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

[Sean Christopherson]

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@xxxxxxxxxx>
> > ---
> >   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.

> 
> I am hoping I am missing something here?