE_SHELL in more situations; see Git.__getattribute__.
shell = super().__getattribute__("USE_SHELL")

if self._safe:
if shell:
raise UnsafeExecutionError(
redacted_command,
"Command cannot be executed in a shell when in safe mode.",
)
if not isinstance(command, Sequence):
raise UnsafeExecutionError(
redacted_command,
"Command must be a Sequence to be executed in safe mode.",
)
if command[0] != self.GIT_PYTHON_GIT_EXECUTABLE:
raise UnsafeExecutionError(
redacted_command,
f'Only "{self.GIT_PYTHON_GIT_EXECUTABLE}" can be executed when in safe mode.',
)
config_args = [
"-c",
"core.askpass=/bin/true",
"-c",
"core.fsmonitor=false",
"-c",
"core.hooksPath=/dev/null",
"-c",
"core.sshCommand=/bin/true",
"-c",
"credential.helper=/bin/true",
"-c",
"http.emptyAuth=true",
"-c",
"protocol.allow=never",
"-c",
"protocol.https.allow=always",
"-c",
"url.https://bitbucket.org/.insteadOf=git@bitbucket.org:",
"-c",
"url.https://codeberg.org/.insteadOf=git@codeberg.org:",
"-c",
"url.https://github.com/.insteadOf=git@github.com:",
"-c",
"url.https://gitlab.com/.insteadOf=git@gitlab.com:",
"-c",
"url.https://.insteadOf=git://",
"-c",
"url.https://.insteadOf=http://",
"-c",
"url.https://.insteadOf=ssh://",
]
command = [command.pop(0)] + config_args + command

# Allow the user to have the command executed in their working dir.
try:
cwd = self._working_dir or os.getcwd() # type: Union[None, str]
Expand All @@ -1244,6 +1345,15 @@ def execute(
# just to be sure.
env["LANGUAGE"] = "C"
env["LC_ALL"] = "C"
# Globally disable things that can execute commands, including password prompts.
if self._safe:
env["GIT_ASKPASS"] = "/bin/true"
env["GIT_EDITOR"] = "/bin/true"
env["GIT_PAGER"] = "/bin/true"
env["GIT_SSH"] = "/bin/true"
env["GIT_SSH_COMMAND"] = "/bin/true"
env["GIT_TERMINAL_PROMPT"] = "false"
env["SSH_ASKPASS"] = "/bin/true"
env.update(self._environment)
if inline_env is not None:
env.update(inline_env)
Expand All @@ -1260,13 +1370,6 @@ def execute(
# END handle

stdout_sink = PIPE if with_stdout else getattr(subprocess, "DEVNULL", None) or open(os.devnull, "wb")
if shell is None:
# Get the value of USE_SHELL with no deprecation warning. Do this without
# warnings.catch_warnings, to avoid a race condition with application code
# configuring warnings. The value could be looked up in type(self).__dict__
# or Git.__dict__, but those can break under some circumstances. This works
# the same as self.USE_SHELL in more situations; see Git.__getattribute__.
shell = super().__getattribute__("USE_SHELL")
_logger.debug(
"Popen(%s, cwd=%s, stdin=%s, shell=%s, universal_newlines=%s)",
redacted_command,
Expand Down
4 changes: 4 additions & 0 deletions git/exc.py
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Original file line number Diff line number Diff line change
Expand Up @@ -159,6 +159,10 @@ def __init__(
super().__init__(command, status, stderr, stdout)


class UnsafeExecutionError(CommandError):
"""Thrown if anything but git is executed when in safe mode."""


class CheckoutError(GitError):
"""Thrown if a file could not be checked out from the index as it contained
changes.
Expand Down
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