[PATCH 10/10] LSM: Blob sharing support for S.A.R.A and LandLock

[Casey Schaufler]

On 9/14/2018 1:05 PM, Kees Cook wrote:
> On Fri, Sep 14, 2018 at 8:57 AM, Casey Schaufler <casey at schaufler-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...

Looking forward to seeing them.

>>> 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.

That shouldn't be difficult.

>> 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.

OK

>> 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. ;)

SELinux set the precedence on that one. Not my fault!

>> /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"?

Sorry. I changed terminology (order and priority) halfway through the
message. Yes, I like order better. We should stick with that.