Date:   Fri, 14 Sep 2018 13:05:07 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Casey Schaufler <casey@...aufler-ca.com>
Cc:     Paul Moore <paul@...l-moore.com>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        James Morris <jmorris@...ei.org>,
        LKML <linux-kernel@...r.kernel.org>,
        SE Linux <selinux@...ho.nsa.gov>,
        John Johansen <john.johansen@...onical.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        "Schaufler, Casey" <casey.schaufler@...el.com>
Subject: Re: [PATCH 10/10] LSM: Blob sharing support for S.A.R.A and LandLock

On Fri, Sep 14, 2018 at 8:57 AM, Casey Schaufler <casey@...aufler-ca.com> wrote:
> On 9/13/2018 5:19 PM, Kees Cook wrote:
>> We already have the minor LSMs that cannot change order.
>
> Are you saying that we don't have a mechanism to change
> the order, or that they wouldn't work right in a different
> order? Well, there's the capability module that has to be
> first.

I just meant their order is explicit in security.c.

>> They aren't
>> part of security= parsing either.
>
> True, but there's no reason now that we couldn't change that.
> Except for capability. Hmm.

Right, we have at least one that MUST be first (and must not be disabled).

>> Should "blob-sharing" LSMs be like major LSMs or minor LSMs?
>
> I like the idea of changing the minor modules to do the full
> registration process. That would make them all the same.
> Except for capability. In any case, the "blob-sharing" LSMs
> need to do the full registration process to account for their
> blobs sizes, and that brings the "major" behavior along with it.

I agree. I'm working on some clean-ups that I'll send out soon, though
I'm worried about some of the various boot-time options...

>> If someone is booting with "security=selinux,tomoyo" and then SARA
>> lands upstream, does that person have to explicitly add "sara" to
>> their boot args, since they're doing a non-default list of LSMs?
>
> Yes. security= is explicit.
>
>> (I actually prefer the answer being "yes" here, FWIW, I just want to
>> nail down the expectations.)
>
> For now let's leave the minor (capability, yama, loadpin) as they are,
> and require all new modules of any flavor to use full registration.

I would even be fine to convert yama and loadpin.

> We could consider something like
>
> security=$lsm           # Stack with $lsm at priority 2 - Existing behavior
> $lsm.stacked=N          # Add $lsm to the stack at priority N. Delete if N == 0
>
> It's OK to specify "selinux.stacked=2" and "sara.stacked=2". Which gets
> called first is left up to the system to decide. Whatever the behavior is
> gets documented. Capability will always be first and have priority 1.
> It's OK to specify "smack.stacked=1".

I'm less excited about this kind of stacking priority, but, whatever
the case, I think my cleanups may help with whatever we decide.

> The default stack is determined by CONFIG_SECURITY_$lsm_STACKED at
> build time. CONFIG_SECURITY_$lsm_STACKED changes from a boolean to
> an integer value to establish the default hook order.
>
> /sys/kernel/security/lsm reports the modules in hook call order.

Didn't I send a patch to new-line terminate this list? I always get
annoyed when I "cat" it. ;)

> /sys/kernel/security/lsm-stack reports the list with the hook call priority
>
>         capability:1,yama:1,selinux:1,sara:5,landlack:17
>
> If stacking is not configured $lsm.stacked=0 is treated as security=none.
> For other values of N $lsm.stacked=N is treated as security=$lsm.

I feel like "order" is bad enough. Can we avoid adding "priority"?

-Kees

-- 
Kees Cook
Pixel Security

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