linux-kernel - Re: [PATCH v3 1/8] KVM: x86/mmu: Bug the VM if KVM attempts to double count an NX huge page

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Message-ID: <YvhHfkbkVCWPy12W@google.com>
Date:   Sun, 14 Aug 2022 00:53:18 +0000
From:   Mingwei Zhang <mizhang@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, David Matlack <dmatlack@...gle.com>,
        Yan Zhao <yan.y.zhao@...el.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH v3 1/8] KVM: x86/mmu: Bug the VM if KVM attempts to
 double count an NX huge page

On Fri, Aug 05, 2022, Sean Christopherson wrote:
> WARN and kill the VM if KVM attempts to double count an NX huge page,
> i.e. attempts to re-tag a shadow page with "NX huge page disallowed".
> KVM does NX huge page accounting only when linking a new shadow page, and
> it should be impossible for a new shadow page to be already accounted.
> E.g. even in the TDP MMU case, where vCPUs can race to install a new
> shadow page, only the "winner" will account the installed page.
> 
> Kill the VM instead of continuing on as either KVM has an egregious bug,
> e.g. didn't zero-initialize the data, or there's host data corruption, in
> which carrying on is dangerous, e.g. could cause silent data corruption
> in the guest.
> 
> Reported-by: David Matlack <dmatlack@...gle.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Mingwei Zhang <mizhang@...gle.com>
> ---
>  arch/x86/kvm/mmu/mmu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 3e1317325e1f..36b898dbde91 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -804,7 +804,7 @@ static void account_shadowed(struct kvm *kvm, struct kvm_mmu_page *sp)
>  
>  void account_huge_nx_page(struct kvm *kvm, struct kvm_mmu_page *sp)
>  {
> -	if (sp->lpage_disallowed)
> +	if (KVM_BUG_ON(sp->lpage_disallowed, kvm))
>  		return;
>  
>  	++kvm->stat.nx_lpage_splits;
> -- 
> 2.37.1.559.g78731f0fdb-goog
>