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

["Huang, Kai" <kai.huang@intel.com>]

On 7/03/2024 12:01 pm, Sean Christopherson wrote:
> On Thu, Mar 07, 2024, Kai Huang 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@google.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?
> 
> Y'all get the same answer we gave the SNP folks: KVM does not yet support TDX,
> so as far is KVM is concerned, there is no existing functionality to support.
> 
> s/firmware/Linux if this is a Linux kernel problem.
> 
>    On Thu, Feb 08, 2024, Paolo Bonzini wrote:
>    > On Thu, Feb 8, 2024 at 6:27 PM Sean Christopherson <seanjc@google.com> wrote:
>    > > No.  KVM does not yet support SNP, so as far as KVM's ABI goes, there are no
>    > > existing guests.  Yes, I realize that I am burying my head in the sand to some
>    > > extent, but it is simply not sustainable for KVM to keep trying to pick up the
>    > > pieces of poorly defined hardware specs and broken guest firmware.
>    >
>    > 101% agreed. There are cases in which we have to and should bend
>    > together backwards for guests (e.g. older Linux kernels), but not for
>    > code that---according to current practices---is chosen by the host
>    > admin.
>    >
>    > (I am of the opinion that "bring your own firmware" is the only sane
>    > way to handle attestation/measurement, but that's not how things are
>    > done currently).

Fair enough, and good to know. :-)

(Still better to hear from Kirill, though.)