[PATCH 16/18] LSM: Allow arbitrary LSM ordering

[Casey Schaufler]

On 9/16/2018 4:00 PM, Kees Cook wrote:
> On Sun, Sep 16, 2018 at 11:49 AM, Casey Schaufler
> <casey at schaufler-ca.com> wrote:
>> On 9/15/2018 5:30 PM, Kees Cook wrote:
>>> To prepare for having a third type of LSM ("shared blob"), this implements
>>> dynamic handling of LSM ordering. The visible change here is that the
>>> "security=" boot commandline is now a comma-separated ordered list of
>>> all LSMs, not just the single "exclusive" LSM. This means that the
>>> "minor" LSMs can now be disabled at boot time by omitting them from the
>>> commandline. Additionally LSM ordering becomes entirely mutable for LSMs
>>> with LSM_ORDER_MUTABLE ("capability" is not mutable and is always enabled
>>> first).
>> Today if I have Yama enabled and use security=apparmor I get a
>> module list of capability,yama,apparmor. With this change I would
>> get a different result, capability,apparmor. I am personally OK with
>> this, but I think others may see it as a violation of compatibility.
> Correct. That is the problem I had asked about earlier: it means
> people with existing security= for specifying the active major LSM
> will _disable_ all the minor LSMs after this change. It makes me
> uncomfortable.
>
>> One solution is to leave security= as is, not affecting "minor"
>> modules and only allowing specification of one major module, and adding
> I would much prefer this, yes.
>
> A question remains: how do we map the existing "security=" selection
> of a "major" LSM against what will be next "exclusive" plus tomoyo,
> and in the extreme case, nothing?
>
> Perhaps as part of deprecating "security=", we could just declare that
> it is selecting between SELinux, AppArmor, Smack, and Tomoyo only?

I'd be happier keeping yama and loadpin as the special cases. Someone
might want to say security=landlock and expect the existing "minor"
module behavior.

>> another boot option security.stack= that overrides a security= option
>> and that takes the list as you've implemented here.
> or "lsm.stack=" that overrides "security=" entirely?

I thought about that. In some ways that would be most sane.

>> An icky alternative would be to say that any security= specification
>> with no commas in it retains the old behavior. So
>>         security=apparmor
>>         security=apparmor,
>> would get you
>>         capability,yama,apparmor
>>         capability,apparmor
>> respectively.
>>
>> Another option would be to require negation on the minor modules,
>> such as
>>         security=apparmor,-loadpin
>>
>> I can't honestly say which I like least or best.
> The trailing comma thing gets us some compatibility, but we still have
> to decide which things should be exclusive-via-"security=" since with
> blob-sharing it already becomes possible to do selinux + tomoyo.
>
> The -$lsm style may make it hard to sensibly order any unspecified
> LSMs. I guess it could just fall back to "follow builtin ordering of
> unspecified LSMs" (unless someone had, maybe, "-all").

That's why I'm not especially happy with either one.

> so, if builtin ordering after blob-sharing is
> capability,integrity,yama,loadpin,{selinux,apparmor,smack},tomoyo
>
> security=apparmor  is  capability,apparmor,integrity,yama,loadpin,tomoyo

I would expect capability,integrity,yama,loadpin,apparmor to reflect
today's behavior.

> security=yama,smack,-all  is  capability,yama,smack

Yes

> security=loadpin,selinux,yama,-integrity  is
> capability,loadpin,selinux,yama,tomoyo

I think that the negation should only apply to
integrity, yama and loadpin. All blob-using modules
must be explicitly stated if you want to use them.

> Whatever we design, it needs to handle both the blob-sharing
> near-future, and have an eye towards "extreme stacking" in the
> some-day future. In both cases, the idea of a "major" LSM starts to
> get very very hazy.

Long term the only distinction is "minor" and blob using. So long as
there's a way to enforce incompatibility (i.e. not Smack and SELinux)
in the sorter term we can adopt that mindset already.

> As for how we classify things, based on hooks...
>
> now:
>     always: capability
>     major: selinux,apparmor,smack,tomoyo
>     minor: yama,loadpin
>     init-only: integrity
>
> blob-sharing:
>     always: capability
>     exclusive: selinux,apparmor,smack
>     sharing: tomoyo,integrity,yama,loadpin
>
> extreme:
>     always: capability
>     sharing: selinux,apparmor,smack,tomoyo,integrity,yama,loadpin
>
> The most special are capability (unconditional, run first) and
> integrity (init-only, no security_add_hooks() call).
>
> Can we classify things as MAC and non-MAC for "major" vs "minor"? SARA
> and Landlock aren't MAC (and neither is integrity), or should we do
> the "-$lsm" thing instead?

I don't like using MAC because the use of the module isn't the issue,
it's the interfaces used. As ugly as it is, I like the -$lsm better.

>
> -Kees
>