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

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Date:   Thu, 14 Jan 2021 09:13:25 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        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>,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH v2 01/14] KVM: SVM: Zero out the VMCB array used to track
 SEV ASID association

On Thu, Jan 14, 2021, Tom Lendacky wrote:
> 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);
> 
> Alternatively, this call could just be changed to kcalloc().

Agreed, kcalloc() is a better option.  I'll do that in v3.