Re: [RESEND PATCH v11 0/3] Add trusted_for(2) (was O_MAYEXEC)

[Mickaël Salaün]

Andrew, could you please merge this into your tree?

On 19/10/2020 18:49, Mickaël Salaün wrote:
> Hi,
> 
> Can you please consider to merge this into the tree?
> 
> 
> Overview
> ========
> 
> The final goal of this patch series is to enable the kernel to be a
> global policy manager by entrusting processes with access control at
> their level.  To reach this goal, two complementary parts are required:
> * user space needs to be able to know if it can trust some file
>   descriptor content for a specific usage;
> * and the kernel needs to make available some part of the policy
>   configured by the system administrator.
> 
> Primary goal of trusted_for(2)
> ==============================
> 
> This new syscall enables user space to ask the kernel: is this file
> descriptor's content trusted to be used for this purpose?  The set of
> usage currently only contains "execution", but other may follow (e.g.
> "configuration", "sensitive_data").  If the kernel identifies the file
> descriptor as trustworthy for this usage, user space should then take
> this information into account.  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.
> 
> 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.
> 
> 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.  For instance,
> this is required to close 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].
> 
> Complementary W^X protections can be brought by SELinux, IPE [5] and
> trampfd [6].
> 
> Prerequisite of its use
> =======================
> 
> User space needs to adapt to take advantage of this new feature.  For
> example, the PEP 578 [7] (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 on their way [8].
> 
> 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 a first LWN article about O_MAYEXEC and a new one about
> trusted_for(2) and its background:
> * https://lwn.net/Articles/820000/
> * https://lwn.net/Articles/832959/
> 
> This patch series can be applied on top of v5.9 .  This can be tested
> with CONFIG_SYSCTL.  I would really appreciate constructive comments on
> this patch series.
> 
> Previous series:
> https://lore.kernel.org/lkml/20201001170232.522331-1-mic@xxxxxxxxxxx/
> 
> [1] https://lore.kernel.org/lkml/1544647356.4028.105.camel@xxxxxxxxxxxxx/
> [2] https://lore.kernel.org/lkml/20190904201933.10736-6-cyphar@xxxxxxxxxx/
> [3] https://lore.kernel.org/lkml/CALCETrVovr8XNZSroey7pHF46O=kj_c5D9K8h=z2T_cNrpvMig@xxxxxxxxxxxxxx/
> [4] https://lore.kernel.org/lkml/CALCETrVeZ0eufFXwfhtaG_j+AdvbzEWE0M3wjXMWVEO7pj+xkw@xxxxxxxxxxxxxx/
> [5] https://lore.kernel.org/lkml/20200406221439.1469862-12-deven.desai@xxxxxxxxxxxxxxxxxxx/
> [6] https://lore.kernel.org/lkml/20200922215326.4603-1-madvenka@xxxxxxxxxxxxxxxxxxx/
> [7] https://www.python.org/dev/peps/pep-0578/
> [8] https://lore.kernel.org/lkml/0c70debd-e79e-d514-06c6-4cd1e021fa8b@xxxxxxxxxx/
> 
> Regards,
> 
> Mickaël Salaün (3):
>   fs: Add trusted_for(2) syscall implementation and related sysctl
>   arch: Wire up trusted_for(2)
>   selftest/interpreter: Add tests for trusted_for(2) policies
> 
>  Documentation/admin-guide/sysctl/fs.rst       |  50 +++
>  arch/alpha/kernel/syscalls/syscall.tbl        |   1 +
>  arch/arm/tools/syscall.tbl                    |   1 +
>  arch/arm64/include/asm/unistd.h               |   2 +-
>  arch/arm64/include/asm/unistd32.h             |   2 +
>  arch/ia64/kernel/syscalls/syscall.tbl         |   1 +
>  arch/m68k/kernel/syscalls/syscall.tbl         |   1 +
>  arch/microblaze/kernel/syscalls/syscall.tbl   |   1 +
>  arch/mips/kernel/syscalls/syscall_n32.tbl     |   1 +
>  arch/mips/kernel/syscalls/syscall_n64.tbl     |   1 +
>  arch/mips/kernel/syscalls/syscall_o32.tbl     |   1 +
>  arch/parisc/kernel/syscalls/syscall.tbl       |   1 +
>  arch/powerpc/kernel/syscalls/syscall.tbl      |   1 +
>  arch/s390/kernel/syscalls/syscall.tbl         |   1 +
>  arch/sh/kernel/syscalls/syscall.tbl           |   1 +
>  arch/sparc/kernel/syscalls/syscall.tbl        |   1 +
>  arch/x86/entry/syscalls/syscall_32.tbl        |   1 +
>  arch/x86/entry/syscalls/syscall_64.tbl        |   1 +
>  arch/xtensa/kernel/syscalls/syscall.tbl       |   1 +
>  fs/open.c                                     |  77 ++++
>  include/linux/fs.h                            |   1 +
>  include/linux/syscalls.h                      |   2 +
>  include/uapi/asm-generic/unistd.h             |   4 +-
>  include/uapi/linux/trusted-for.h              |  18 +
>  kernel/sysctl.c                               |  12 +-
>  tools/testing/selftests/Makefile              |   1 +
>  .../testing/selftests/interpreter/.gitignore  |   2 +
>  tools/testing/selftests/interpreter/Makefile  |  21 +
>  tools/testing/selftests/interpreter/config    |   1 +
>  .../selftests/interpreter/trust_policy_test.c | 362 ++++++++++++++++++
>  30 files changed, 567 insertions(+), 4 deletions(-)
>  create mode 100644 include/uapi/linux/trusted-for.h
>  create mode 100644 tools/testing/selftests/interpreter/.gitignore
>  create mode 100644 tools/testing/selftests/interpreter/Makefile
>  create mode 100644 tools/testing/selftests/interpreter/config
>  create mode 100644 tools/testing/selftests/interpreter/trust_policy_test.c
> 
> 
> base-commit: bbf5c979011a099af5dc76498918ed7df445635b
>