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

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Date:   Fri, 12 Aug 2022 12:33:05 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Yuan Yao <yuan.yao@...ux.intel.com>
Cc:     Yuan Yao <yuan.yao@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Jon Cargille <jcargill@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Oliver Upton <oupton@...gle.com>
Subject: Re: [PATCH 1/1] kvm: nVMX: Checks "VMCS shadowing" with VMCS link
 pointer for non-root mode VM{READ,WRITE}

On Thu, Aug 11, 2022 at 7:02 PM Yuan Yao <yuan.yao@...ux.intel.com> 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@...el.com>
> > ---
> >  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).