linux-kernel - Re: [PATCH v16 27/51] KVM: x86: Disable support for IBT and SHSTK if allow_smaller

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Message-ID: <aNGYKwtO7PgDpxxi@google.com>
Date: Mon, 22 Sep 2025 11:40:43 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Tom Lendacky <thomas.lendacky@....com>, Mathias Krause <minipli@...ecurity.net>, 
	John Allen <john.allen@....com>, Rick Edgecombe <rick.p.edgecombe@...el.com>, 
	Chao Gao <chao.gao@...el.com>, Binbin Wu <binbin.wu@...ux.intel.com>, 
	Xiaoyao Li <xiaoyao.li@...el.com>, Maxim Levitsky <mlevitsk@...hat.com>, 
	Zhang Yi Z <yi.z.zhang@...ux.intel.com>, Xin Li <xin@...or.com>
Subject: Re: [PATCH v16 27/51] KVM: x86: Disable support for IBT and SHSTK if
 allow_smaller_maxphyaddr is true

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@...gle.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
>