linux-kernel - Re: [PATCH v4 06/19] KVM: VMX: Set FRED MSR interception

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: <aFrD-Pn9cmHcVxWs@google.com>
Date: Tue, 24 Jun 2025 08:27:52 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: "Xin Li (Intel)" <xin@...or.com>
Cc: pbonzini@...hat.com, kvm@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, corbet@....net, tglx@...utronix.de, 
	mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org, 
	hpa@...or.com, andrew.cooper3@...rix.com, luto@...nel.org, 
	peterz@...radead.org, chao.gao@...el.com, xin3.li@...el.com
Subject: Re: [PATCH v4 06/19] KVM: VMX: Set FRED MSR interception

On Fri, Mar 28, 2025, Xin Li (Intel) wrote:
> @@ -7935,6 +7945,34 @@ static void update_intel_pt_cfg(struct kvm_vcpu *vcpu)
>  		vmx->pt_desc.ctl_bitmask &= ~(0xfULL << (32 + i * 4));
>  }
>  
> +static void vmx_set_intercept_for_fred_msr(struct kvm_vcpu *vcpu)
> +{

This function should short-circult on

	if (!kvm_cpu_cap_has(X86_FEATURE_FRED))
		return;

Functionally, it shouldn't matter.  It's mostly for documentation purposes, and
to avoid doing unnecessary work.

> +	bool flag = !guest_cpu_cap_has(vcpu, X86_FEATURE_FRED);

"flag" is unnecessarily ambiguous (eww, I see that the exiting PT code does that).
I like "set", as it has (hopefully) obvious polarity, and aligns with the function
being called.

> +
> +	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_RSP1, MSR_TYPE_RW, flag);
> +	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_RSP2, MSR_TYPE_RW, flag);
> +	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_RSP3, MSR_TYPE_RW, flag);
> +	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_STKLVLS, MSR_TYPE_RW, flag);
> +	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_SSP1, MSR_TYPE_RW, flag);
> +	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_SSP2, MSR_TYPE_RW, flag);
> +	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_SSP3, MSR_TYPE_RW, flag);
> +	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_CONFIG, MSR_TYPE_RW, flag);
> +
> +	/*
> +	 * IA32_FRED_RSP0 and IA32_PL0_SSP (a.k.a. IA32_FRED_SSP0) are only used
> +	 * for delivering events when running userspace, while KVM always runs in
> +	 * kernel mode (the CPL is always 0 after any VM exit), thus KVM can run
> +	 * safely with guest IA32_FRED_RSP0 and IA32_PL0_SSP.
> +	 *
> +	 * As a result, no need to intercept IA32_FRED_RSP0 and IA32_PL0_SSP.
> +	 *
> +	 * Note, save and restore of IA32_PL0_SSP belong to CET supervisor context
> +	 * management no matter whether FRED is enabled or not.  So leave its
> +	 * state management to CET code.
> +	 */
> +	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_RSP0, MSR_TYPE_RW, flag);
> +}