Re: [PATCH v4 14/20] KVM:VMX: Set up interception for CET MSRs

[Yang, Weijiang]

On 7/26/2023 4:30 PM, Chao Gao wrote:
> On Thu, Jul 20, 2023 at 11:03:46PM -0400, Yang Weijiang wrote:
> > Pass through CET MSRs when the associated feature is enabled.
> > Shadow Stack feature requires all the CET MSRs to make it
> > architectural support in guest. IBT feature only depends on
> > MSR_IA32_U_CET and MSR_IA32_S_CET to enable both user and
> > supervisor IBT.
> If a guest supports SHSTK only, KVM has no way to prevent guest from
> enabling IBT because the U/S_CET are pass-thru'd. it is a problem.

Yes, this is a CET architectural defect when only SHSTK is enabled. But 
it shouldn't

bring issues, right? I will highlight it in commit log.

> I am wondering if it is necessary to pass-thru U/S_CET MSRs. Probably
> the answer is yes at least for U_CET MSR because the MSR is per-task.

Agree,  also xsaves/xrstors in guest could fail if they're not pass-thrued.

> > Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> > ---
> > arch/x86/kvm/vmx/vmx.c | 35 +++++++++++++++++++++++++++++++++++
> > 1 file changed, 35 insertions(+)
> >
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index b29817ec6f2e..85cb7e748a89 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -709,6 +709,10 @@ static bool is_valid_passthrough_msr(u32 msr)
> > 	case MSR_LBR_CORE_TO ... MSR_LBR_CORE_TO + 8:
> > 		/* LBR MSRs. These are handled in vmx_update_intercept_for_lbr_msrs() */
> > 		return true;
> > +	case MSR_IA32_U_CET:
> > +	case MSR_IA32_S_CET:
> > +	case MSR_IA32_PL0_SSP ... MSR_IA32_INT_SSP_TAB:
> > +		return true;
> > 	}
> >
> > 	r = possible_passthrough_msr_slot(msr) != -ENOENT;
> > @@ -7758,6 +7762,34 @@ static void update_intel_pt_cfg(struct kvm_vcpu *vcpu)
> > 		vmx->pt_desc.ctl_bitmask &= ~(0xfULL << (32 + i * 4));
> > }
> >
> > +static void vmx_update_intercept_for_cet_msr(struct kvm_vcpu *vcpu)
> > +{
> > +	if (guest_can_use(vcpu, X86_FEATURE_SHSTK)) {
> > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_U_CET,
> > +					  MSR_TYPE_RW, false);
> > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_S_CET,
> > +					  MSR_TYPE_RW, false);
> > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL0_SSP,
> > +					  MSR_TYPE_RW, false);
> > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL1_SSP,
> > +					  MSR_TYPE_RW, false);
> > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL2_SSP,
> > +					  MSR_TYPE_RW, false);
> > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL3_SSP,
> > +					  MSR_TYPE_RW, false);
> > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_INT_SSP_TAB,
> > +					  MSR_TYPE_RW, false);
> > +		return;
> > +	}
> > +
> > +	if (guest_can_use(vcpu, X86_FEATURE_IBT)) {
> > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_U_CET,
> > +					  MSR_TYPE_RW, false);
> > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_S_CET,
> > +					  MSR_TYPE_RW, false);
> > +	}
> This is incorrect. see
>
> https://lore.kernel.org/all/ZJYzPn7ipYfO0fLZ@xxxxxxxxxx/

Yes, will add the lost counterpart, thanks!

> > +}
> > +
> > static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> > {
> > 	struct vcpu_vmx *vmx = to_vmx(vcpu);
> > @@ -7825,6 +7857,9 @@ static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> >
> > 	/* Refresh #PF interception to account for MAXPHYADDR changes. */
> > 	vmx_update_exception_bitmap(vcpu);
> > +
> > +	if (kvm_is_cet_supported())
> Nit: this check is not necessary. here isn't a hot path. and if
> kvm_is_cet_supported() is false, guest_can_use(., X86_FEATURE_SHSTK/IBT)
> should be false.

Yes, I think it can be removed.

> > +		vmx_update_intercept_for_cet_msr(vcpu);
> > }
> >
> > static u64 vmx_get_perf_capabilities(void)
> > --
> > 2.27.0