Re: [PATCH v7 0/7] Add support for O_MAYEXEC

Kees Cook
2020-07-24T19:06:59Z

I think this looks good now.

Andrew, since you're already carrying my exec clean-ups (repeated here
in patch 1-3), can you pick the rest of this series too?

Thanks!

-Kees

On Thu, Jul 23, 2020 at 07:12:20PM +0200, Mickaël Salaün wrote:
> Hi,
>
> This seventh patch series do not set __FMODE_EXEC for the sake of
> simplicity. A notification feature could be added later if needed. The
> handling of all file types is now well defined and tested: by default,
> when opening a path, access to a directory is denied (with EISDIR),
> access to a regular file depends on the sysctl policy, and access to
> other file types (i.e. fifo, device, socket) is denied if there is any
> enforced policy. There is new tests covering all these cases (cf.
> test_file_types() ).
>
> As requested by Mimi Zohar, I completed the series with one of her
> patches for IMA. I also picked Kees Cook's patches to consolidate exec
> permission checking into do_filp_open()'s flow.
>
>
> # Goal of O_MAYEXEC
>
> The goal of this patch series is to enable to control script execution
> with interpreters help. A new O_MAYEXEC flag, usable through
> openat2(2), is added to enable userspace script interpreters to delegate
> to the kernel (and thus the system security policy) the permission to
> interpret/execute scripts or other files containing what can be seen as
> commands.
>
> A simple system-wide security policy can be enforced by the system
> administrator through a sysctl configuration consistent with the mount
> points or the file access rights. The documentation patch explains the
> prerequisites.
>
> Furthermore, the security policy can also be delegated to an LSM, either
> a MAC system or an integrity system. For instance, the new kernel
> MAY_OPENEXEC flag closes a major IMA measurement/appraisal interpreter
> integrity gap by bringing the ability to check the use of scripts [1].
> Other uses are expected, such as for magic-links [2], SGX integration
> [3], bpffs [4] or IPE [5].
>
>
> # Prerequisite of its use
>
> Userspace needs to adapt to take advantage of this new feature. For
> example, 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
> withou -c, stdin piping of code) are on their way [7].
>
>
> # Examples
>
> The initial idea comes from CLIP OS 4 and the original implementation
> has been used for more than 12 years:
> https://github.com/clipos-archive/clipos4_doc
> Chrome OS has a similar approach:
> https://chromium.googlesource.com/chromiumos/docs/+/master/security/noexec_shell_scripts.md
>
> Userland patches can be found here:
> https://github.com/clipos-archive/clipos4_portage-overlay/search?q=O_MAYEXEC
> Actually, 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
>
> 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
> See also an overview article: https://lwn.net/Articles/820000/
>
>
> This patch series can be applied on top of v5.8-rc5 . This can be tested
> with CONFIG_SYSCTL. I would really appreciate constructive comments on
> this patch series.
>
> Previous version:
> https://lore.kernel.org/lkml/20200505153156.925111-1-mic@digikod.net/
>
>
> [1] https://lore.kernel.org/lkml/1544647356.4028.105.camel@linux.ibm.com/
> [2] https://lore.kernel.org/lkml/20190904201933.10736-6-cyphar@cyphar.com/
> [3] https://lore.kernel.org/lkml/CALCETrVovr8XNZSroey7pHF46O=kj_c5D9K8h=z2T_cNrpvMig@mail.gmail.com/
> [4] https://lore.kernel.org/lkml/CALCETrVeZ0eufFXwfhtaG_j+AdvbzEWE0M3wjXMWVEO7pj+xkw@mail.gmail.com/
> [5] https://lore.kernel.org/lkml/20200406221439.1469862-12-deven.desai@linux.microsoft.com/
> [6] https://www.python.org/dev/peps/pep-0578/
> [7] https://lore.kernel.org/lkml/0c70debd-e79e-d514-06c6-4cd1e021fa8b@python.org/
>
> Regards,
>
> Kees Cook (3):
> exec: Change uselib(2) IS_SREG() failure to EACCES
> exec: Move S_ISREG() check earlier
> exec: Move path_noexec() check earlier
>
> Mickaël Salaün (3):
> fs: Introduce O_MAYEXEC flag for openat2(2)
> fs,doc: Enable to enforce noexec mounts or file exec through O_MAYEXEC
> selftest/openat2: Add tests for O_MAYEXEC enforcing
>
> Mimi Zohar (1):
> ima: add policy support for the new file open MAY_OPENEXEC flag
>
> Documentation/ABI/testing/ima_policy | 2 +-
> Documentation/admin-guide/sysctl/fs.rst | 49 +++
> fs/exec.c | 23 +-
> fs/fcntl.c | 2 +-
> fs/namei.c | 36 +-
> fs/open.c | 12 +-
> include/linux/fcntl.h | 2 +-
> include/linux/fs.h | 3 +
> include/uapi/asm-generic/fcntl.h | 7 +
> kernel/sysctl.c | 12 +-
> security/integrity/ima/ima_main.c | 3 +-
> security/integrity/ima/ima_policy.c | 15 +-
> tools/testing/selftests/kselftest_harness.h | 3 +
> tools/testing/selftests/openat2/Makefile | 3 +-
> tools/testing/selftests/openat2/config | 1 +
> tools/testing/selftests/openat2/helpers.h | 1 +
> .../testing/selftests/openat2/omayexec_test.c | 325 ++++++++++++++++++
> 17 files changed, 470 insertions(+), 29 deletions(-)
> create mode 100644 tools/testing/selftests/openat2/config
> create mode 100644 tools/testing/selftests/openat2/omayexec_test.c
>
> --
> 2.27.0
>

--
Kees Cook