linux-kernel - Re: [PATCH v14 04/22] KVM: x86: Refresh CPUID on write to guest MSR_IA32

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <ad6a3c7c-92df-421a-aee2-f9fe6afda8e9@intel.com>
Date: Wed, 10 Sep 2025 17:23:18 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Chao Gao <chao.gao@...el.com>, kvm@...r.kernel.org,
 linux-kernel@...r.kernel.org
Cc: acme@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
 hpa@...or.com, john.allen@....com, mingo@...nel.org, mingo@...hat.com,
 minipli@...ecurity.net, mlevitsk@...hat.com, namhyung@...nel.org,
 pbonzini@...hat.com, prsampat@....com, rick.p.edgecombe@...el.com,
 seanjc@...gle.com, shuah@...nel.org, tglx@...utronix.de,
 weijiang.yang@...el.com, x86@...nel.org, xin@...or.com
Subject: Re: [PATCH v14 04/22] KVM: x86: Refresh CPUID on write to guest
 MSR_IA32_XSS

On 9/9/2025 5:39 PM, Chao Gao wrote:
> From: Yang Weijiang <weijiang.yang@...el.com>
> 
> Update CPUID.(EAX=0DH,ECX=1).EBX to reflect current required xstate size
> due to XSS MSR modification.
> CPUID(EAX=0DH,ECX=1).EBX reports the required storage size of all enabled
> xstate features in (XCR0 | IA32_XSS). The CPUID value can be used by guest
> before allocate sufficient xsave buffer.
> 
> Note, KVM does not yet support any XSS based features, i.e. supported_xss
> is guaranteed to be zero at this time.
> 
> Opportunistically skip CPUID updates if XSS value doesn't change.
> 
> Suggested-by: Sean Christopherson <seanjc@...gle.com>
> Co-developed-by: Zhang Yi Z <yi.z.zhang@...ux.intel.com>
> Signed-off-by: Zhang Yi Z <yi.z.zhang@...ux.intel.com>
> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
> Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
> Reviewed-by: Chao Gao <chao.gao@...el.com>
> Tested-by: Mathias Krause <minipli@...ecurity.net>
> Tested-by: John Allen <john.allen@....com>
> Tested-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> Signed-off-by: Chao Gao <chao.gao@...el.com>

Reviewed-by: Xiaoyao Li <xiaoyao.li@...el.com>

> ---
>   arch/x86/kvm/cpuid.c | 3 ++-
>   arch/x86/kvm/x86.c   | 2 ++
>   2 files changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 46cf616663e6..b5f87254ced7 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -316,7 +316,8 @@ static void kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu)
>   	best = kvm_find_cpuid_entry_index(vcpu, 0xD, 1);
>   	if (best && (cpuid_entry_has(best, X86_FEATURE_XSAVES) ||
>   		     cpuid_entry_has(best, X86_FEATURE_XSAVEC)))
> -		best->ebx = xstate_required_size(vcpu->arch.xcr0, true);
> +		best->ebx = xstate_required_size(vcpu->arch.xcr0 |
> +						 vcpu->arch.ia32_xss, true);
>   }
>   
>   static bool kvm_cpuid_has_hyperv(struct kvm_vcpu *vcpu)
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 6c167117018c..bbae3bf405c7 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4020,6 +4020,8 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>   		 */
>   		if (data & ~vcpu->arch.guest_supported_xss)
>   			return 1;
> +		if (vcpu->arch.ia32_xss == data)
> +			break;
>   		vcpu->arch.ia32_xss = data;
>   		vcpu->arch.cpuid_dynamic_bits_dirty = true;
>   		break;