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

[MickaÃl SalaÃn]

On 06/05/2020 15:58, Lev R. Oshvang . wrote:
> On Tue, May 5, 2020 at 6:36 PM MickaÃl SalaÃn <mic@xxxxxxxxxxx> wrote:
>>
>>
>> On 05/05/2020 17:31, MickaÃl SalaÃn 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)

Aleksa replied to this.

> 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.

This patch series doesn't change the way IMA handles mount points.

> 
> Regards,
> LEv
>