Re: [PATCH v5 0/6] Add support for O_MAYEXEC

["Lev R. Oshvang ." <levonshe@gmail.com>]

On Tue, May 5, 2020 at 6:36 PM Mickaël Salaün <mic@digikod.net> wrote:
>
>
> On 05/05/2020 17:31, Mickaël Salaün wrote:
> > Hi,
> >
> > This fifth patch series add new kernel configurations (OMAYEXEC_STATIC,
> > OMAYEXEC_ENFORCE_MOUNT, and OMAYEXEC_ENFORCE_FILE) to enable to
> > configure the security policy at kernel build time.  As requested by
> > Mimi Zohar, I completed the series with one of her patches for IMA.
> >
> > 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 interpreter 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 openat2(2) [2], SGX integration
> > [3], bpffs [4] or IPE [5].
> >
> > 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.
> >
> > The initial idea come from CLIP OS 4 and the original implementation has
> > been used for more than 12 years:
> > https://github.com/clipos-archive/clipos4_doc
> >
> > 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
> >
> > This patch series can be applied on top of v5.7-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/20200428175129.634352-1-mic@digikod.net/
>
> The previous version (v4) is
> https://lore.kernel.org/lkml/20200430132320.699508-1-mic@digikod.net/


Hi Michael

I have couple of question
1. Why you did not add O_MAYEXEC to open()?
Some time ago (around v4.14) open() did not return EINVAL when
VALID_OPEN_FLAGS check failed.
Now it does, so I do not see a problem that interpreter will use
simple open(),  ( Although that path might be manipulated, but file
contents will be verified by IMA)
2. When you apply a new flag to mount, it means that IMA will check
all files under this mount and it does not matter whether the file in
question is a script or not.
IMHO it is too hard overhead for performance reasons.

Regards,
LEv