linux-kernel - Re: [PATCH v2 01/32] KVM: SVM: Disable interception of SPEC

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: Windows password security audit tool. GUI, reports in PDF.

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

Message-ID: <629e0dc3-49b4-45f5-aeaa-8a9109e81d14@linux.intel.com>
Date: Wed, 11 Jun 2025 12:38:17 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Chao Gao <chao.gao@...el.com>, Borislav Petkov <bp@...en8.de>,
 Xin Li <xin@...or.com>, Dapeng Mi <dapeng1.mi@...ux.intel.com>,
 Francesco Lavra <francescolavra.fl@...il.com>,
 Manali Shukla <Manali.Shukla@....com>
Subject: Re: [PATCH v2 01/32] KVM: SVM: Disable interception of SPEC_CTRL iff
 the MSR exists for the guest



On 6/11/2025 6:57 AM, Sean Christopherson wrote:
> Disable interception of SPEC_CTRL when the CPU virtualizes (i.e. context
> switches) SPEC_CTRL if and only if the MSR exists according to the vCPU's
> CPUID model.  Letting the guest access SPEC_CTRL is generally benign, but
> the guest would see inconsistent behavior if KVM happened to emulate an
> access to the MSR.
>
> Fixes: d00b99c514b3 ("KVM: SVM: Add support for Virtual SPEC_CTRL")
> Reported-by: Chao Gao <chao.gao@...el.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/kvm/svm/svm.c | 9 ++++++---
>   1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 0ad1a6d4fb6d..21e745acebc3 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1362,11 +1362,14 @@ static void init_vmcb(struct kvm_vcpu *vcpu)
>   	svm_recalc_instruction_intercepts(vcpu, svm);
>   
>   	/*
> -	 * If the host supports V_SPEC_CTRL then disable the interception
> -	 * of MSR_IA32_SPEC_CTRL.
> +	 * If the CPU virtualizes MSR_IA32_SPEC_CTRL, i.e. KVM doesn't need to
> +	 * manually context switch the MSR, immediately configure interception
> +	 * of SPEC_CTRL, without waiting for the guest to access the MSR.
>   	 */
>   	if (boot_cpu_has(X86_FEATURE_V_SPEC_CTRL))
> -		set_msr_interception(vcpu, svm->msrpm, MSR_IA32_SPEC_CTRL, 1, 1);
> +		set_msr_interception(vcpu, svm->msrpm, MSR_IA32_SPEC_CTRL,
> +				     guest_has_spec_ctrl_msr(vcpu),
> +				     guest_has_spec_ctrl_msr(vcpu));
Side topic, not related to this patch directly.

Setting to 1 for set_msr_interception() means to disable interception.
The name of the function seems a bit counterintuitive to me.
Maybe some description for the function can help people not familiar with
SVM code without further checking the implementation?


>   
>   	if (kvm_vcpu_apicv_active(vcpu))
>   		avic_init_vmcb(svm, vmcb);