Re: [PATCH v6 2/3] KVM: selftests: Add a test to get/set triple fault event

[Chenyi Qiang]

On 5/19/2022 3:20 AM, Sean Christopherson wrote:
> On Thu, Apr 21, 2022, Chenyi Qiang wrote:
> > +#ifndef __x86_64__
> > +# error This test is 64-bit only
>
> No need, all of KVM selftests are 64-bit only.
>
> > +#endif
> > +
> > +#define VCPU_ID			0
> > +#define ARBITRARY_IO_PORT	0x2000
> > +
> > +/* The virtual machine object. */
> > +static struct kvm_vm *vm;
> > +
> > +static void l2_guest_code(void)
> > +{
> > +	/*
> > +	 * Generate an exit to L0 userspace, i.e. main(), via I/O to an
> > +	 * arbitrary port.
> > +	 */
>
> I think we can test a "real" triple fault without too much effort by abusing
> vcpu->run->request_interrupt_window.  E.g. map the run struct into L2, clear
> PIN_BASED_EXT_INTR_MASK in vmcs12, and then in l2_guest_code() do:
>
> 	asm volatile("cli");
>
> 	run->request_interrupt_window = true;

Maybe, A GUEST_SYNC to main() to change the request_interrupt_window 
also works.

> 	asm volatile("sti; ud2");

How does the "real" triple fault occur here? Through UD2?

> 	asm volatile("inb %%dx, %%al"
> 		     : : [port] "d" (ARBITRARY_IO_PORT) : "rax"); 	
>
> The STI shadow will block the IRQ-window VM-Exit until after the ud2, i.e. until
> after the triple fault is pending.
>
> And then main() can
>
>    1. verify it got KVM_EXIT_IRQ_WINDOW_OPEN with a pending triple fault
>    2. clear the triple fault and re-enter L1+l2
>    3. continue with the existing code, e.g. verify it got KVM_EXIT_IO, stuff a
>       pending triple fault, etc...
>
> That said, typing that out makes me think we should technically handle/prevent this
> in KVM since triple fault / shutdown is kinda sorta just a special exception.
>
> Heh, and a potentially bad/crazy idea would be to use a reserved/magic vector in
> kvm_vcpu_events.exception to save/restore triple fault, e.g. we could steal
> NMI_VECTOR.
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index f2d0ee9296b9..d58c0cfd3cd3 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4743,7 +4743,8 @@ static int kvm_vcpu_ready_for_interrupt_injection(struct kvm_vcpu *vcpu)
>          return (kvm_arch_interrupt_allowed(vcpu) &&
>                  kvm_cpu_accept_dm_intr(vcpu) &&
>                  !kvm_event_needs_reinjection(vcpu) &&
> -               !vcpu->arch.exception.pending);
> +               !vcpu->arch.exception.pending &&
> +               !kvm_test_request(KVM_REQ_TRIPLE_FAULT, vcpu));
>   }
>
>   static int kvm_vcpu_ioctl_interrupt(struct kvm_vcpu *vcpu,