linux-kernel - Re: [PATCH 0/2] Introduce security_create_user

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Message-ID: <83b9774f-5cda-d05f-e62d-7bf7547ae7ba@cloudflare.com>
Date:   Tue, 28 Jun 2022 11:44:47 -0500
From:   Frederick Lawler <fred@...udflare.com>
To:     KP Singh <kpsingh@...nel.org>,
        Casey Schaufler <casey@...aufler-ca.com>
Cc:     Paul Moore <paul@...l-moore.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Christian Brauner <brauner@...nel.org>, revest@...omium.org,
        jackmanb@...omium.org, ast@...nel.org, andrii@...nel.org,
        kafai@...com, songliubraving@...com, yhs@...com,
        john.fastabend@...il.com, jmorris@...ei.org, serge@...lyn.com,
        bpf@...r.kernel.org, linux-security-module@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...udflare.com
Subject: Re: [PATCH 0/2] Introduce security_create_user_ns()

On 6/28/22 11:12 AM, KP Singh wrote:
> On Tue, Jun 28, 2022 at 6:02 PM Casey Schaufler <casey@...aufler-ca.com> wrote:
>>
>> On 6/28/2022 8:14 AM, Frederick Lawler wrote:
>>> On 6/27/22 6:18 PM, Casey Schaufler wrote:
>>>> On 6/27/2022 3:27 PM, Paul Moore wrote:
>>>>> On Mon, Jun 27, 2022 at 6:15 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>>>>>> On 6/27/22 11:56 PM, Paul Moore wrote:
>>>>>>> On Mon, Jun 27, 2022 at 8:11 AM Christian Brauner <brauner@...nel.org> wrote:
>>>>>>>> On Thu, Jun 23, 2022 at 11:21:37PM -0400, Paul Moore wrote:
>>>>>>> ...
>>>>>>>
>>>>>>>>> This is one of the reasons why I usually like to see at least one LSM
>>>>>>>>> implementation to go along with every new/modified hook.  The
>>>>>>>>> implementation forces you to think about what information is necessary
>>>>>>>>> to perform a basic access control decision; sometimes it isn't always
>>>>>>>>> obvious until you have to write the access control :)
>>>>>>>> I spoke to Frederick at length during LSS and as I've been given to
>>>>>>>> understand there's a eBPF program that would immediately use this new
>>>>>>>> hook. Now I don't want to get into the whole "Is the eBPF LSM hook
>>>>>>>> infrastructure an LSM" but I think we can let this count as a legitimate
>>>>>>>> first user of this hook/code.
>>>>>>> Yes, for the most part I don't really worry about the "is a BPF LSM a
>>>>>>> LSM?" question, it's generally not important for most discussions.
>>>>>>> However, there is an issue unique to the BPF LSMs which I think is
>>>>>>> relevant here: there is no hook implementation code living under
>>>>>>> security/.  While I talked about a hook implementation being helpful
>>>>>>> to verify the hook prototype, it is also helpful in providing an
>>>>>>> in-tree example for other LSMs; unfortunately we don't get that same
>>>>>>> example value when the initial hook implementation is a BPF LSM.
>>>>>> I would argue that such a patch series must come together with a BPF
>>>>>> selftest which then i) contains an in-tree usage example, ii) adds BPF
>>>>>> CI test coverage. Shipping with a BPF selftest at least would be the
>>>>>> usual expectation.
>>>>> I'm not going to disagree with that, I generally require matching
>>>>> tests for new SELinux kernel code, but I was careful to mention code
>>>>> under 'security/' and not necessarily just a test implementation :)  I
>>>>> don't want to get into a big discussion about it, but I think having a
>>>>> working implementation somewhere under 'security/' is more
>>>>> discoverable for most LSM folks.
>>>>
>>>> I agree. It would be unfortunate if we added a hook explicitly for eBPF
>>>> only to discover that the proposed user needs something different. The
>>>> LSM community should have a chance to review the code before committing
>>>> to all the maintenance required in supporting it.
>>>>
>>>> Is there a reference on how to write an eBPF security module?
>>>
>>> There's a documentation page that briefly touches on a BPF LSM implementation [1].
>>
>> That's a brief touch, alright. I'll grant that the LSM interface isn't
>> especially well documented for C developers, but we have done tutorials
>> and have multiple examples. I worry that without an in-tree example for
>> eBPF we might well be setting developers up for spectacular failure.
>>
> 
> Casey, Daniel and I are recommending an in-tree example, it will be
> in BPF selftests and we will CC you on the reviews.
> 
> Frederick, is that okay with you?

Yep.

> 
>>>
>>>> There should be something out there warning the eBPF programmer of the
>>>> implications of providing a secid_to_secctx hook for starters.
>>>>
>>>
>>> Links:
>>> 1. https://docs.kernel.org/bpf/prog_lsm.html?highlight=bpf+lsm#
>>>