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

[Andy Lutomirski]

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.