Re: [PATCH v6 19/25] KVM: VMX: Emulate read and write to CET MSRs

[Maxim Levitsky]

On Wed, 2023-11-01 at 09:31 -0700, Sean Christopherson wrote:
> On Tue, Oct 31, 2023, Maxim Levitsky wrote:
> > On Thu, 2023-09-14 at 02:33 -0400, Yang Weijiang wrote:
> > > Add emulation interface for CET MSR access. The emulation code is split
> > > into common part and vendor specific part. The former does common check
> > > for MSRs and reads/writes directly from/to XSAVE-managed MSRs via the
> > > helpers while the latter accesses the MSRs linked to VMCS fields.
> > > 
> > > Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> > > ---
> 
> ...
> 
> > > +	case MSR_IA32_PL0_SSP ... MSR_IA32_PL3_SSP:
> > > +	case MSR_KVM_SSP:
> > > +		if (host_msr_reset && kvm_cpu_cap_has(X86_FEATURE_SHSTK))
> > > +			break;
> > > +		if (!guest_can_use(vcpu, X86_FEATURE_SHSTK))
> > > +			return 1;
> > > +		if (index == MSR_KVM_SSP && !host_initiated)
> > > +			return 1;
> > > +		if (is_noncanonical_address(data, vcpu))
> > > +			return 1;
> > > +		if (index != MSR_IA32_INT_SSP_TAB && !IS_ALIGNED(data, 4))
> > > +			return 1;
> > > +		break;
> > Once again I'll prefer to have an ioctl for setting/getting SSP, this will
> > make the above code simpler (e.g there will be no need to check that write
> > comes from the host/etc).
> 
> I don't think an ioctl() would be simpler overall, especially when factoring in
> userspace.  With a synthetic MSR, we get the following quite cheaply:
> 
>  1. Enumerating support to userspace.
>  2. Save/restore of the value, e.g. for live migration.
>  3. Vendor hooks for propagating values to/from the VMCS/VMCB.
> 
> For an ioctl(), 
> #1 would require a capability, #2 (and #1 to some extent) would
> require new userspace flows, and #3 would require new kvm_x86_ops hooks.
> 
> The synthetic MSR adds a small amount of messiness, as does bundling 
> MSR_IA32_INT_SSP_TAB with the other shadow stack MSRs.  The bulk of the mess comes
> from the need to allow userspace to write '0' when KVM enumerated supported to
> userspace.


Let me put it this way - all hacks start like that, and in this case this is API/ABI hack
so we will have to live with it forever.

Once there is a precedent, trust me there will be 10s of new 'fake' msrs added, and the
interface will become one big mess.

As I suggested, if you don't want to add new capability/ioctl and vendor callback per new
x86 arch register, then let's implement KVM_GET_ONE_REG/KVM_SET_ONE_REG and then it will
be really easy to add new regs without confusing users, and without polluting msr
namespace with msrs that don't exist.


Best regards,
	Maxim Levitsky


> 
> If we isolate MSR_IA32_INT_SSP_TAB, that'll help with the synthetic MSR and with
> MSR_IA32_INT_SSP_TAB.  For the unfortunate "host reset" behavior, the best idea I
> came up with is to add a helper.  It's still a bit ugly, but the ugliness is
> contained in a helper and IMO makes it much easier to follow the case statements.
> 
> get:
> 
> 	case MSR_IA32_INT_SSP_TAB:
> 		if (!guest_can_use(vcpu, X86_FEATURE_SHSTK) ||
> 		    !guest_cpuid_has(vcpu, X86_FEATURE_LM))
> 			return 1;
> 		break;
> 	case MSR_KVM_SSP:
> 		if (!host_initiated)
> 			return 1;
> 		fallthrough;
> 	case MSR_IA32_PL0_SSP ... MSR_IA32_PL3_SSP:
> 		if (!guest_can_use(vcpu, X86_FEATURE_SHSTK))
> 			return 1;
> 		break;
> 
> static bool is_set_cet_msr_allowed(struct kvm_vcpu *vcpu, u32 index, u64 data,
> 				   bool host_initiated)
> {
> 	bool any_cet = index == MSR_IA32_S_CET || index == MSR_IA32_U_CET;
> 
> 	if (guest_can_use(vcpu, X86_FEATURE_SHSTK))
> 		return true;
> 
> 	if (any_cet && guest_can_use(vcpu, X86_FEATURE_IBT))
> 		return true;
> 
> 	/* 
> 	 * If KVM supports the MSR, i.e. has enumerated the MSR existence to
> 	 * userspace, then userspace is allowed to write '0' irrespective of
> 	 * whether or not the MSR is exposed to the guest.
> 	 */
> 	if (!host_initiated || data)
> 		return false;
> 
> 	if (kvm_cpu_cap_has(X86_FEATURE_SHSTK))
> 		return true;
> 
> 	return any_cet && kvm_cpu_cap_has(X86_FEATURE_IBT);
> }
> 
> set:
> 	case MSR_IA32_U_CET:
> 	case MSR_IA32_S_CET:
> 		if (!is_set_cet_msr_allowed(vcpu, index, data, host_initiated))
> 			return 1;
> 		if (data & CET_US_RESERVED_BITS)
> 			return 1;
> 		if (!guest_can_use(vcpu, X86_FEATURE_SHSTK) &&
> 		    (data & CET_US_SHSTK_MASK_BITS))
> 			return 1;
> 		if (!guest_can_use(vcpu, X86_FEATURE_IBT) &&
> 		    (data & CET_US_IBT_MASK_BITS))
> 			return 1;
> 		if (!IS_ALIGNED(CET_US_LEGACY_BITMAP_BASE(data), 4))
> 			return 1;
> 
> 		/* IBT can be suppressed iff the TRACKER isn't WAIT_ENDBR. */
> 		if ((data & CET_SUPPRESS) && (data & CET_WAIT_ENDBR))
> 			return 1;
> 		break;
> 	case MSR_IA32_INT_SSP_TAB:
> 		if (!guest_cpuid_has(vcpu, X86_FEATURE_LM))
> 			return 1;
> 
> 		if (is_noncanonical_address(data, vcpu))
> 			return 1;
> 		break;
> 	case MSR_KVM_SSP:
> 		if (!host_initiated)
> 			return 1;
> 		fallthrough;
> 	case MSR_IA32_PL0_SSP ... MSR_IA32_PL3_SSP:
> 		if (!is_set_cet_msr_allowed(vcpu, index, data, host_initiated))
> 			return 1;
> 		if (is_noncanonical_address(data, vcpu))
> 			return 1;
> 		if (!IS_ALIGNED(data, 4))
> 			return 1;
> 		break;
> 	}
>