Subject: Re: [PATCH v3 08/12] KVM: Propagate vcpu explicitly to mark_page_dirty_in_slot()

From: Sean Christopherson <[email-masked]>
Date: Thu, 18 Nov 2021 18:50:55 +0000

On Thu, Nov 18, 2021, Sean Christopherson wrote:
> On Thu, Nov 18, 2021, David Woodhouse wrote:
> > That leaves the one in TDP MMU handle_changed_spte_dirty_log() which
> > AFAICT can trigger the same crash seen by butt3rflyh4ck — can't that
> > happen from a thread where kvm_get_running_vcpu() is NULL too? For that
> > one I'm not sure.
> 
> I think could be trigger in the TDP MMU via kvm_mmu_notifier_release()
> -> kvm_mmu_zap_all(), e.g. if the userspace VMM exits while dirty logging is
> enabled.  That should be easy to (dis)prove via a selftest.

Scratch that, the dirty log update is guarded by the new_spte being present, so
zapping of any kind won't trigger it.

Currently, I believe the only path that would create a present SPTE without an
active vCPU is mmu_notifer.change_pte, but that squeaks by because its required
to be wrapped with invalidate_range_{start,end}(MMU_NOTIFY_CLEAR), and KVM zaps
in that situation.

Ben's series to promote pages on disabling of dirty logging will also sqeuak by
because dirty logging is obviously disabled.