Re: [PATCH v2 01/21] KVM: nVMX: Unconditionally purge queued/injected events on nested "exit"

[Maxim Levitsky]

On Tue, 2022-06-14 at 20:47 +0000, Sean Christopherson wrote:
> Drop pending exceptions and events queued for re-injection when leaving
> nested guest mode, even if the "exit" is due to VM-Fail, SMI, or forced
> by host userspace.  Failure to purge events could result in an event
> belonging to L2 being injected into L1.
> 
> This _should_ never happen for VM-Fail as all events should be blocked by
> nested_run_pending, but it's possible if KVM, not the L1 hypervisor, is
> the source of VM-Fail when running vmcs02.
> 
> SMI is a nop (barring unknown bugs) as recognition of SMI and thus entry
> to SMM is blocked by pending exceptions and re-injected events.
> 
> Forced exit is definitely buggy, but has likely gone unnoticed because
> userspace probably follows the forced exit with KVM_SET_VCPU_EVENTS (or
> some other ioctl() that purges the queue).
> 
> Fixes: 4f350c6dbcb9 ("kvm: nVMX: Handle deferred early VMLAUNCH/VMRESUME failure properly")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/vmx/nested.c | 19 +++++++++++--------
>  1 file changed, 11 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 7d8cd0ebcc75..ee6f27dffdba 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -4263,14 +4263,6 @@ static void prepare_vmcs12(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
>  			nested_vmx_abort(vcpu,
>  					 VMX_ABORT_SAVE_GUEST_MSR_FAIL);
>  	}
> -
> -	/*
> -	 * Drop what we picked up for L2 via vmx_complete_interrupts. It is
> -	 * preserved above and would only end up incorrectly in L1.
> -	 */
> -	vcpu->arch.nmi_injected = false;
> -	kvm_clear_exception_queue(vcpu);
> -	kvm_clear_interrupt_queue(vcpu);
>  }
>  
>  /*
> @@ -4609,6 +4601,17 @@ void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,
>  		WARN_ON_ONCE(nested_early_check);
>  	}
>  
> +	/*
> +	 * Drop events/exceptions that were queued for re-injection to L2
> +	 * (picked up via vmx_complete_interrupts()), as well as exceptions
> +	 * that were pending for L2.  Note, this must NOT be hoisted above
> +	 * prepare_vmcs12(), events/exceptions queued for re-injection need to
> +	 * be captured in vmcs12 (see vmcs12_save_pending_event()).
> +	 */
> +	vcpu->arch.nmi_injected = false;
> +	kvm_clear_exception_queue(vcpu);
> +	kvm_clear_interrupt_queue(vcpu);
> +
>  	vmx_switch_vmcs(vcpu, &vmx->vmcs01);
>  
>  	/* Update any VMCS fields that might have changed while L2 ran */

Makes sense.

Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>

Best regards,
	Maxim Levitsky