Re: [PATCH 1/2] KVM: TDX: Handle TDG.VP.VMCALL<GetQuote>

[Binbin Wu]

On 4/2/2025 8:53 PM, Binbin Wu wrote:
[...]
> > > diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> > > index b952bc673271..535200446c21 100644
> > > --- a/arch/x86/kvm/vmx/tdx.c
> > > +++ b/arch/x86/kvm/vmx/tdx.c
> > > @@ -1463,6 +1463,39 @@ static int tdx_get_td_vm_call_info(struct kvm_vcpu *vcpu)
> > >       return 1;
> > >   }
> > >   +static int tdx_complete_get_quote(struct kvm_vcpu *vcpu)
> > > +{
> > > +    tdvmcall_set_return_code(vcpu, vcpu->run->tdx_get_quote.ret);
> > > +    return 1;
> > > +}
> > > +
> > > +static int tdx_get_quote(struct kvm_vcpu *vcpu)
> > > +{
> > > +    struct vcpu_tdx *tdx = to_tdx(vcpu);
> > > +
> > > +    u64 gpa = tdx->vp_enter_args.r12;
> > > +    u64 size = tdx->vp_enter_args.r13;
> > > +
> > > +    /* The buffer must be shared memory. */
> > > +    if (vt_is_tdx_private_gpa(vcpu->kvm, gpa) || size == 0) {
> > > +        tdvmcall_set_return_code(vcpu, TDVMCALL_STATUS_INVALID_OPERAND);
> > > +        return 1;
> > > +    }
> > It is a little bit confusing about the shared buffer check here.  There are two
> > perspectives here:
> >
> > 1) the buffer has already been converted to shared, i.e., the attributes are
> > stored in the Xarray.
> > 2) the GPA passed in the GetQuote must have the shared bit set.
> >
> > The key is we need 1) here.  From the spec, we need the 2) as well because it
> > *seems* that the spec requires GetQuote to provide the GPA with shared bit set,
> > as it says "Shared GPA as input".
> >
> > The above check only does 2).  I think we need to check 1) as well, because once
> > you forward this GetQuote to userspace, userspace is able to access it freely.
>
> Right.
>
> Another discussion is whether KVM should skip the sanity checks for GetQuote
> and let the userspace take the job.
> Considering checking the buffer is shared memory or not, KVM seems to be a
> better place.
A second thought. If the userspace could do the shared memory check, the
whole sanity checks can be done in userspace to keep KVM as small as possible.

> > As a result, the comment
> >
> >    /* The buffer must be shared memory. */
> >
> > should also be updated to something like:
> >
> >    /*
> >     * The buffer must be shared. GetQuote requires the GPA to have
> >     * shared bit set.
> >     */