linux-kernel - Re: [PATCH 4/5] KVM: x86/mmu: Don't install TDP MMU SPTE if SP has unexpected level

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Date:   Tue, 20 Dec 2022 10:24:22 -0800
From:   David Matlack <dmatlack@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Robert Hoo <robert.hu@...ux.intel.com>,
        Greg Thelen <gthelen@...gle.com>,
        Ben Gardon <bgardon@...gle.com>,
        Mingwei Zhang <mizhang@...gle.com>
Subject: Re: [PATCH 4/5] KVM: x86/mmu: Don't install TDP MMU SPTE if SP has
 unexpected level

On Tue, Dec 13, 2022 at 06:15:56PM +0000, Sean Christopherson wrote:
> On Tue, Dec 13, 2022, David Matlack wrote:
> > On Mon, Dec 12, 2022 at 7:30 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > >
> > > Don't install a leaf TDP MMU SPTE if the parent page's level doesn't
> > > match the target level of the fault, and instead have the vCPU retry the
> > > faulting instruction after warning.  Continuing on is completely
> > > unnecessary as the absolute worst case scenario of retrying is DoSing
> > > the vCPU, whereas continuing on all but guarantees bigger explosions, e.g.
> > 
> > Would it make sense to kill the VM instead via KVM_BUG()?
> 
> No, because if bug that hits this escapes to a release, odds are quite high that
> retrying will succeed.  E.g. the fix earlier in this series is for a rare corner
> case that I was able to hit consistently only by hacking KVM to effectively
> synchronize the page fault and zap.  Other than an extra page fault, no harm has
> been done to the guest, e.g. there's no need to kill the VM to protect it from
> data corruption.

Good points, agreed!