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

From: Casey Schaufler
Date: Mon Sep 17 2018 - 13:13:34 EST

On 9/17/2018 9:24 AM, Kees Cook wrote:
> On Mon, Sep 17, 2018 at 8:06 AM, Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote:
>>> 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.
> If that's desired then we have to declare tomoyo as "exclusive" even
> though it doesn't use blobs. But then what happens in the extreme
> stacking case? Do we add "lsm.extreme=1" to change how security= is
> parsed?

TOMOYO uses the cred blob pointer. When the blob is shared TOMOYO
has to be allocated a pointer size chunk to store the pointer in.
Smack has the same behavior on file blobs.

>>> 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.
> What about tomoyo, though? It's presently considered a major LSM (i.e.
> security=tomoyo disables the other majors), but it doesn't use blobs.
>>> 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.
> Given that tomoyo doesn't share blobs and integrity doesn't register
> hooks, how would they be considered in that world? Or rather, what
> distinguishes a "minor" LSM? It seems there are three cases: uses
> blobs with sharing, uses blobs without sharing, uses no blobs. What
> happens if an LSM grows a feature that needs blob sharing? If "uses no
> blobs" should be considered "shares blobs", then there is no
> distinction between "minor" and "blob sharing".

Today the distinction is based on how the module registers hooks.
Modules that use blobs (including TOMOYO) use security_module_enable()
and those that don't just use security_add_hooks(). The "pick one"
policy is enforced in security_module_enable(), which is why you can
have as many non-blob users as you like. You could easily have a
non-blob using module that was exclusive simply by using

In the stacking case you could have integrity_init() call
security_module_enable() but not security_add_hooks(). You wouldn't
want to do that without stacking configured, because that would
make integrity exclusive.

>>> 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.
> Agreed on MAC. And yes, I think -$lsm is best here. Should we overload
> "security=" or add "lsm.stacking="?

Keep security=$lsm with the existing exclusive behavior.

Add lsm=$lsm1,...,$lsmN which requires a full list of modules

If you want to be fancy (I don't!) you could add

lsm.add=$lsm1,...,$lsmN which adds the modules to the stack
lsm.delete=$lsm1,...,$lsmN which deletes modules from the stack