Re: [PATCH v3 4/8] KVM: x86/mmu: Properly account NX huge page workaround for nonpaging MMUs

[Mingwei Zhang <mizhang@google.com>]

On Fri, Aug 05, 2022, Sean Christopherson wrote:
> Account and track NX huge pages for nonpaging MMUs so that a future
> enhancement to precisely check if a shadow page can't be replaced by a NX
> huge page doesn't get false positives.  Without correct tracking, KVM can
> get stuck in a loop if an instruction is fetching and writing data on the
> same huge page, e.g. KVM installs a small executable page on the fetch
> fault, replaces it with an NX huge page on the write fault, and faults
> again on the fetch.
> 
> Alternatively, and perhaps ideally, KVM would simply not enforce the
> workaround for nonpaging MMUs.  The guest has no page tables to abuse
> and KVM is guaranteed to switch to a different MMU on CR0.PG being
> toggled so there's no security or performance concerns.  However, getting
> make_spte() to play nice now and in the future is unnecessarily complex.
> 
> In the current code base, make_spte() can enforce the mitigation if TDP
> is enabled or the MMU is indirect, but make_spte() may not always have a
> vCPU/MMU to work with, e.g. if KVM were to support in-line huge page
> promotion when disabling dirty logging.
> 
> Without a vCPU/MMU, KVM could either pass in the correct information
> and/or derive it from the shadow page, but the former is ugly and the
> latter subtly non-trivial due to the possibility of direct shadow pages
> in indirect MMUs.  Given that using shadow paging with an unpaged guest
> is far from top priority _and_ has been subjected to the workaround since
> its inception, keep it simple and just fix the accounting glitch.
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> Reviewed-by: David Matlack <dmatlack@google.com>
Reviewed-by: Mingwei Zhang <mizhang@google.com>
> ---
>  arch/x86/kvm/mmu/mmu.c  |  2 +-
>  arch/x86/kvm/mmu/spte.c | 12 ++++++++++++
>  2 files changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 53d0dafa68ff..345b6b22ab68 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3123,7 +3123,7 @@ static int __direct_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>  			continue;
>  
>  		link_shadow_page(vcpu, it.sptep, sp);
> -		if (fault->is_tdp && fault->huge_page_disallowed)
> +		if (fault->huge_page_disallowed)
>  			account_nx_huge_page(vcpu->kvm, sp,
>  					     fault->req_level >= it.level);
>  	}
> diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
> index 7314d27d57a4..52186b795bce 100644
> --- a/arch/x86/kvm/mmu/spte.c
> +++ b/arch/x86/kvm/mmu/spte.c
> @@ -147,6 +147,18 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
>  	if (!prefetch)
>  		spte |= spte_shadow_accessed_mask(spte);
>  
> +	/*
> +	 * For simplicity, enforce the NX huge page mitigation even if not
> +	 * strictly necessary.  KVM could ignore the mitigation if paging is
> +	 * disabled in the guest, as the guest doesn't have an page tables to
> +	 * abuse.  But to safely ignore the mitigation, KVM would have to
> +	 * ensure a new MMU is loaded (or all shadow pages zapped) when CR0.PG
> +	 * is toggled on, and that's a net negative for performance when TDP is
> +	 * enabled.  When TDP is disabled, KVM will always switch to a new MMU
> +	 * when CR0.PG is toggled, but leveraging that to ignore the mitigation
> +	 * would tie make_spte() further to vCPU/MMU state, and add complexity
> +	 * just to optimize a mode that is anything but performance critical.
> +	 */
>  	if (level > PG_LEVEL_4K && (pte_access & ACC_EXEC_MASK) &&
>  	    is_nx_huge_page_enabled(vcpu->kvm)) {
>  		pte_access &= ~ACC_EXEC_MASK;
> -- 
> 2.37.1.559.g78731f0fdb-goog
>