Re: [PATCH v16 27/51] KVM: x86: Disable support for IBT and SHSTK if allow_smaller_maxphyaddr is true

[Xiaoyao Li]

On 9/20/2025 6:32 AM, Sean Christopherson wrote:
> Make IBT and SHSTK virtualization mutually exclusive with "officially"
> supporting setups with guest.MAXPHYADDR < host.MAXPHYADDR, i.e. if the
> allow_smaller_maxphyaddr module param is set.  Running a guest with a
> smaller MAXPHYADDR requires intercepting #PF, and can also trigger
> emulation of arbitrary instructions.  Intercepting and reacting to #PFs
> doesn't play nice with SHSTK, as KVM's MMU hasn't been taught to handle
> Shadow Stack accesses, and emulating arbitrary instructions doesn't play
> nice with IBT or SHSTK, as KVM's emulator doesn't handle the various side
> effects, e.g. doesn't enforce end-branch markers or model Shadow Stack
> updates.
>
> Note, hiding IBT and SHSTK based solely on allow_smaller_maxphyaddr is
> overkill, as allow_smaller_maxphyaddr is only problematic if the guest is
> actually configured to have a smaller MAXPHYADDR.  However, KVM's ABI
> doesn't provide a way to express that IBT and SHSTK may break if enabled
> in conjunction with guest.MAXPHYADDR < host.MAXPHYADDR.  I.e. the
> alternative is to do nothing in KVM and instead update documentation and
> hope KVM users are thorough readers.  

KVM_SET_CPUID* can return error to userspace. So KVM can return -EINVAL 
when userspace sets a smaller maxphyaddr with SHSTK/IBT enabled.

> Go with the conservative-but-correct
> approach; worst case scenario, this restriction can be dropped if there's
> a strong use case for enabling CET on hosts with allow_smaller_maxphyaddr.
>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>   arch/x86/kvm/cpuid.c | 10 ++++++++++
>   1 file changed, 10 insertions(+)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 499c86bd457e..b5c4cb13630c 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -963,6 +963,16 @@ void kvm_set_cpu_caps(void)
>   	if (!tdp_enabled)
>   		kvm_cpu_cap_clear(X86_FEATURE_SHSTK);
>   
> +	/*
> +	 * Disable support for IBT and SHSTK if KVM is configured to emulate
> +	 * accesses to reserved GPAs, as KVM's emulator doesn't support IBT or
> +	 * SHSTK, nor does KVM handle Shadow Stack #PFs (see above).
> +	 */
> +	if (allow_smaller_maxphyaddr) {
> +		kvm_cpu_cap_clear(X86_FEATURE_SHSTK);
> +		kvm_cpu_cap_clear(X86_FEATURE_IBT);
> +	}
> +
>   	kvm_cpu_cap_init(CPUID_7_EDX,
>   		F(AVX512_4VNNIW),
>   		F(AVX512_4FMAPS),