Re: [PATCH v6 5/7] KVM: x86: VMX: Prevent MSR passthrough when MSR access is denied

[Aaron Lewis <aaronlewis@google.com>]

> >
> >> +
> >>   /*
> >>    * These 2 parameters are used to config the controls for Pause-Loop Exiting:
> >>    * ple_gap:    upper bound on the amount of time between two successive
> >> @@ -622,6 +642,41 @@ static inline bool report_flexpriority(void)
> >>          return flexpriority_enabled;
> >>   }
> >
> > One thing that seems to be missing is removing MSRs from the
> > permission bitmap or resetting the permission bitmap to its original
> > state before adding changes on top of it.  This would be needed on
> > subsequent calls to kvm_vm_ioctl_set_msr_filter().  When that happens
> > the original changes made by KVM_REQ_MSR_FILTER_CHANGED need to be
> > backed out before applying the new set.
>
> I'm not sure I follow. Subsequent calls to set_msr_filter() will invoke
> the "please reset the whole MSR passthrough bitmap to a consistent
> state" which will then reapply the in-kvm desired state through the
> bitmap and filter state on top on each of those.
>

Yes, you're correct.  I discovered this after the fact by adding a
test to the selftest I wrote for the deny list system which I revamped
to work for your filter system.  It proved the permission bitmaps are
in fact set as expected on subsequent calls.  You can disregard this
comment.

As a side note, I'm happy to share the test if you'd like. I also used
it to uncover an issue in the first commit of this series.