Re: [PATCH] KVM: x86/mmu: Reject direct bits in gpa passed to KVM_PRE_FAULT_MEMORY

[Sean Christopherson <seanjc@google.com>]

On Fri, Jun 13, 2025, Yan Zhao wrote:
> On Thu, Jun 12, 2025 at 12:49:43AM -0400, Paolo Bonzini wrote:
> > Only let userspace pass the same addresses that were used in KVM_SET_USER_MEMORY_REGION
> > (or KVM_SET_USER_MEMORY_REGION2); gpas in the the upper half of the address space
> > are an implementation detail of TDX and KVM.
> > 
> > Extracted from a patch by Sean Christopherson <seanjc@google.com>.
> > 
> > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> > ---
> >  arch/x86/kvm/mmu/mmu.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index a4040578b537..4e06e2e89a8f 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -4903,6 +4903,9 @@ long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
> >  	if (!vcpu->kvm->arch.pre_fault_allowed)
> >  		return -EOPNOTSUPP;
> >  
> > +	if (kvm_is_gfn_alias(vcpu->kvm, gpa_to_gfn(range->gpa)))
> > +		return -EINVAL;
> > +
> Do we need the same check in kvm_vm_ioctl_set_mem_attributes()?

Yeah, we probably should disallow aliases there.  It should be benign?  Because
memslots disallow aliases, and so the aliased gfn entries in the xarray will never
actually be consumed.