Re: [PATCH v2] KVM: VMX: Enable Notify VM exit

[Xiaoyao Li]

On 9/10/2021 2:47 AM, Sean Christopherson wrote:
> On Tue, Sep 07, 2021, Xiaoyao Li wrote:
> > On 9/3/2021 12:29 AM, Sean Christopherson wrote:
> > > > After syncing internally, we know that the internal threshold is not
> > > > architectural but a model-specific value. It will be published in some place
> > > > in future.
> > >
> > > Any chance it will also be discoverable, e.g. via an MSR?
> >
> > I also hope we can expose it via MSR. If not, we can maintain a table per
> > FMS in KVM to get the internal threshold. However, per FMS info is not
> > friendly to be virtualized (when we are going to enable the nested support).
>
> Yeah, FMS is awful.  If the built-in buffer isn't discoverable, my vote is to
> assume the worst, i.e. a built-in buffer of '0', and have the notify_window
> param default to a safe value, e.g. 25k or maybe even 150k (to go above what the
> hardware folks apparently deemed safe for SPR).  It's obviously not idea, but
> it's better than playing FMS guessing games.
>
> > I'll try to persuade internal to expose it via MSR, but I guarantee nothing.
>
> ...
>
> > > On a related topic, this needs tests.  One thought would be to stop unconditionally
> > > intercepting #AC if NOTIFY_WINDOW is enabled, and then have the test set up the
> > > infinite #AC vectoring scenario.
> >
> > yes, we have already tested with this case with notify_window set to 0. No
> > false positive.
>
> Can you send a selftest or kvm-unit-test?

Actually we implement the attacking case of CVE-2015-5307 with 
kvm-unit-test, while manually disabling the intercept of #AC.

First, it requires modification of KVM that only posting the 
kvm-unit-test doesn't help.

Second, release the attacking case is not the correct action.