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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Message-ID: <CALMp9eT6jSz02L89RBoKuiz3fAo-EqS-Q2B0qF3HrcE1SZ-izw@mail.gmail.com>
Date:   Fri, 12 Aug 2022 17:30:53 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Yao Yuan <yaoyuan0329os@...il.com>
Cc:     Yuan Yao <yuan.yao@...ux.intel.com>, 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 Fri, Aug 12, 2022 at 4:08 PM Yao Yuan <yaoyuan0329os@...il.com> 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@...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).
>
> 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.