linux-kernel - Re: [PATCH 11/16] KVM: x86/mmu: Explicitly disallow private accesses to emulated MMIO

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Message-ID: <0a05d5ec-5352-4bab-96ae-2fa35235477c@intel.com>
Date: Thu, 7 Mar 2024 11:35:00 +1300
From: "Huang, Kai" <kai.huang@...el.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini
	<pbonzini@...hat.com>
CC: <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>, Yan Zhao
	<yan.y.zhao@...el.com>, Isaku Yamahata <isaku.yamahata@...el.com>, "Michael
 Roth" <michael.roth@....com>, Yu Zhang <yu.c.zhang@...ux.intel.com>, Chao
 Peng <chao.p.peng@...ux.intel.com>, Fuad Tabba <tabba@...gle.com>, David
 Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH 11/16] KVM: x86/mmu: Explicitly disallow private accesses
 to emulated MMIO



On 28/02/2024 3:41 pm, Sean Christopherson wrote:
> Explicitly detect and disallow private accesses to emulated MMIO in
> kvm_handle_noslot_fault() instead of relying on kvm_faultin_pfn_private()
> to perform the check.  This will allow the page fault path to go straight
> to kvm_handle_noslot_fault() without bouncing through __kvm_faultin_pfn().
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/kvm/mmu/mmu.c | 5 +++++
>   1 file changed, 5 insertions(+)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 5c8caab64ba2..ebdb3fcce3dc 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3314,6 +3314,11 @@ static int kvm_handle_noslot_fault(struct kvm_vcpu *vcpu,
>   {
>   	gva_t gva = fault->is_tdp ? 0 : fault->addr;
>   
> +	if (fault->is_private) {
> +		kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
> +		return -EFAULT;
> +	}
> +

As mentioned in another reply in this series, unless I am mistaken, for 
TDX guest the _first_ MMIO access would still cause EPT violation with 
MMIO GFN being private.

Returning to userspace cannot really help here because the MMIO mapping 
is inside the guest.

I am hoping I am missing something here?