Re: [PATCH v12 17/24] KVM: VMX: Set up interception for CET MSRs

[Chao Gao <chao.gao@intel.com>]

On Tue, Aug 19, 2025 at 11:45:42AM -0700, Sean Christopherson wrote:
>On Tue, Aug 19, 2025, Xin Li wrote:
>> On 8/19/2025 9:11 AM, Sean Christopherson wrote:
>> > On Mon, Aug 11, 2025, Chao Gao wrote:
>> > > From: Yang Weijiang <weijiang.yang@intel.com>
>> > > 
>> > > Enable/disable CET MSRs interception per associated feature configuration.
>> > > 
>> > > Shadow Stack feature requires all CET MSRs passed through to guest to make
>> > > it supported in user and supervisor mode
>> > 
>> > I doubt that SS _requires_ CET MSRs to be passed through.  IIRC, the actual
>> > reason for passing through most of the MSRs is that they are managed via XSAVE,
>> > i.e. _can't_ be intercepted without also intercepting XRSTOR.

Agreed. Will update the changelog.

>> > 
>> > > while IBT feature only depends on
>> > > MSR_IA32_{U,S}_CETS_CET to enable user and supervisor IBT.
>> > > 
>> > > Note, this MSR design introduced an architectural limitation of SHSTK and
>> > > IBT control for guest, i.e., when SHSTK is exposed, IBT is also available
>> > > to guest from architectural perspective since IBT relies on subset of SHSTK
>> > > relevant MSRs.
>> > > 
>> > > Suggested-by: Sean Christopherson <seanjc@google.com>
>> > > Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
>> > > Tested-by: Mathias Krause <minipli@grsecurity.net>
>> > > Tested-by: John Allen <john.allen@amd.com>
>> > > Signed-off-by: Chao Gao <chao.gao@intel.com>
>> > > ---
>> > >   arch/x86/kvm/vmx/vmx.c | 20 ++++++++++++++++++++
>> > >   1 file changed, 20 insertions(+)
>> > > 
>> > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> > > index bd572c8c7bc3..130ffbe7dc1a 100644
>> > > --- a/arch/x86/kvm/vmx/vmx.c
>> > > +++ b/arch/x86/kvm/vmx/vmx.c
>> > > @@ -4088,6 +4088,8 @@ void pt_update_intercept_for_msr(struct kvm_vcpu *vcpu)
>> > >   void vmx_recalc_msr_intercepts(struct kvm_vcpu *vcpu)
>> > >   {
>> > > +	bool set;
>> > 
>> > s/set/intercept
>> > 
>> 
>> Maybe because you asked me to change "flag" to "set" when reviewing FRED
>> patches, however "intercept" does sound better, and I just changed it :)
>
>Ah crud.  I had a feeling I was flip-flopping.  I obviously don't have a strong
>preference.

Anyway, I will use "intercept".

>
>> > > +
>> > >   	if (!cpu_has_vmx_msr_bitmap())
>> > >   		return;
>> > > @@ -4133,6 +4135,24 @@ void vmx_recalc_msr_intercepts(struct kvm_vcpu *vcpu)
>> > >   		vmx_set_intercept_for_msr(vcpu, MSR_IA32_FLUSH_CMD, MSR_TYPE_W,
>> > >   					  !guest_cpu_cap_has(vcpu, X86_FEATURE_FLUSH_L1D));
>> > > +	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK)) {
>> > > +		set = !guest_cpu_cap_has(vcpu, X86_FEATURE_SHSTK);
>> > > +
>> > > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL0_SSP, MSR_TYPE_RW, set);
>> > > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL1_SSP, MSR_TYPE_RW, set);
>> > > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL2_SSP, MSR_TYPE_RW, set);
>> > > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL3_SSP, MSR_TYPE_RW, set);
>> > > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_INT_SSP_TAB, MSR_TYPE_RW, set);
>> > 
>> > MSR_IA32_INT_SSP_TAB isn't managed via XSAVE, so why is it being passed through?
>> 
>> It's managed in VMCS host and guest areas, i.e. HOST_INTR_SSP_TABLE and
>> GUEST_INTR_SSP_TABLE, if the "load CET" bits are set in both VM entry
>> and exit controls.
>
>Ah, "because it's essentially free".  Unless there's a true need to pass it through,
>I think it makes sense to intercept.  Merging KVM's bitmap with vmcs12's bitmap
>isn't completely free (though it's quite cheap).  More importantly, this is technically
>wrong due to MSR_IA32_INT_SSP_TAB not existing if the vCPU doesn't have LM.  That's
>obviously easy to solve, I just don't see the point.

Sure. I will leave MSR_IA32_INT_SSP_TAB intercept and add a LM check on its emulation.