linux-kernel - Re: [PATCH v2 1/2] KVM: nVMX: use vmx_segment_cache

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Message-ID: <ZpbhItUq-p_emFUT@google.com>
Date: Tue, 16 Jul 2024 14:07:46 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Maxim Levitsky <mlevitsk@...hat.com>
Cc: kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>, 
	Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org, 
	Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 1/2] KVM: nVMX: use vmx_segment_cache_clear

On Mon, Jul 15, 2024, Maxim Levitsky wrote:
> In prepare_vmcs02_rare, call vmx_segment_cache_clear, instead
> of setting the segment_cache.bitmask directly.
> 
> No functional change intended.
> 
> Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
> ---
>  arch/x86/kvm/vmx/nested.c | 5 +++--
>  arch/x86/kvm/vmx/vmx.c    | 4 ----
>  arch/x86/kvm/vmx/vmx.h    | 5 +++++
>  3 files changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 643935a0f70ab..d3ca1a772ae67 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -2469,6 +2469,9 @@ static void prepare_vmcs02_rare(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12)
>  
>  	if (!hv_evmcs || !(hv_evmcs->hv_clean_fields &
>  			   HV_VMX_ENLIGHTENED_CLEAN_FIELD_GUEST_GRP2)) {
> +
> +		vmx_segment_cache_clear(vmx);
> +
>  		vmcs_write16(GUEST_ES_SELECTOR, vmcs12->guest_es_selector);
>  		vmcs_write16(GUEST_CS_SELECTOR, vmcs12->guest_cs_selector);
>  		vmcs_write16(GUEST_SS_SELECTOR, vmcs12->guest_ss_selector);
> @@ -2505,8 +2508,6 @@ static void prepare_vmcs02_rare(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12)
>  		vmcs_writel(GUEST_TR_BASE, vmcs12->guest_tr_base);
>  		vmcs_writel(GUEST_GDTR_BASE, vmcs12->guest_gdtr_base);
>  		vmcs_writel(GUEST_IDTR_BASE, vmcs12->guest_idtr_base);
> -
> -		vmx->segment_cache.bitmask = 0;

This actually exacerbates the bug that you're trying fix in patch 2.  Clearing
segment_cache.bitmask _after_ writing the relevant state limits the stale data
to only the accessor that's running in IRQ context (kvm_arch_vcpu_put()).

Clearing segment_cache.bitmask _before_ writing the relevant statement means
that kvm_arch_vcpu_put() _and_ all future readers will be exposed to the stale
data, because the stale data cached by kvm_arch_vcpu_put() won't mark it invalid.