linux-kernel - Re: [PATCH] KVM: x86: allow KVM_REQ_GET_NESTED_STATE

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Date:   Mon, 25 Jan 2021 11:16:46 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: allow KVM_REQ_GET_NESTED_STATE_PAGES outside
 guest mode for VMX

On Mon, Jan 25, 2021, Paolo Bonzini wrote:
> +static bool nested_get_vmcs12_pages(struct kvm_vcpu *vcpu)
> +{
> +	struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
> +	struct vcpu_vmx *vmx = to_vmx(vcpu);
> +	struct kvm_host_map *map;
> +	struct page *page;
> +	u64 hpa;
> +
> +	if (!nested_get_evmcs_page(vcpu))
> +		return false;
>  
>  	if (nested_cpu_has2(vmcs12, SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES)) {
>  		/*
> @@ -3224,6 +3233,17 @@ static bool nested_get_vmcs12_pages(struct kvm_vcpu *vcpu)
>  	return true;
>  }
>  
> +static bool vmx_get_nested_state_pages(struct kvm_vcpu *vcpu)
> +{
> +	if (!nested_get_evmcs_page(vcpu))
> +		return false;
> +
> +	if (is_guest_mode(vcpu) && !nested_get_vmcs12_pages(vcpu))
> +		return false;

nested_get_evmcs_page() will get called twice in the common case of
is_guest_mode() == true.  I can't tell if that will ever be fatal, but it's
definitely weird.  Maybe this?

	if (!is_guest_mode(vcpu))
		return nested_get_evmcs_page(vcpu);

	return nested_get_vmcs12_pages(vcpu);

> +
> +	return true;
> +}
> +
>  static int nested_vmx_write_pml_buffer(struct kvm_vcpu *vcpu, gpa_t gpa)
>  {
>  	struct vmcs12 *vmcs12;
> @@ -6602,7 +6622,7 @@ struct kvm_x86_nested_ops vmx_nested_ops = {
>  	.hv_timer_pending = nested_vmx_preemption_timer_pending,
>  	.get_state = vmx_get_nested_state,
>  	.set_state = vmx_set_nested_state,
> -	.get_nested_state_pages = nested_get_vmcs12_pages,
> +	.get_nested_state_pages = vmx_get_nested_state_pages,
>  	.write_log_dirty = nested_vmx_write_pml_buffer,
>  	.enable_evmcs = nested_enable_evmcs,
>  	.get_evmcs_version = nested_get_evmcs_version,
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 9a8969a6dd06..b910aa74ee05 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8802,9 +8802,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  
>  	if (kvm_request_pending(vcpu)) {
>  		if (kvm_check_request(KVM_REQ_GET_NESTED_STATE_PAGES, vcpu)) {
> -			if (WARN_ON_ONCE(!is_guest_mode(vcpu)))
> -				;
> -			else if (unlikely(!kvm_x86_ops.nested_ops->get_nested_state_pages(vcpu))) {
> +			if (unlikely(!kvm_x86_ops.nested_ops->get_nested_state_pages(vcpu))) {
>  				r = 0;
>  				goto out;
>  			}
> -- 
> 2.26.2
>