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

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Message-ID: <d70d3b2d-6c3f-b1fc-f40c-f5ec01a627c0@schaufler-ca.com>
Date:   Tue, 28 Jun 2022 09:02:13 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Frederick Lawler <fred@...udflare.com>,
        Paul Moore <paul@...l-moore.com>,
        Daniel Borkmann <daniel@...earbox.net>
Cc:     Christian Brauner <brauner@...nel.org>, kpsingh@...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,
        Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [PATCH 0/2] Introduce security_create_user_ns()

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.

>
>> 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#
>