Re: [PATCH v3 1/1] security: Add CONFIG_LSM_AUTO to handle default LSM stack ordering

[Casey Schaufler <casey@schaufler-ca.com>]

On 2/22/2021 1:12 PM, Nicolas Iooss wrote:
> On Mon, Feb 22, 2021 at 9:32 PM Casey Schaufler <casey@schaufler-ca.com> wrote:
>> On 2/22/2021 10:31 AM, Mickaël Salaün wrote:
>>> On 22/02/2021 17:51, Casey Schaufler wrote:
>>>> On 2/22/2021 7:06 AM, Mickaël Salaün wrote:
>>>>> From: Mickaël Salaün <mic@linux.microsoft.com>
>>>>>
>>>>> Add a new option CONFIG_LSM_AUTO to enable users to delegate default LSM
>>>>> stacking order to kernel developers.  This enable to keep a consistent
>>>>> order of enabled LSM when changing the LSM selection, especially when a
>>>>> new LSM is added to the kernel.
>>>> TL;DR - NAK
>>>>
>>>> Do you think that we might have considered this when stacking was
>>>> introduced?
>>> I didn't dig the detailed history of LSM stacking, but you are in Cc
>>> because I know that you know. I may have though that the main goal of
>>> the current LSM stacking implementation was to enable to stack existing
>>> LSMs, which works well with this CONFIG_LSM list, but doesn't work as
>>> well for new LSMs.
>> It works just fine for new LSMs if you treat them as significant
>> features which may have significant impact on the behavior of the
>> system.
>>
>>>> Did you even consider the implications before sending
>>>> the patch?
>>> Yes, and it doesn't change much the current behavior without user
>>> interaction. However, it gives the choice to users to choose how they
>>> want their configuration to evolve.
>> Automatic inclusions of new LSMs would be counter to existing practice.
>> It won't work for "major" LSMs.
>>
>>
>>>> This only makes any sense if you want to compile in
>>>> AppArmor and/or Smack but always use SELinux. The existing Kconfig
>>>> model handles that perfectly well.
>>> This patch series doesn't change this behavior if the user doesn't want
>>> it to change.
>> Well, there's the question. If a distribution/system uses the new scheme
>> "users" are going to get new LSMs spontaniously. If they don't it's up to
>> the "user". Unsophisticated users won't want this, and the others don't
>> need it.
> Hello, sorry if I missed something simple but I did not understand
> what "Automatic inclusions of new LSMs " and "get new LSMs
> spontaniously" is about. If I understood the kernel practice
> development correctly, when a new LSM will be included, it will have a
> dedicated "config SECURITY_MYNEWLSM" which will be default to "n" in
> order to respect the "principle of least astonishment". How could such
> a new LSM be automatically/spontaneously added to the LSM list?

It wouldn't. But compiling the new LSM mynewlsm doesn't add it to
the list, either. Today no one should expect a LSM to be active if
it hasn't been added to the CONFIG_LSM list. The proposed addition
of CONFIG_LSM_AUTO would change that. "make oldconfig" would add
security modules that are built to the list. This is unnecessary
since whoever changed CONFIG_SECURITY_MYNEWLSM to "y" could easily
have added it to CONFIG_LSM. In the right place.

> I understand that this is a tough issue and that the subject might
> have been discussed a few years ago, and if that's the case, it would
> be nice to have pointers to some clear documentation or past emails
> (and it would be very very nice if the kernel documentation was
> updated to document the current state of LSM stacking:

I'm not going to argue against that.

>  for example
> https://www.kernel.org/doc/html/v5.11/admin-guide/LSM/index.html still
> documents the "security=" kernel parameter even though it conflicts
> with CONFIG_LSM and can be ignored by the kernel in practise).

You can still select one "major" module using security= if you
don't use lsm= to specify a full list. We put real effort into
being backward compatible. 

>
> Thanks,
> Nicolas
>