[PATCH 4/5] LSM: Define SELinux function to measure security state

[Mimi Zohar]

On Mon, 2020-06-15 at 16:18 -0700, Casey Schaufler wrote:
> On 6/15/2020 10:44 AM, Mimi Zohar wrote:
> > (Cc'ing John)
> >
> > On Mon, 2020-06-15 at 10:33 -0700, Casey Schaufler wrote:
> >> On 6/15/2020 9:45 AM, Lakshmi Ramasubramanian wrote:
> >>> On 6/15/20 4:57 AM, Stephen Smalley wrote:
> >>>
> >>> Hi Stephen,
> >>>
> >>> Thanks for reviewing the patches.
> >>>
> >>>>> +void security_state_change(char *lsm_name, void *state, int state_len)
> >>>>> +{
> >>>>> +       ima_lsm_state(lsm_name, state, state_len);
> >>>>> +}
> >>>>> +
> >>>> What's the benefit of this trivial function instead of just calling
> >>>> ima_lsm_state() directly?
> >>> One of the feedback Casey Schaufler had given earlier was that calling an IMA function directly from SELinux (or, any of the Security Modules) would be a layering violation.
> >> Hiding the ima_lsm_state() call doesn't address the layering.
> >> The point is that SELinux code being called from IMA (or the
> >> other way around) breaks the subsystem isolation. Unfortunately,
> >> it isn't obvious to me how you would go about what you're doing
> >> without integrating the subsystems.
> > Casey, I'm not sure why you think there is a layering issue here.
> 
> I don't think there is, after further review. If the IMA code called
> selinux_dosomething() directly I'd be very concerned, but calling
> security_dosomething() which then calls selinux_dosomething() is fine.
> If YAMA called security_dosomething() I'd be very concerned, but that's
> not what's happening here.

As long as the call to IMA is not an LSM hook, there shouldn't be a
problem with an LSM calling IMA directly.  A perfect example is
measuring, appraising and/or auditing LSM policies.

Mimi

> 
> >  There were multiple iterations of IMA before it was upstreamed.  One
> > iteration had separate integrity hooks(LIM).  Only when the IMA calls
> > and the security hooks are co-located, are they combined, as requested
> > by Linus.
> >
> > There was some AppArmour discussion about calling IMA directly, but I
> > haven't heard about it in a while or seen the patch.