Re: [PATCH v2 7/8] KVM: x86: Reject fixeds-size Hyper-V hypercalls with non-zero "var_cnt"

[Sean Christopherson]

On Mon, Nov 01, 2021, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@xxxxxxxxxx> writes:
> > @@ -2331,6 +2331,11 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
> >  			ret = HV_STATUS_OPERATION_DENIED;
> >  			break;
> >  		}
> > +		if (unlikely(hc.var_cnt)) {
> > +			ret = HV_STATUS_INVALID_HYPERCALL_INPUT;
> > +			break;
> > +		}
> > +
> 
> Probably true for HVCALL_RESET_DEBUG_SESSION but I'm not sure about
> HVCALL_POST_DEBUG_DATA/HVCALL_RETRIEVE_DEBUG_DATA (note 'fallthrough'
> above) -- these are not described well in TLFS.

I'll drop the check for all the DEBUG hypercalls and add a note in the changelog
to call out that they're probably not supposed to use var_cnt, but that the TLFS
documentation isn't clear one way or the other.