Re: [PATCH v7 22/26] KVM: VMX: Set up interception for CET MSRs

[Maxim Levitsky <mlevitsk@redhat.com>]

On Fri, 2023-12-01 at 14:33 +0800, Chao Gao wrote:
> On Thu, Nov 30, 2023 at 07:44:45PM +0200, Maxim Levitsky wrote:
> > On Fri, 2023-11-24 at 00:53 -0500, Yang Weijiang wrote:
> > > 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 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 architectual perspective since IBT relies on subset of SHSTK
> > > relevant MSRs.
> > > 
> > > Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
> > > ---
> > >  arch/x86/kvm/vmx/vmx.c | 42 ++++++++++++++++++++++++++++++++++++++++++
> > >  1 file changed, 42 insertions(+)
> > > 
> > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > > index 554f665e59c3..e484333eddb0 100644
> > > --- a/arch/x86/kvm/vmx/vmx.c
> > > +++ b/arch/x86/kvm/vmx/vmx.c
> > > @@ -699,6 +699,10 @@ 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;
> > > @@ -7766,6 +7770,42 @@ 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)
> > > +{
> > > +	bool incpt;
> > > +
> > > +	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK)) {
> > > +		incpt = !guest_cpuid_has(vcpu, X86_FEATURE_SHSTK);
> > > +
> > > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_U_CET,
> > > +					  MSR_TYPE_RW, incpt);
> > > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_S_CET,
> > > +					  MSR_TYPE_RW, incpt);
> > > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL0_SSP,
> > > +					  MSR_TYPE_RW, incpt);
> > > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL1_SSP,
> > > +					  MSR_TYPE_RW, incpt);
> > > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL2_SSP,
> > > +					  MSR_TYPE_RW, incpt);
> > > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL3_SSP,
> > > +					  MSR_TYPE_RW, incpt);
> > > +		if (guest_cpuid_has(vcpu, X86_FEATURE_LM))
> > > +			vmx_set_intercept_for_msr(vcpu, MSR_IA32_INT_SSP_TAB,
> > > +						  MSR_TYPE_RW, incpt);
> > > +		if (!incpt)
> > > +			return;
> > > +	}
> > > +
> > > +	if (kvm_cpu_cap_has(X86_FEATURE_IBT)) {
> > > +		incpt = !guest_cpuid_has(vcpu, X86_FEATURE_IBT);
> > > +
> > > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_U_CET,
> > > +					  MSR_TYPE_RW, incpt);
> > > +		vmx_set_intercept_for_msr(vcpu, MSR_IA32_S_CET,
> > > +					  MSR_TYPE_RW, incpt);
> > > +	}
> > > +}
> > > +
> > >  static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> > >  {
> > >  	struct vcpu_vmx *vmx = to_vmx(vcpu);
> > > @@ -7843,6 +7883,8 @@ static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> > >  
> > >  	/* Refresh #PF interception to account for MAXPHYADDR changes. */
> > >  	vmx_update_exception_bitmap(vcpu);
> > > +
> > > +	vmx_update_intercept_for_cet_msr(vcpu);
> > >  }
> > >  
> > >  static u64 vmx_get_perf_capabilities(void)
> > 
> > My review feedback from the previous patch still applies as well,
> > 
> > I still think that we should either try a best effort approach to plug
> > this virtualization hole, or we at least should fail guest creation
> > if the virtualization hole is present as I said:
> > 
> > "Another, much simpler option is to fail the guest creation if the shadow stack + indirect branch tracking
> > state differs between host and the guest, unless both are disabled in the guest.
> > (in essence don't let the guest be created if (2) or (3) happen)"
> 
> Enforcing a "none" or "all" policy is a temporary solution. in future, if some
> reserved bits in S/U_CET MSRs are extended for new features, there will be:
> 
> 	platform A supports SS + IBT
> 	platform B supports SS + IBT + new feature
> 
> Guests running on B inevitably have the same virtualization hole. and if kvm
> continues enforcing the policy on B, then VM migration from A to B would be
> impossible.
> 
> To me, intercepting S/U_CET MSR and CET_S/U xsave components is intricate and
> yields marginal benefits. And I also doubt any reasonable OS implementation
> would depend on #GP of WRMSR to S/U_CET MSRs for functionalities. So, I vote
> to leave the patch as-is.

To some extent I do agree with you but this can become a huge mess in the future.
I think we need at least to tell Intel/AMD about this to ensure that they don't make this thing worse
than it already is.

Also the very least we can do if we opt to keep things as is, 
is to document this virtualization hole - we have Documentation/virt/kvm/x86/errata.rst for that.

Best regards,
	Maxim Levitsky

> 
> > Please at least tell me what do you think about this.
> > Best regards,
> > 	Maxim Levitsky
> > 
> > 
> >