KVM: x86/mmu: Add lockdep assert to enforce safe usage of kvm_unmap_gfn_range()

Add a lockdep assertion in kvm_unmap_gfn_range() to ensure that either
mmu_invalidate_in_progress is elevated, or that the range is being zapped
due to memslot removal (loosely detected by slots_lock being held).
Zapping SPTEs without mmu_invalidate_{in_progress,seq} protection is unsafe
as KVM's page fault path snapshots state before acquiring mmu_lock, and
thus can create SPTEs with stale information if vCPUs aren't forced to
retry faults (due to seeing an in-progress or past MMU invalidation).

Memslot removal is a special case, as the memslot is retrieved outside of
mmu_invalidate_seq, i.e. doesn't use the "standard" protections, and
instead relies on SRCU synchronization to ensure any in-flight page faults
are fully resolved before zapping SPTEs.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20241009192345.1148353-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -1556,6 +1556,17 @@ bool kvm_unmap_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range)
 {
 	bool flush = false;
 
+	/*
+	 * To prevent races with vCPUs faulting in a gfn using stale data,
+	 * zapping a gfn range must be protected by mmu_invalidate_in_progress
+	 * (and mmu_invalidate_seq).  The only exception is memslot deletion;
+	 * in that case, SRCU synchronization ensures that SPTEs are zapped
+	 * after all vCPUs have unlocked SRCU, guaranteeing that vCPUs see the
+	 * invalid slot.
+	 */
+	lockdep_assert_once(kvm->mmu_invalidate_in_progress ||
+			    lockdep_is_held(&kvm->slots_lock));
+
 	if (kvm_memslots_have_rmaps(kvm))
 		flush = __kvm_rmap_zap_gfn_range(kvm, range->slot,
 						 range->start, range->end,