CVE-2023-6507 (Python, published 2023-12-08T19:15Z)
Description: value: An issue was found in CPython 3.12.0 `subprocess` module on POSIX platforms. The issue was fixed in CPython 3.12.1 and does not affect other stable releases.

When using the `extra_groups=` parameter with an empty list as a value (ie `extra_groups=[]`) the logic regressed to not call `setgroups(0, NULL)` before calling `exec()`, thus not dropping the original processes' groups before starting the new process. There is no issue when the parameter isn't used or when any value is used besides an empty list.

This issue only impacts CPython processes run with sufficient privilege to make the `setgroups` system call (typically `root`).


Severity: nan (CVSS v3 base score: 4.9)
Weakness: NVD-CWE-noinfo \u2014 Insufficient Information
There is insufficient information about the issue to classify it; details are unkown or unspecified.
Repository: https://github.com/python/cpython
Files changed:
.github/workflows/posix-deps-apt.sh
Lib/test/support/script_helper.py
Lib/test/test_subprocess.py
Misc/NEWS.d/next/Tests/2023-12-04-15-56-11.gh-issue-112334.FFc9Ti.rst
Commit message: gh-112334: Regression test that vfork is used when expected. (#112734)

Regression test that vfork is used when expected by subprocess.

This is written integration test style, it uses strace if it is present and appears to work to find out what system call actually gets used in different scenarios.

Test coverage is added for the default behavior and that of each of the specific arguments that must disable the use of vfork.  obviously not an entire test matrix, but it covers the most important aspects.

If there are ever issues with this test being flaky or failing on new platforms, rather than try and adapt it for all possible platforms, feel free to narrow the range it gets tested on when appropriate. That is not likely to reduce coverage.

Vulnerable code:
    @unittest.skipIf(not sysconfig.get_config_var("HAVE_VFORK"),
                     "vfork() not enabled by configure.")
    @mock.patch("subprocess._fork_exec")
    def test__use_vfork(self, mock_fork_exec):
        self.assertTrue(subprocess._USE_VFORK)  # The default value regardless.
        mock_fork_exec.side_effect = RuntimeError("just testing args")
        with self.assertRaises(RuntimeError):
            subprocess.run([sys.executable, "-c", "pass"])
        mock_fork_exec.assert_called_once()
        self.assertTrue(mock_fork_exec.call_args.args[-1])
        with mock.patch.object(subprocess, '_USE_VFORK', False):
            with self.assertRaises(RuntimeError):
                subprocess.run([sys.executable, "-c", "pass"])
            self.assertFalse(mock_fork_exec.call_args_list[-1].args[-1])


Fixed code:
    strace \
    """Used to implement assert_python_*.

    *args are the command line flags to pass to the python interpreter.
    **env_vars keyword arguments are environment variables to set on the process.

    If __run_using_command= is supplied, it must be a list of
    command line arguments to prepend to the command line used.
    Useful when you want to run another command that should launch the
    python interpreter via its own arguments. ["/bin/echo", "--"] for
    example could print the unquoted python command line instead of
    run it.
    """
    run_using_command = env_vars.pop('__run_using_command', None)
    if run_using_command:
        cmd_line = run_using_command + cmd_line
    @unittest.skipIf(not sysconfig.get_config_var("HAVE_VFORK"),
                     "vfork() not enabled by configure.")
    @mock.patch("subprocess._fork_exec")
    def test__use_vfork(self, mock_fork_exec):
        self.assertTrue(subprocess._USE_VFORK)  # The default value regardless.
        mock_fork_exec.side_effect = RuntimeError("just testing args")
        with self.assertRaises(RuntimeError):
            subprocess.run([sys.executable, "-c", "pass"])
        mock_fork_exec.assert_called_once()
        # NOTE: These assertions are *ugly* as they require the last arg
        # to remain the have_vfork boolean. We really need to refactor away
        # from the giant "wall of args" internal C extension API.
        self.assertTrue(mock_fork_exec.call_args.args[-1])
        with mock.patch.object(subprocess, '_USE_VFORK', False):
            with self.assertRaises(RuntimeError):
                subprocess.run([sys.executable, "-c", "pass"])
            self.assertFalse(mock_fork_exec.call_args_list[-1].args[-1])

    @unittest.skipIf(not sysconfig.get_config_var("HAVE_VFORK"),
                     "vfork() not enabled by configure.")
    @unittest.skipIf(sys.platform != "linux", "Linux only, requires strace.")
    def test_vfork_used_when_expected(self):
        # This is a performance regression test to ensure we default to using
        # vfork() when possible.
        strace_binary = "/usr/bin/strace"
        # The only system calls we are interested in.
        strace_filter = "--trace=clone,clone2,clone3,fork,vfork,exit,exit_group"
        true_binary = "/bin/true"
        strace_command = [strace_binary, strace_filter]

        try:
            does_strace_work_process = subprocess.run(
                    strace_command + [true_binary],
                    stderr=subprocess.PIPE,
                    stdout=subprocess.DEVNULL,
            )
            rc = does_strace_work_process.returncode
            stderr = does_strace_work_process.stderr
        except OSError:
            rc = -1
            stderr = ""
        if rc or (b"+++ exited with 0 +++" not in stderr):
            self.skipTest("strace not found or not working as expected.")

        with self.subTest(name="default_is_vfork"):
            vfork_result = assert_python_ok(
                    "-c",
                    textwrap.dedent(f"""\
                    import subprocess
                    subprocess.check_call([{true_binary!r}])"""),
                    __run_using_command=strace_command,
            )
            # Match both vfork() and clone(..., flags=...|CLONE_VFORK|...)
            self.assertRegex(vfork_result.err, br"(?i)vfork")
            # Do NOT check that fork() or other clones did not happen.
            # If the OS denys the vfork it'll fallback to plain fork().

        # Test that each individual thing that would disable the use of vfork
        # actually disables it.
        for sub_name, preamble, sp_kwarg, expect_permission_error in (
                ("!use_vfork", "subprocess._USE_VFORK = False", "", False),
                ("preexec", "", "preexec_fn=lambda: None", False),
                ("setgid", "", f"group={os.getgid()}", True),
                ("setuid", "", f"user={os.getuid()}", True),
                ("setgroups", "", "extra_groups=[]", True),
        ):
            with self.subTest(name=sub_name):
                non_vfork_result = assert_python_ok(
                    "-c",
                    textwrap.dedent(f"""\
                    import subprocess
                    {preamble}
                    try:
                        subprocess.check_call(
                                [{true_binary!r}], **dict({sp_kwarg}))
                    except PermissionError:
                        if not {expect_permission_error}:
                            raise"""),
                    __run_using_command=strace_command,
                )
                # Ensure neither vfork() or clone(..., flags=...|CLONE_VFORK|...).
                self.assertNotRegex(non_vfork_result.err, br"(?i)vfork")

Adds a regression test to verify that ``vfork()`` is used when expected by
:mod:`subprocess` on vfork enabled POSIX systems (Linux).