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

[Kees Cook]

On Mon, Sep 17, 2018 at 10:13 AM, Casey Schaufler
<casey at schaufler-ca.com> wrote:
> 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.

Oh dang, yes, I got confused over secid and other "extreme" shared things.

So one change of my series would be to declare tomoyo as "exclusive" too.

> 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
> security_module_enable().

True. With my removal of security_module_enable(), yes, it makes sense
to mark all LSMs that were calling it before as exclusive, rather than
focusing on whether they would be exclusive under the blob-sharing
situation.

> 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

We've got two issues: ordering and enablement. It's been strongly
suggested that we should move away from per-LSM enable/disable flags
(to which I agree). If ordering should be separate from enablement (to
avoid the "booted kernel with new LSM built in, but my lsm="..." line
didn't include it so it's disabled case), then I think we need to
split the logic (otherwise we just reinvented "security=" with similar
problems).

Should "lsm=" allow arbitrary ordering? (I think yes.)

Should "lsm=" imply implicit enable/disable? (I think no: unlisted
LSMs are implicitly auto-appended to the explicit list)

So then we could have "lsm.enable=..." and "lsm.disable=...".

If builtin list was:
capability,yama,loadpin,integrity,{selinux,smack,tomoyo,apparmor}
then:

    lsm.disable=loadpin lsm=smack

becomes

    capability,smack,yama,integrity

and

    CONFIG_SECURITY_LOADPIN_DEFAULT_ENABLED=n
    selinux.enable=0 lsm.add=loadpin lsm.disable=smack,tomoyo lsm=integrity

becomes

    capability,integrity,yama,loadpin,apparmor


If "lsm=" _does_ imply enablement, then how does it interact with
per-LSM disabling? i.e. what does "apparmor.enabled=0
lsm=yama,apparmor" mean? If it means "turn on apparmor" how do I turn
on a CONFIG-default-off LSM without specifying all the other LSMs too?

-Kees

-- 
Kees Cook
Pixel Security