Message-Id: <20200910164612.114215-1-mic@digikod.net>
Date:   Thu, 10 Sep 2020 18:46:09 +0200
From:   Mickaël Salaün <mic@...ikod.net>
To:     linux-kernel@...r.kernel.org
Cc:     Mickaël Salaün <mic@...ikod.net>,
        Aleksa Sarai <cyphar@...har.com>,
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        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: [RFC PATCH v9 0/3] Add introspect_access(2) (was O_MAYEXEC)

Hi,

This ninth patch series rework the previous AT_INTERPRETED and O_MAYEXEC
series with a new syscall: introspect_access(2) .  Access check are now
only possible on a file descriptor, which enable to avoid possible race
conditions in user space.

For now, the only LSM hook triggered by introspect_access(2) is
inode_permission() which takes a struct inode as argument.  However,
struct path is still available in this syscall, which enables to add a
new hook to fit the needs of IMA and other path-based LSMs.

Goal of introspect_access(2)
============================

The goal of this patch series is to enable to control script execution
with interpreters help.  A new introspect_access() system call is added
to enable user space 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_INTROSPECTION_EXEC flag 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] or IPE [5].

Possible extended usage
=======================

For now, only the X_OK mode is compatible with introspect_access(2).
This enables to restrict the addition of new control flows in a process.
Using R_OK or W_OK with introspect_access(2) returns -EINVAL.

Possible future use-cases for R_OK with introspect_access(2) may be to
check configuration files that may impact the behavior of applications
(i.e.  influence critical part of the current control flow).  Those
should then be trusted as well.  The W_OK with introspect_access(2)
could be used to check that a file descriptor is allowed to receive
sensitive data such as debug logs.

Prerequisite of its use
=======================

User space 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
without -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 v5.9-rc4 .  This can be tested
with CONFIG_SYSCTL.  I would really appreciate constructive comments on
this patch series.

Previous version:
https://lore.kernel.org/lkml/20200908075956.1069018-1-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,

Mickaël Salaün (3):
  fs: Add introspect_access(2) syscall implementation and related sysctl
  arch: Wire up introspect_access(2)
  selftest/interpreter: Add tests for introspect_access(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                                     |  79 ++++
 include/linux/fs.h                            |   3 +
 include/linux/syscalls.h                      |   1 +
 include/uapi/asm-generic/unistd.h             |   4 +-
 kernel/sysctl.c                               |  12 +-
 .../testing/selftests/interpreter/.gitignore  |   2 +
 tools/testing/selftests/interpreter/Makefile  |  18 +
 tools/testing/selftests/interpreter/config    |   1 +
 .../interpreter/introspection_policy_test.c   | 361 ++++++++++++++++++
 28 files changed, 547 insertions(+), 4 deletions(-)
 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/introspection_policy_test.c

-- 
2.28.0

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