linux-kernel - Re: [PATCH 42/43] KVM: VMX: Drop VMWRITEs to zero fields at vCPU RESET

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Date:   Tue, 25 May 2021 01:02:53 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 42/43] KVM: VMX: Drop VMWRITEs to zero fields at vCPU
 RESET

On Mon, May 24, 2021, Jim Mattson wrote:
> On Mon, May 24, 2021 at 3:28 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > That said, I'm not against switching to VMWRITE for everything, but regardless
> > of which route we choose, we should commit to one or the other.  I.e. double down
> > on memset() and bet that Intel won't break KVM, or replace the memset() in
> > alloc_vmcs_cpu() with a sequence that writes all known (possible?) fields.  The
> > current approach of zeroing the memory in software but initializing _some_ fields
> > is the worst option, e.g. I highly doubt vmcs01 and vmcs02 do VMWRITE(..., 0) on
> > the same fields.
> 
> The memset should probably be dropped, unless it is there to prevent
> information leakage. However, it is not necessary to VMWRITE all known
> (or possible) fields--just those that aren't guarded by an enable bit.

Yeah, I was thinking of defense-in-depth, e.g. better to have VM-Enter consume
'0' than random garbage because KVM botched an enabling sequence.  We essentially
get that today via the memset().  I'll fiddle with the sequence and see how much
overhead a paranoid and/or really paranoid approach would incur.