linux-kernel - Re: [PATCH v5 16/17] KVM: x86: nSVM: always intercept x2apic msrs

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Date:   Wed, 18 May 2022 20:18:31 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     pbonzini@...hat.com, seanjc@...gle.com, joro@...tes.org,
        jon.grimm@....com, wei.huang2@....com, terry.bowman@....com
Subject: Re: [PATCH v5 16/17] KVM: x86: nSVM: always intercept x2apic msrs

On Wed, 2022-05-18 at 11:26 -0500, Suravee Suthikulpanit wrote:
> From: Maxim Levitsky <mlevitsk@...hat.com>
> 
> As a preparation for x2avic, this patch ensures that x2apic msrs
> are always intercepted for the nested guest.
> 
> Reviewed-by: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
> Tested-by: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
> Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
> ---
>  arch/x86/kvm/svm/nested.c | 5 +++++
>  arch/x86/kvm/svm/svm.h    | 9 +++++++++
>  2 files changed, 14 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index f209c1ca540c..b61f8939c210 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -230,6 +230,11 @@ static bool nested_svm_vmrun_msrpm(struct vcpu_svm *svm)
>  			break;
>  
>  		p      = msrpm_offsets[i];
> +
> +		/* x2apic msrs are intercepted always for the nested guest */
> +		if (is_x2apic_msrpm_offset(p))
> +			continue;
> +
>  		offset = svm->nested.ctl.msrpm_base_pa + (p * 4);
>  
>  		if (kvm_vcpu_read_guest(&svm->vcpu, offset, &value, 4))
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 818817b11f53..309445619756 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -517,6 +517,15 @@ static inline bool nested_npt_enabled(struct vcpu_svm *svm)
>  	return svm->nested.ctl.nested_ctl & SVM_NESTED_CTL_NP_ENABLE;
>  }
>  
> +static inline bool is_x2apic_msrpm_offset(u32 offset)
> +{
> +	/* 4 msrs per u8, and 4 u8 in u32 */
> +	u32 msr = offset * 16;
> +
> +	return (msr >= APIC_BASE_MSR) &&
> +	       (msr < (APIC_BASE_MSR + 0x100));
> +}
> +
>  /* svm.c */
>  #define MSR_INVALID				0xffffffffU
>  

Just one thing, this patch should be earlier in the series (or even first one),
to avoid having a commit window where the problem exists, where malicious
L1 can get access to L0's apic msrs this way.

Best regards,
	Maxim Levitsky