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

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Date: Thu, 7 Mar 2024 11:49:11 +1300
From: "Huang, Kai" <kai.huang@...el.com>
To: Sean Christopherson <seanjc@...gle.com>, "Kirill A. Shutemov"
	<kirill.shutemov@...ux.intel.com>
CC: Paolo Bonzini <pbonzini@...hat.com>, <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 7/03/2024 11:43 am, Sean Christopherson wrote:
> On Thu, Mar 07, 2024, Kai Huang wrote:
>>
>>
>> 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.
> 
> That's a guest bug.  The guest *knows* it's a TDX VM, it *has* to know.  Accessing
> emulated MMIO and thus taking a #VE before enabling paging is nonsensical.  Either
> enable paging and setup MMIO regions as shared, or go straight to TDCALL.

+Kirill,

I kinda forgot the detail, but what I am afraid is there might be bunch 
of existing TDX guests (since TDX guest code is upstream-ed) using 
unmodified drivers, which doesn't map MMIO regions as shared I suppose.

Kirill,

Could you clarify whether TDX guest code maps MMIO regions as shared 
since beginning?