From: Sean Christopherson Date: Fri, 19 Sep 2025 22:32:33 +0000 (-0700) Subject: KVM: x86: Disable support for Shadow Stacks if TDP is disabled X-Git-Tag: v6.18-rc1~55^2~5^2~25 X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=1f6f68fcfe43cf26fc0a98fe14e0454cc5c75416;p=thirdparty%2Fkernel%2Flinux.git 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=1 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). Reviewed-by: Binbin Wu Reviewed-by: Xiaoyao Li Link: https://lore.kernel.org/r/20250919223258.1604852-27-seanjc@google.com Signed-off-by: Sean Christopherson --- diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c index 32fde9e80c286..499c86bd457e1 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),