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

[Tao Xu]

On 11/3/20 12:43 AM, Andy Lutomirski wrote:
> On Sun, Nov 1, 2020 at 10:14 PM Tao Xu <tao3.xu@xxxxxxxxx> wrote:
> > There are some cases that malicious virtual machines can cause CPU stuck
> > (event windows don't open up), e.g., infinite loop in microcode when
> > nested #AC (CVE-2015-5307). No event window obviously means no events,
> > e.g. NMIs, SMIs, and IRQs will all be blocked, may cause the related
> > hardware CPU can't be used by host or other VM.
> >
> > To resolve those cases, it can enable a notify VM exit if no
> > event window occur in VMX non-root mode for a specified amount of
> > time (notify window).
> >
> > Expose a module param for setting notify window, default setting it to
> > the time as 1/10 of periodic tick, and user can set it to 0 to disable
> > this feature.
> >
> > TODO:
> > 1. The appropriate value of notify window.
> > 2. Another patch to disable interception of #DB and #AC when notify
> > VM-Exiting is enabled.
>
> Whoa there.
>
> A VM control that says "hey, CPU, if you messed up and livelocked for
> a long time, please break out of the loop" is not a substitute for
> fixing the livelocks.  So I don't think you get do disable
> interception of #DB and #AC.  I also think you should print a loud
> warning and have some intelligent handling when this new exit
> triggers.
>
> > +static int handle_notify(struct kvm_vcpu *vcpu)
> > +{
> > +       unsigned long exit_qualification = vmcs_readl(EXIT_QUALIFICATION);
> > +
> > +       /*
> > +        * Notify VM exit happened while executing iret from NMI,
> > +        * "blocked by NMI" bit has to be set before next VM entry.
> > +        */
> > +       if (exit_qualification & NOTIFY_VM_CONTEXT_VALID) {
> > +               if (enable_vnmi &&
> > +                   (exit_qualification & INTR_INFO_UNBLOCK_NMI))
> > +                       vmcs_set_bits(GUEST_INTERRUPTIBILITY_INFO,
> > +                                     GUEST_INTR_STATE_NMI);
>
> This needs actual documentation in the SDM or at least ISE please.
Notify VM-Exit is defined in ISE, chapter 9.2:
https://software.intel.com/content/dam/develop/external/us/en/documents/architecture-instruction-set-extensions-programming-reference.pdf

I will add this information into commit message. Thank you for reminding me.