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

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Date: Fri, 8 Mar 2024 13:09:22 +1300
From: "Huang, Kai" <kai.huang@...el.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
CC: Sean Christopherson <seanjc@...gle.com>, 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 8/03/2024 6:10 am, Kirill A. Shutemov wrote:
> On Thu, Mar 07, 2024 at 11:49:11AM +1300, Huang, Kai wrote:
>>
>>
>> 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.
> 
> Unmodified drivers gets their MMIO regions mapped with ioremap() that sets
> shared bit, unless asked explicitly to make it private (encrypted).
> 

Thanks for clarification.  Obviously I had bad memory.