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

[Tao Xu <tao3.xu@intel.com>]

On 11/3/20 12:43 AM, Andy Lutomirski wrote:
> On Sun, Nov 1, 2020 at 10:14 PM Tao Xu <tao3.xu@intel.com> 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.