linux-kernel - Re: [PATCH 2/2] KVM: SVM: Skip VMSA init in sev_es_init

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Message-ID: <eb0eab09-3625-d3f2-d1bf-ef6595fb04e1@amd.com>
Date:   Fri, 25 Aug 2023 12:26:48 +0200
From:   "Gupta, Pankaj" <pankaj.gupta@....com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Peter Gonda <pgonda@...gle.com>
Subject: Re: [PATCH 2/2] KVM: SVM: Skip VMSA init in sev_es_init_vmcb() if
 pointer is NULL

On 8/25/2023 4:23 AM, Sean Christopherson wrote:
> Skip initializing the VMSA physical address in the VMCB if the VMSA is
> NULL, which occurs during intrahost migration as KVM initializes the VMCB
> before copying over state from the source to the destination (including
> the VMSA and its physical address).
> 
> In normal builds, __pa() is just math, so the bug isn't fatal, but with
> CONFIG_DEBUG_VIRTUAL=y, the validity of the virtual address is verified
> and passing in NULL will make the kernel unhappy.
> 
> Fixes: 6defa24d3b12 ("KVM: SEV: Init target VMCBs in sev_migrate_from")
> Cc: stable@...r.kernel.org
> Cc: Peter Gonda <pgonda@...gle.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/kvm/svm/sev.c | 7 +++++--
>   1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index acc700bcb299..5585a3556179 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -2975,9 +2975,12 @@ static void sev_es_init_vmcb(struct vcpu_svm *svm)
>   	/*
>   	 * An SEV-ES guest requires a VMSA area that is a separate from the
>   	 * VMCB page. Do not include the encryption mask on the VMSA physical
> -	 * address since hardware will access it using the guest key.
> +	 * address since hardware will access it using the guest key.  Note,
> +	 * the VMSA will be NULL if this vCPU is the destination for intrahost
> +	 * migration, and will be copied later.
>   	 */
> -	svm->vmcb->control.vmsa_pa = __pa(svm->sev_es.vmsa);
> +	if (svm->sev_es.vmsa)
> +		svm->vmcb->control.vmsa_pa = __pa(svm->sev_es.vmsa);
>   
>   	/* Can't intercept CR register access, HV can't modify CR registers */
>   	svm_clr_intercept(svm, INTERCEPT_CR0_READ);

Reviewed-by: Pankaj Gupta <pankaj.gupta@....com>