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

[Aleksa Sarai]

On 2020-05-06, Lev R. Oshvang . <levonshe@xxxxxxxxx> wrote:
> On Tue, May 5, 2020 at 6:36 PM Mickaël Salaün <mic@xxxxxxxxxxx> 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@xxxxxxxxxxx/
> >
> > The previous version (v4) is
> > https://lore.kernel.org/lkml/20200430132320.699508-1-mic@xxxxxxxxxxx/
> 
> 
> 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)

You don't get -EINVAL from open() in the case of unknown flags, that's
something only openat2() does in the open*() family. Hence why it's only
introduced for openat2().

> 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


-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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