[v20,0/6] Script execution control (was O_MAYEXEC)

Message

Mickaël Salaün Oct. 11, 2024, 6:44 p.m. UTC

Hi,

The ultimate goal of this patch series is to be able to ensure that
direct file execution (e.g. ./script.sh) and indirect file execution
(e.g. sh script.sh) lead to the same result, especially from a security
point of view.

The main changes from the previous version are new securebits with a
better semantic, an improved documentation, new user space code examples
including an enlighten interpreter and an extensive tests suite.  As
discussed, I also removed the ptrace checks which doesn't make sense
anymore.  You'll now find a quick FAQ at the end of this cover letter.
The current status is summarized in this article:
https://lwn.net/Articles/982085/
I also gave a talk at LPC last month:
https://lpc.events/event/18/contributions/1692/
And here is a proof of concept for Python (for now, for the previous
version: v19): https://github.com/zooba/spython/pull/12

Overview
--------

This patch series is a new approach of the initial O_MAYEXEC feature,
and a revamp of the previous patch series.  Taking into account the last
reviews [1], we now stick to the kernel semantic for file executability.
One major change is the clear split between access check and policy
management.

The first patch brings the AT_CHECK flag to execveat(2).  The goal is to
enable user space to check if a file could be executed (by the kernel).
Unlike stat(2) that only checks file permissions, execveat2(2) +
AT_CHECK take into account the full context, including mount points
(noexec), caller's limits, and all potential LSM extra checks (e.g.
argv, envp, credentials).

The second patch brings two new securebits used to set or get a security
policy for a set of processes.  For this to be meaningful, all
executable code needs to be trusted.  In practice, this means that
(malicious) users can be restricted to only run scripts provided (and
trusted) by the system.

[1] https://lore.kernel.org/r/CAHk-=[e-mail]

Script execution
----------------

One important thing to keep in mind is that the goal of this patch
series is to get the same security restrictions with these commands:
* ./script.py
* python script.py
* python < script.py
* python -m script.py

However, on secure systems, we should be able to forbid these commands
because there is no way to reliably identify the origin of the script:
* xargs -a script.py -d '\r' -- python -c
* cat script.py | python
* python

Background
----------

Compared to the previous patch series, there is no more dedicated
syscall nor sysctl configuration.  This new patch series only add new
flags: one for execveat(2) and four for prctl(2).

This kind of script interpreter restriction may already be used in
hardened systems, which may need to fork interpreters and install
different versions of the binaries.  This mechanism should enable to
avoid the use of duplicate binaries (and potential forked source code)
for secure interpreters (e.g. secure Python [2]) by making it possible
to dynamically enforce restrictions or not.

The ability to control script execution is also required to close a
major IMA measurement/appraisal interpreter integrity [3].

This new execveat + AT_CHECK should not be confused with the O_EXEC flag
(for open) which is intended for execute-only, which obviously doesn't
work for scripts.

I gave a talk about controlling script execution where I explain the
previous approaches [4].  The design of the WIP RFC I talked about
changed quite a bit since then.

[2] https://github.com/zooba/spython
[3] https://lore.kernel.org/lkml/[e-mail]/
[4] https://lssna2023.sched.com/event/1K7bO

Execution policy
----------------

The "execution" usage means that the content of the file descriptor is
trusted according to the system policy to be executed by user space,
which means that it interprets the content or (try to) maps it as
executable memory.

It is important to note that this can only enable to extend access
control managed by the kernel.  Hence it enables current access control
mechanism to be extended and become a superset of what they can
currently control.  Indeed, the security policy could also be delegated
to an LSM, either a MAC system or an integrity system.

Complementary W^X protections can be brought by SELinux or IPE [5].

Being able to restrict execution also enables to protect the kernel by
restricting arbitrary syscalls that an attacker could perform with a
crafted binary or certain script languages.  It also improves multilevel
isolation by reducing the ability of an attacker to use side channels
with specific code.  These restrictions can natively be enforced for ELF
binaries (with the noexec mount option) but require this kernel
extension to properly handle scripts (e.g. Python, Perl).  To get a
consistent execution policy, additional memory restrictions should also
be enforced (e.g. thanks to SELinux).

[5] https://lore.kernel.org/lkml/[e-mail]/

Prerequisite for security use
-----------------------------

Because scripts might not currently have the executable permission and
still run well as is, or because we might want specific users to be
allowed to run arbitrary scripts, we also need a configuration
mechanism.

According to the threat model, to get a secure execution environment on
top of these changes, it might be required to configure and enable
existing security mechanisms such as secure boot, restrictive mount
points (e.g. with rw AND noexec), correct file permissions (including
executable libraries), IMA/EVM, SELinux policy...

The first thing to patch is the libc to check loaded libraries (e.g. see
chromeOS changes).  The second thing to patch are the script
interpreters by checking direct scripts executability and by checking
their own libraries (e.g. Python's imported files or argument-passed
modules).  For instance, the PEP 578 [6] (Runtime Audit Hooks) enables
Python 3.8 to be extended with policy enforcement points related to code
interpretation, which can be used to align with the PowerShell audit
features.  Additional Python security improvements (e.g. a limited
interpreter without -c, stdin piping of code) are developed [2] [7].

[6] https://www.python.org/dev/peps/pep-0578/
[7] https://lore.kernel.org/lkml/[e-mail]/

libc patch
----------

Dynamic linking needs still need to check the libraries the same way
interpreters need to check scripts.

chromeOS patches glibc with a fstatvfs check [8] [9]. This enables to
check against noexec mount points, which is OK but doesn't fit with
execve semantics.  Moreover, the kernel is not aware of such check, so
all access control checks are not performed (e.g. file permission, LSMs
security policies, integrity and authenticity checks), it is not handled
with audit, and more importantly this would not work on generic
distributions because of the strict requirement and chromeOS-specific
assumptions.

[8] https://issuetracker.google.com/issues/40054993
[9] https://chromium.googlesource.com/chromiumos/overlays/chromiumos-overlay/+/6abfc9e327241a5f684b8b941c899b7ca8b6dbc1/sys-libs/glibc/files/local/glibc-2.37/0007-Deny-LD_PRELOAD-of-files-in-NOEXEC-mount.patch

Examples
--------

The initial idea comes from CLIP OS 4 and the original implementation
has been used for more than a decade:
https://github.com/clipos-archive/clipos4_doc
Chrome OS has a similar approach:
https://www.chromium.org/chromium-os/developer-library/guides/security/noexec-shell-scripts/

User space patches can be found here:
https://github.com/clipos-archive/clipos4_portage-overlay/search?q=O_MAYEXEC
There is more than the O_MAYEXEC changes (which matches this search)
e.g., to prevent Python interactive execution. There are patches for
Bash, Wine, Java (Icedtea), Busybox's ash, Perl and Python. There are
also some related patches which do not directly rely on O_MAYEXEC but
which restrict the use of browser plugins and extensions, which may be
seen as scripts too:
https://github.com/clipos-archive/clipos4_portage-overlay/tree/master/www-client

Past talks and articles
-----------------------

Closing the script execution control gap at Linux Plumbers Conference
2024: https://lpc.events/event/18/contributions/1692/

An introduction to O_MAYEXEC was given at the Linux Security Summit
Europe 2018 - Linux Kernel Security Contributions by ANSSI:
https://www.youtube.com/watch?v=chNjCRtPKQY&t=17m15s

The "write xor execute" principle was explained at Kernel Recipes 2018 -
CLIP OS: a defense-in-depth OS:
https://www.youtube.com/watch?v=PjRE0uBtkHU&t=11m14s

LWN articles:
* https://lwn.net/Articles/982085/
* https://lwn.net/Articles/832959/
* https://lwn.net/Articles/820000/

FAQ
Link: https://lore.kernel.org/r/[e-mail]
---

Q: Why not extend open(2) or openat2(2) with a new flag like O_MAYEXEC?
A: Because it is not flexible enough:
https://lore.kernel.org/r/CAG48ez0NAV5gPgmbDaSjo=zzE=FgnYz=-OHuXwu0Vts=[e-mail]

Q: Why not only allowing file descriptor to avoid TOCTOU?
A: Because there are different use cases:
https://lore.kernel.org/r/CAHk-=whb=XuU=[e-mail]

Q: We can copy a script into a memfd and use it as an executable FD.
   Wouldn't that bypass the purpose of this patch series?
A: If an attacker can create a memfd it means that a
   malicious/compromised code is already running and it's too late for
   script execution control to help.  This patch series makes it more
   difficult for an attacker to execute arbitrary code on a trusted
   system in the first place:
https://lore.kernel.org/all/[e-mail]/

Q: What about ROP?
A: See previous answer. If ROP is exploited then the attacker already
   controls some code:
https://lore.kernel.org/all/[e-mail]/

Q: What about LD_PRELOAD environment variable?
A: The dynamic linker should be enlighten to check if libraries are
   allowed to be loaded.

Q: What about The PATH environment variable?
A: All programs allowed to be executed are deemed trusted.

Q: Should we check seccomp filters too?
A: Yes, they should be considered as executable code because they can
   change the behavior of processes, similarly to code injection:
https://lore.kernel.org/all/[e-mail]/

Q: Could that be used for role transition?
A: That would be risky and difficult to implement correctly:
https://lore.kernel.org/all/[e-mail]/

Previous versions
-----------------

v19: https://lore.kernel.org/r/[e-mail]
v18: https://lore.kernel.org/r/[e-mail]
v17: https://lore.kernel.org/r/[e-mail]
v16: https://lore.kernel.org/r/[e-mail]
v15: https://lore.kernel.org/r/[e-mail]
v14: https://lore.kernel.org/r/[e-mail]
v13: https://lore.kernel.org/r/[e-mail]
v12: https://lore.kernel.org/r/[e-mail]
v11: https://lore.kernel.org/r/[e-mail]
v10: https://lore.kernel.org/r/[e-mail]
v9: https://lore.kernel.org/r/[e-mail]
v8: https://lore.kernel.org/r/[e-mail]
v7: https://lore.kernel.org/r/[e-mail]
v6: https://lore.kernel.org/r/[e-mail]
v5: https://lore.kernel.org/r/[e-mail]
v4: https://lore.kernel.org/r/[e-mail]
v3: https://lore.kernel.org/r/[e-mail]
v2: https://lore.kernel.org/r/[e-mail]
v1: https://lore.kernel.org/r/[e-mail]

Regards,

Mickaël Salaün (6):
  exec: Add a new AT_CHECK flag to execveat(2)
  security: Add EXEC_RESTRICT_FILE and EXEC_DENY_INTERACTIVE securebits
  selftests/exec: Add 32 tests for AT_CHECK and exec securebits
  selftests/landlock: Add tests for execveat + AT_CHECK
  samples/check-exec: Add set-exec
  samples/check-exec: Add an enlighten "inc" interpreter and 28 tests

 fs/exec.c                                     |  18 +-
 include/linux/binfmts.h                       |   7 +-
 include/uapi/linux/fcntl.h                    |  31 ++
 include/uapi/linux/securebits.h               | 113 ++++-
 kernel/audit.h                                |   1 +
 kernel/auditsc.c                              |   1 +
 samples/Kconfig                               |   8 +
 samples/Makefile                              |   1 +
 samples/check-exec/.gitignore                 |   2 +
 samples/check-exec/Makefile                   |  15 +
 samples/check-exec/inc.c                      | 204 ++++++++
 samples/check-exec/run-script-ask.inc         |   8 +
 samples/check-exec/script-ask.inc             |   4 +
 samples/check-exec/script-exec.inc            |   3 +
 samples/check-exec/script-noexec.inc          |   3 +
 samples/check-exec/set-exec.c                 |  85 ++++
 security/commoncap.c                          |  29 +-
 security/security.c                           |  10 +
 tools/testing/selftests/exec/.gitignore       |   4 +
 tools/testing/selftests/exec/Makefile         |  19 +-
 .../selftests/exec/check-exec-tests.sh        | 205 ++++++++
 tools/testing/selftests/exec/check-exec.c     | 446 ++++++++++++++++++
 tools/testing/selftests/exec/config           |   2 +
 tools/testing/selftests/exec/false.c          |   5 +
 .../selftests/kselftest/ktap_helpers.sh       |   2 +-
 tools/testing/selftests/landlock/fs_test.c    |  26 +
 26 files changed, 1240 insertions(+), 12 deletions(-)
 create mode 100644 samples/check-exec/.gitignore
 create mode 100644 samples/check-exec/Makefile
 create mode 100644 samples/check-exec/inc.c
 create mode 100755 samples/check-exec/run-script-ask.inc
 create mode 100755 samples/check-exec/script-ask.inc
 create mode 100755 samples/check-exec/script-exec.inc
 create mode 100644 samples/check-exec/script-noexec.inc
 create mode 100644 samples/check-exec/set-exec.c
 create mode 100755 tools/testing/selftests/exec/check-exec-tests.sh
 create mode 100644 tools/testing/selftests/exec/check-exec.c
 create mode 100644 tools/testing/selftests/exec/config
 create mode 100644 tools/testing/selftests/exec/false.c


base-commit: 8cf0b93919e13d1e8d4466eb4080a4c4d9d66d7b