[v15,0/3] Add trusted_for(2) (was O_MAYEXEC)

Message

Mickaël Salaün Oct. 12, 2021, 7:24 p.m. UTC

Hi,

This new patch series, rebase on v5.15-rc4, adds a full syscall
documentation as requested by Andrew Morton.  This cover letter also
briefly describes the syscall.  Andrew, can you please consider to merge
this into your 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 set by the system
administrator through a sysctl configuration consistent with the mount
points or the file access rights.  The documentation 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].

System call description
=======================

trusted_for(int fd, enum trusted_for_usage usage, u32 flags);

@fd is the file descriptor to check.

@usage identifies the user space usage intended for @fd: only
TRUSTED_FOR_EXECUTION for now, but trusted_for_usage could be extended
to identify other usages (e.g. configuration, sensitive data).

@flags must be 0 for now but it could be used in the future to do
complementary checks (e.g. signature or integrity requirements, origin
of the file).

This system call returns 0 on success, or -EACCES if the kernel policy
denies the specified usage (which should be enforced by the caller).

The first patch contains the full syscall and sysctl documentation.

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 13 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 v5.10-rc6 .  This can be
tested with CONFIG_SYSCTL.  I would really appreciate constructive
comments on this patch series.

Previous series:
https://lore.kernel.org/r/[e-mail]/

[1] https://lore.kernel.org/lkml/[e-mail]/
[2] https://lore.kernel.org/lkml/[e-mail]/
[3] https://lore.kernel.org/lkml/CALCETrVovr8XNZSroey7pHF46O=kj_c5D9K8h=[e-mail]/
[4] https://lore.kernel.org/lkml/[e-mail]/
[5] https://lore.kernel.org/lkml/[e-mail]/
[6] https://lore.kernel.org/lkml/[e-mail]/
[7] https://www.python.org/dev/peps/pep-0578/
[8] https://lore.kernel.org/lkml/[e-mail]/

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                                     | 110 ++++++
 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, 600 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: 9e1ff307c779ce1f0f810c7ecce3d95bbae40896