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

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Date:   Mon, 7 Nov 2022 13:35:04 +0100
From:   Mickaël Salaün <mic@...ikod.net>
To:     Casey Schaufler <casey@...aufler-ca.com>,
        Paul Moore <paul@...l-moore.com>,
        Kees Cook <keescook@...omium.org>
Cc:     Nicolas Iooss <nicolas.iooss@....org>,
        James Morris <jmorris@...ei.org>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        Mickaël Salaün <mic@...ux.microsoft.com>
Subject: Re: [PATCH v3 1/1] security: Add CONFIG_LSM_AUTO to handle default
 LSM stack ordering


On 04/11/2022 18:20, Casey Schaufler wrote:
> On 11/4/2022 9:29 AM, Mickaël Salaün wrote:
>>
>> On 18/10/2022 21:31, Paul Moore wrote:
>>> On Tue, Oct 18, 2022 at 1:55 AM Kees Cook <keescook@...omium.org> wrote:
>>>> On Mon, Oct 17, 2022 at 09:45:21PM -0400, Paul Moore wrote:
>>
>> [...]
>>
>>>>> We can have defaults, like we do know, but I'm in no hurry to remove
>>>>> the ability to allow admins to change the ordering at boot time.
>>>>
>>>> My concern is with new LSMs vs the build system. A system builder will
>>>> be prompted for a new CONFIG_SECURITY_SHINY, but won't be prompted
>>>> about making changes to CONFIG_LSM to include it.
>>>
>>> I would argue that if an admin/builder doesn't understand what a shiny
>>> new LSM does, they shouldn't be enabling that shiny new LSM.  Adding
>>> new, potentially restrictive, controls to your kernel build without a
>>> basic understanding of those controls is a recipe for disaster and I
>>> try to avoid recommending disaster as a planned course of action :)
>>
>> It depends on what this shiny new LSMs do *by default*. In the case of
>> Landlock, it do nothing unless a process does specific system calls
>> (same as for most new kernel features: sysfs entries, syscall flags…).
>> I guess this is the same for most LSMs.
> 
> "By default" is somewhat ambiguous. Smack will always enforce its
> basic policy. If files aren't labeled and the Smack process label
> isn't explicitly set there won't be any problems. However, if files
> have somehow gotten labels assigned and there are no rules defined
> things can go sideways.

Right, it should then mean without effect whatever kernel-mediated 
persistent data (e.g. FS's xattr), but I agree that the limit with an 
explicit configuration can be blurry. I guess we could explicitly mark 
LSMs with a property that specify if they consider safe (for the system) 
to be implicitly enabled without explicit run time configuration.