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

[Paolo Bonzini]

On Thu, Jun 12, 2025 at 7:29 AM Xiaoyao Li <xiaoyao.li@xxxxxxxxx> wrote:
>
> On 6/12/2025 12:49 PM, 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@xxxxxxxxxx>.
> >
> > Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > ---
> >   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 to worry about the case (range->gpa + range->size) becomes alias?

No, because the function only processes a single page and everything
in the non-aliased part of the address space *can* be prefaulted.

KVM's generic kvm_vcpu_pre_fault_memory() call will see the EINVAL on
a later invocation and will stop processing the part of the request
that is has the shared/direct bit set.

Paolo

>
> >       /*
> >        * reload is efficient when called repeatedly, so we can do it on
> >        * every iteration.
>