Re: [PATCH v3 59/75] x86/sev-es: Handle MONITOR/MONITORX Events

Tom Lendacky
2020-06-11T19:33:25Z

On 6/11/20 12:13 PM, Sean Christopherson wrote:
> On Thu, Jun 11, 2020 at 03:10:45PM +0200, Joerg Roedel wrote:
>> On Tue, May 19, 2020 at 11:38:45PM -0700, Sean Christopherson wrote:
>>> On Tue, Apr 28, 2020 at 05:17:09PM +0200, Joerg Roedel wrote:
>>>> +static enum es_result vc_handle_monitor(struct ghcb *ghcb,
>>>> + struct es_em_ctxt *ctxt)
>>>> +{
>>>> + phys_addr_t monitor_pa;
>>>> + pgd_t *pgd;
>>>> +
>>>> + pgd = __va(read_cr3_pa());
>>>> + monitor_pa = vc_slow_virt_to_phys(ghcb, ctxt->regs->ax);
>>>> +
>>>> + ghcb_set_rax(ghcb, monitor_pa);
>>>> + ghcb_set_rcx(ghcb, ctxt->regs->cx);
>>>> + ghcb_set_rdx(ghcb, ctxt->regs->dx);
>>>> +
>>>> + return sev_es_ghcb_hv_call(ghcb, ctxt, SVM_EXIT_MONITOR, 0, 0);
>>>
>>> Why? If SVM has the same behavior as VMX, the MONITOR will be disarmed on
>>> VM-Enter, i.e. the VMM can't do anything useful for MONITOR/MWAIT. I
>>> assume that's the case given that KVM emulates MONITOR/MWAIT as NOPs on
>>> SVM.
>>
>> Not sure if it is disarmed on VMRUN, but the MONITOR/MWAIT instructions
>> are part of the GHCB spec, so they are implemented here.
>
> Even if MONITOR/MWAIT somehow works across VMRUN I'm not sure it's something
> the guest should enable by default as it leaks GPAs to the untrusted host,
> with no benefit to the guest except in specific configurations. Yeah, the
> VMM can muck with page tables to trace guest to the some extent, but the
> guest shouldn't be unnecessarily volunteering information to the host.
>
> If MONITOR/MWAIT is effectively a NOP then removing this code is a no
> brainer.
>
> Can someone from AMD chime in?

I don't think there is any guarantee that MONITOR/MWAIT would work within
a guest (I'd have to dig some more on that to get a definitive answer, but
probably not necessary to do). As you say, if KVM emulates it as a NOP,
there's no sense in exposing the GPA - make it a NOP in the handler. I
just need to poke some other hypervisor vendors and hear what they have to
say.

Thanks,
Tom

>