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

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Message-ID: <f0fb4c6e-cf79-45ad-846c-9dc67bd91f5f@linux.intel.com>
Date: Mon, 22 Sep 2025 15:45:40 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: 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>,
 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 26/51] KVM: x86: Disable support for Shadow Stacks if
 TDP is disabled



On 9/20/2025 6:32 AM, Sean Christopherson wrote:
> 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

Writable=0,Dirty=0 -> Writable=0,Dirty=1

Otherwise,
Reviewed-by: Binbin Wu <binbin.wu@...ux.intel.com>

> 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),