On Thu, Nov 18, 2021, Sean Christopherson wrote:

On Thu, Nov 18, 2021, David Woodhouse wrote:

quoted

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.