linux-kernel - [PATCH v16 26/51] KVM: x86: Disable support for Shadow Stacks if TDP is disabled

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Message-ID: <20250919223258.1604852-27-seanjc@google.com>
Date: Fri, 19 Sep 2025 15:32:33 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson <seanjc@...gle.com>
Cc: 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: [PATCH v16 26/51] KVM: x86: Disable support for Shadow Stacks if TDP
 is disabled

Make TDP a hard requirement for Shadow Stacks, as there are no plans to
add Shadow Stack support to the Shadow MMU.  E.g. KVM hasn't been taught
to understand the magic Writable=0,Dirty=0 combination that is required
for Shadow Stack accesses, and so enabling Shadow Stacks when using
shadow paging will put the guest into an infinite #PF loop (KVM thinks the
shadow page tables have a valid mapping, hardware says otherwise).

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/cpuid.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 32fde9e80c28..499c86bd457e 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -955,6 +955,14 @@ void kvm_set_cpu_caps(void)
 	if (!tdp_enabled || !boot_cpu_has(X86_FEATURE_OSPKE))
 		kvm_cpu_cap_clear(X86_FEATURE_PKU);
 
+	/*
+	 * Shadow Stacks aren't implemented in the Shadow MMU.  Shadow Stack
+	 * accesses require "magic" Writable=0,Dirty=1 protection, which KVM
+	 * doesn't know how to emulate or map.
+	 */
+	if (!tdp_enabled)
+		kvm_cpu_cap_clear(X86_FEATURE_SHSTK);
+
 	kvm_cpu_cap_init(CPUID_7_EDX,
 		F(AVX512_4VNNIW),
 		F(AVX512_4FMAPS),
-- 
2.51.0.470.ga7dc726c21-goog