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

Sean Christopherson
2025-09-22T18:40:45Z

On Fri, Sep 19, 2025, 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. 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@google.com>
> ---
> 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);
> + }

Ugh, testing fail. F(IBT) is initialized in CPUID_7_EDX, clearing IBT here has
no effect.

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index b861a88083e1..d290dbc96831 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -964,16 +964,6 @@ 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),
@@ -994,6 +984,16 @@ void kvm_set_cpu_caps(void)
F(IBT),
);

+ /*
+ * 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);
+ }
+
if (boot_cpu_has(X86_FEATURE_AMD_IBPB_RET) &&
boot_cpu_has(X86_FEATURE_AMD_IBPB) &&
boot_cpu_has(X86_FEATURE_AMD_IBRS))

> +
> kvm_cpu_cap_init(CPUID_7_EDX,
> F(AVX512_4VNNIW),
> F(AVX512_4FMAPS),
> --
> 2.51.0.470.ga7dc726c21-goog
>