linux-kernel - Re: [PATCH] KVM: x86/mmu: Do not create SPTEs for GFNs that exceed host.MAXPHYADDR

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Message-ID: <2d10ff16d3f5fa03886721c24f2db10d79759ed2.camel@intel.com>
Date:   Mon, 02 May 2022 23:12:52 +1200
From:   Kai Huang <kai.huang@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Maxim Levitsky <mlevitsk@...hat.com>,
        Ben Gardon <bgardon@...gle.com>,
        David Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH] KVM: x86/mmu: Do not create SPTEs for GFNs that exceed
 host.MAXPHYADDR

On Thu, 2022-04-28 at 23:34 +0000, Sean Christopherson wrote:
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 904f0faff218..c10ae25135e3 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3010,9 +3010,15 @@ static bool handle_abnormal_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fa
>  		/*
>  		 * If MMIO caching is disabled, emulate immediately without
>  		 * touching the shadow page tables as attempting to install an
> -		 * MMIO SPTE will just be an expensive nop.
> +		 * MMIO SPTE will just be an expensive nop.  Do not cache MMIO
> +		 * whose gfn is greater than host.MAXPHYADDR, any guest that
> +		 * generates such gfns is either malicious or in the weeds.
> +		 * Note, it's possible to observe a gfn > host.MAXPHYADDR if
> +		 * and only if host.MAXPHYADDR is inaccurate with respect to
> +		 * hardware behavior, e.g. if KVM itself is running as a VM.
>  		 */
> -		if (unlikely(!enable_mmio_caching)) {
> +		if (unlikely(!enable_mmio_caching) ||
> +		    unlikely(fault->gfn > kvm_mmu_max_gfn_host())) {

Shouldn't we check fault->gfn against cpuid_maxphyaddr(vcpu) instead of
kvm_mmu_max_gfn_host() here?

>  			*ret_val = RET_PF_EMULATE;
>  			return true;
>  		}


-- 
Thanks,
-Kai