Re: [PATCH 1/1] kvm: nVMX: Checks "VMCS shadowing" with VMCS link pointer for non-root mode VM{READ,WRITE}

[Yao Yuan]

On Fri, Aug 12, 2022 at 05:30:53PM -0700, Jim Mattson wrote:
> On Fri, Aug 12, 2022 at 4:08 PM Yao Yuan <yaoyuan0329os@xxxxxxxxx> wrote:
> >
> > On Fri, Aug 12, 2022 at 12:33:05PM -0700, Jim Mattson wrote:
> > > On Thu, Aug 11, 2022 at 7:02 PM Yuan Yao <yuan.yao@xxxxxxxxxxxxxxx> wrote:
> > > >
> > > > On Fri, Aug 12, 2022 at 09:47:06AM +0800, Yuan Yao wrote:
> > > > > Add checking to VMCS12's "VMCS shadowing", make sure the checking of
> > > > > VMCS12's vmcs_link_pointer for non-root mode VM{READ,WRITE} happens
> > > > > only if VMCS12's "VMCS shadowing" is 1.
> > > > >
> > > > > SDM says that for non-root mode the VMCS's "VMCS shadowing" must be 1
> > > > > (and the corresponding bits in VMREAD/VMWRITE bitmap must be 0) when
> > > > > condition checking of [B] is reached(please refer [A]), which means
> > > > > checking to VMCS link pointer for non-root mode VM{READ,WRITE} should
> > > > > happen only when "VMCS shadowing" = 1.
> > > > >
> > > > > Description from SDM Vol3(April 2022) Chapter 30.3 VMREAD/VMWRITE:
> > > > >
> > > > > IF (not in VMX operation)
> > > > >    or (CR0.PE = 0)
> > > > >    or (RFLAGS.VM = 1)
> > > > >    or (IA32_EFER.LMA = 1 and CS.L = 0)
> > > > > THEN #UD;
> > > > > ELSIF in VMX non-root operation
> > > > >       AND (“VMCS shadowing” is 0 OR
> > > > >            source operand sets bits in range 63:15 OR
> > > > >            VMREAD bit corresponding to bits 14:0 of source
> > > > >            operand is 1)  <------[A]
> > > > > THEN VMexit;
> > > > > ELSIF CPL > 0
> > > > > THEN #GP(0);
> > > > > ELSIF (in VMX root operation AND current-VMCS pointer is not valid) OR
> > > > >       (in VMX non-root operation AND VMCS link pointer is not valid)
> > > > > THEN VMfailInvalid;  <------ [B]
> > > > > ...
> > > > >
> > > > > Fixes: dd2d6042b7f4 ("kvm: nVMX: VMWRITE checks VMCS-link pointer before VMCS field")
> > > > > Signed-off-by: Yuan Yao <yuan.yao@xxxxxxxxx>
> > > > > ---
> > > > >  arch/x86/kvm/vmx/nested.c | 2 ++
> > > > >  1 file changed, 2 insertions(+)
> > > > >
> > > > > diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> > > > > index ddd4367d4826..30685be54c5d 100644
> > > > > --- a/arch/x86/kvm/vmx/nested.c
> > > > > +++ b/arch/x86/kvm/vmx/nested.c
> > > > > @@ -5123,6 +5123,7 @@ static int handle_vmread(struct kvm_vcpu *vcpu)
> > > > >                */
> > > > >               if (vmx->nested.current_vmptr == INVALID_GPA ||
> > > > >                   (is_guest_mode(vcpu) &&
> > > > > +                  nested_cpu_has_shadow_vmcs(vcpu) &&
> > > >
> > > > Oops, should be "nested_cpu_has_shadow_vmcs(get_vmcs12(vcpu))".
> > > >
> > > > >                    get_vmcs12(vcpu)->vmcs_link_pointer == INVALID_GPA))
> > > > >                       return nested_vmx_failInvalid(vcpu);
> > > > >
> > > > > @@ -5233,6 +5234,7 @@ static int handle_vmwrite(struct kvm_vcpu *vcpu)
> > > > >        */
> > > > >       if (vmx->nested.current_vmptr == INVALID_GPA ||
> > > > >           (is_guest_mode(vcpu) &&
> > > > > +          nested_cpu_has_shadow_vmcs(vcpu) &&
> > > >
> > > > Ditto.
> > > >
> > > > >            get_vmcs12(vcpu)->vmcs_link_pointer == INVALID_GPA))
> > > > >               return nested_vmx_failInvalid(vcpu);
> > > > >
> > > > > --
> > >
> > > These checks are redundant, aren't they?
> > >
> > > That is, nested_vmx_exit_handled_vmcs_access() has already checked
> > > nested_cpu_has_shadow_vmcs(vmcs12).
> >
> > Ah, you're right it does there.
> >
> > That means in L0 we handle this for vmcs12 which has shadow VMCS
> > setting and the corresponding bit in the bitmap is 0(so no vmexit to
> > L1 and the read/write should from/to vmcs12's shadow vmcs, we handle
> > this here to emulate this), so we don't need to check the shdaow VMCS
> > setting here again. Is this the right understanding ?
>
> That is correct.

I got it, Thanks a lot for your correction!