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 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@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.
> 
> +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@xxxxxxxxxx> 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).