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

[David Matlack <dmatlack@google.com>]

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@google.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!