On Wed, Mar 4, 2020 at 3:58 PM Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
>
> Jon Doron <arilou@xxxxxxxxx> writes:
>
> > Microsoft's kdvm.dll dbgtransport module does not respect the hypercall
> > page and simply identifies the CPU being used (AMD/Intel) and according
> > to it simply makes hypercalls with the relevant instruction
> > (vmmcall/vmcall respectively).
> >
> > The relevant function in kdvm is KdHvConnectHypervisor which first checks
> > if the hypercall page has been enabled via HV_X64_MSR_HYPERCALL_ENABLE,
> > and in case it was not it simply sets the HV_X64_MSR_GUEST_OS_ID to
> > 0x1000101010001 which means:
> > build_number = 0x0001
> > service_version = 0x01
> > minor_version = 0x01
> > major_version = 0x01
> > os_id = 0x00 (Undefined)
> > vendor_id = 1 (Microsoft)
> > os_type = 0 (A value of 0 indicates a proprietary, closed source OS)
> >
> > and starts issuing the hypercall without setting the hypercall page.
> >
> > To resolve this issue simply enable hypercalls if the guest_os_id is
> > not 0.
> >
> > Signed-off-by: Jon Doron <arilou@xxxxxxxxx>
> > ---
> > arch/x86/kvm/hyperv.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> > index 13176ec23496..7ec962d433af 100644
> > --- a/arch/x86/kvm/hyperv.c
> > +++ b/arch/x86/kvm/hyperv.c
> > @@ -1615,7 +1615,7 @@ static u64 kvm_hv_send_ipi(struct kvm_vcpu *current_vcpu, u64 ingpa, u64 outgpa,
> >
> > bool kvm_hv_hypercall_enabled(struct kvm *kvm)
> > {
> > - return READ_ONCE(kvm->arch.hyperv.hv_hypercall) & HV_X64_MSR_HYPERCALL_ENABLE;
> > + return READ_ONCE(kvm->arch.hyperv.hv_guest_os_id) != 0;
> > }
> >
> > static void kvm_hv_hypercall_set_result(struct kvm_vcpu *vcpu, u64 result)
>
> I would've enabled it in both cases,
>
> return (READ_ONCE(kvm->arch.hyperv.hv_hypercall) &
> HV_X64_MSR_HYPERCALL_ENABLE) || (READ_ONCE(kvm->arch.hyperv.hv_guest_os_id) != 0);
>
> to be safe. We can also check what genuine Hyper-V does but I bet it has
> hypercalls always enabled. Also, the function can be made inline,
> there's a single caller.
I dont have any Hyper-V setup at the moment to validate this, i
believe your hunch is correct but ill do the
implementation you have suggested.
>
> --
> Vitaly
>
