linux-kernel - Re: [PATCH 1/5] x86: KVM: SVM: fix for x2avic CVE-2023-5090

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Date:   Thu, 28 Sep 2023 08:53:41 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     kvm@...r.kernel.org, Will Deacon <will@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paolo Bonzini <pbonzini@...hat.com>, x86@...nel.org,
        Robin Murphy <robin.murphy@....com>, iommu@...ts.linux.dev,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <joro@...tes.org>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH 1/5] x86: KVM: SVM: fix for x2avic CVE-2023-5090

KVM: SVM: for the shortlog scope

And my preference is to have the shortlog explain the code change and leave the
CVE reference to a line in the changelog.  CVE numbers are meaningless without
context, e.g. listing the CVE isn't going to be at all helpful for future readers
that look at shortlogs.

E.g.

  KVM: SVM: Always refresh x2APIC MSR intercepts when x2AVIC is enabled

or 

  KVM: SVM: Update MSR intercepts for x2AVIC when guest disables x2APIC

On Thu, Sep 28, 2023, Maxim Levitsky wrote:
> The following problem exists since the x2avic was enabled in the KVM:

Just "x2avic"

> svm_set_x2apic_msr_interception is called to enable the interception of

() after functions

> the x2apic msrs.
> 
> In particular it is called at the moment the guest resets its apic.
> 
> Assuming that the guest's apic was in x2apic mode, the reset will bring
> it back to the xapic mode.
> 
> The svm_set_x2apic_msr_interception however has an erroneous check for
> '!apic_x2apic_mode()' which prevents it from doing anything in this case.
> 
> As a result of this, all x2apic msrs are left unintercepted, and that
> exposes the bare metal x2apic (if enabled) to the guest.
> Oops.
> 
> Remove the erroneous '!apic_x2apic_mode()' check to fix that.
> 
> Cc: stable@...r.kernel.org

Fixes: 4d1d7942e36a ("KVM: SVM: Introduce logic to (de)activate x2AVIC mode")

> Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
> ---
>  arch/x86/kvm/svm/svm.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 9507df93f410a63..acdd0b89e4715a3 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -913,8 +913,7 @@ void svm_set_x2apic_msr_interception(struct vcpu_svm *svm, bool intercept)
>  	if (intercept == svm->x2avic_msrs_intercepted)
>  		return;
>  
> -	if (!x2avic_enabled ||
> -	    !apic_x2apic_mode(svm->vcpu.arch.apic))
> +	if (!x2avic_enabled)
>  		return;
>  
>  	for (i = 0; i < MAX_DIRECT_ACCESS_MSRS; i++) {
> -- 
> 2.26.3
>