Re: [PATCH 1/2] KVM: VMX: avoid running vmx_handle_exit_irqoff in case of emulation

[Maxim Levitsky]

On Thu, 2021-08-26 at 16:01 +0000, Sean Christopherson wrote:
> On Thu, Aug 26, 2021, Maxim Levitsky wrote:
> > If we are emulating an invalid guest state, we don't have a correct
> > exit reason, and thus we shouldn't do anything in this function.
> > 
> > Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> 
> This should have Cc: stable.  I believe userspace could fairly easily trick KVM
> into "handling" a spurious IRQ, e.g. trigger SIGALRM and stuff invalid state.
> For all those evil folks running CPUs that are almost old enough to drive :-)
> 
> > ---
> >  arch/x86/kvm/vmx/vmx.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index fada1055f325..0c2c0d5ae873 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -6382,6 +6382,9 @@ static void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu)
> >  {
> >  	struct vcpu_vmx *vmx = to_vmx(vcpu);
> >  
> > +	if (vmx->emulation_required)
> > +		return;
> 
> Rather than play whack-a-mole with flows consuming stale state, I'd much prefer
> to synthesize a VM-Exit(INVALID_GUEST_STATE).  Alternatively, just skip ->run()
> entirely by adding hooks in vcpu_enter_guest(), but that's a much larger change
> and probably not worth the risk at this juncture.
> 
> ---
>  arch/x86/kvm/vmx/vmx.c | 17 ++++++++++++++---
>  1 file changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 32e3a8b35b13..12fe63800889 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6618,10 +6618,21 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
>  		     vmx->loaded_vmcs->soft_vnmi_blocked))
>  		vmx->loaded_vmcs->entry_time = ktime_get();
>  
> -	/* Don't enter VMX if guest state is invalid, let the exit handler
> -	   start emulation until we arrive back to a valid state */
> -	if (vmx->emulation_required)
> +	/*
> +	 * Don't enter VMX if guest state is invalid, let the exit handler
> +	 * start emulation until we arrive back to a valid state.  Synthesize a
> +	 * consistency check VM-Exit due to invalid guest state and bail.
> +	 */
> +	if (unlikely(vmx->emulation_required)) {
> +		vmx->fail = 0;
> +		vmx->exit_reason.full = EXIT_REASON_INVALID_STATE;
> +		vmx->exit_reason.failed_vmentry = 1;
> +		kvm_register_mark_available(vcpu, VCPU_EXREG_EXIT_INFO_1);
> +		vmx->exit_qualification = ENTRY_FAIL_DEFAULT;
> +		kvm_register_mark_available(vcpu, VCPU_EXREG_EXIT_INFO_2);
> +		vmx->exit_intr_info = 0;
>  		return EXIT_FASTPATH_NONE;
> +	}

I was thinking exactly about this when I wrote the patch, and in fact first
version of it did roughly what you suggest.

But I was afraid that this will also introduce a whack-a-mole as now
it "appears" as if VM entry failed and we should thus kill the guest.

But I'll try that.

Thanks a lot for the review!

Best regards,
	Maxim Levitsky


>  
>  	trace_kvm_entry(vcpu);
>  
> --
> 
> or the beginnings of an aggressive refactor...





> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index cf8fb6eb676a..a4fe0f78898a 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9509,6 +9509,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>                 goto cancel_injection;
>         }
> 
> +       if (unlikely(static_call(kvm_x86_emulation_required)(vcpu)))
> +               return static_call(kvm_x86_emulate_invalid_guest_state)(vcpu);
> +
>         preempt_disable();
> 
>         static_call(kvm_x86_prepare_guest_switch)(vcpu);
> 
> > +
> >  	if (vmx->exit_reason.basic == EXIT_REASON_EXTERNAL_INTERRUPT)
> >  		handle_external_interrupt_irqoff(vcpu);
> >  	else if (vmx->exit_reason.basic == EXIT_REASON_EXCEPTION_NMI)
> > -- 
> > 2.26.3
> >