linux-kernel - Re: [PATCH v2 01/14] KVM: SVM: Zero out the VMCB array used to track SEV ASID association

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Message-ID: <6d329b0c-e34e-6fed-8cc5-10b9ece82409@amd.com>
Date:   Thu, 14 Jan 2021 14:57:51 -0600
From:   Brijesh Singh <brijesh.singh@....com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     brijesh.singh@....com, Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Borislav Petkov <bp@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH v2 01/14] KVM: SVM: Zero out the VMCB array used to track
 SEV ASID association


On 1/13/21 6:36 PM, Sean Christopherson wrote:
> Zero out the array of VMCB pointers so that pre_sev_run() won't see
> garbage when querying the array to detect when an SEV ASID is being
> associated with a new VMCB.  In practice, reading random values is all
> but guaranteed to be benign as a false negative (which is extremely
> unlikely on its own) can only happen on CPU0 on the first VMRUN and would
> only cause KVM to skip the ASID flush.  For anything bad to happen, a
> previous instance of KVM would have to exit without flushing the ASID,
> _and_ KVM would have to not flush the ASID at any time while building the
> new SEV guest.
>
> Cc: Borislav Petkov <bp@...e.de>
> Cc: Tom Lendacky <thomas.lendacky@....com>
> Cc: Brijesh Singh <brijesh.singh@....com>
> Fixes: 70cd94e60c73 ("KVM: SVM: VMRUN should use associated ASID when SEV is enabled")
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/svm/svm.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 7ef171790d02..ccf52c5531fb 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -573,7 +573,7 @@ static int svm_cpu_init(int cpu)
>  	if (svm_sev_enabled()) {
>  		sd->sev_vmcbs = kmalloc_array(max_sev_asid + 1,
>  					      sizeof(void *),
> -					      GFP_KERNEL);
> +					      GFP_KERNEL | __GFP_ZERO);
>  		if (!sd->sev_vmcbs)
>  			goto free_save_area;
>  	}


Reviewed-by: Brijesh Singh <brijesh.singh@....com>