[PATCH 04/34] KVM: WARN if there are dangling MMU invalidations at VM destruction

[Paolo Bonzini]

From: Sean Christopherson <seanjc@xxxxxxxxxx>

Add an assertion that there are no in-progress MMU invalidations when a
VM is being destroyed, with the exception of the scenario where KVM
unregisters its MMU notifier between an .invalidate_range_start() call and
the corresponding .invalidate_range_end().

KVM can't detect unpaired calls from the mmu_notifier due to the above
exception waiver, but the assertion can detect KVM bugs, e.g. such as the
bug that *almost* escaped initial guest_memfd development.

Link: https://lore.kernel.org/all/e397d30c-c6af-e68f-d18e-b4e3739c5389@xxxxxxxxxxxxxxx
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Reviewed-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Reviewed-by: Fuad Tabba <tabba@xxxxxxxxxx>
Tested-by: Fuad Tabba <tabba@xxxxxxxxxx>
Message-Id: <20231027182217.3615211-5-seanjc@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 virt/kvm/kvm_main.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 9cc57b23ec81..5422ce20dcba 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1358,9 +1358,16 @@ static void kvm_destroy_vm(struct kvm *kvm)
 	 * No threads can be waiting in kvm_swap_active_memslots() as the
 	 * last reference on KVM has been dropped, but freeing
 	 * memslots would deadlock without this manual intervention.
+	 *
+	 * If the count isn't unbalanced, i.e. KVM did NOT unregister its MMU
+	 * notifier between a start() and end(), then there shouldn't be any
+	 * in-progress invalidations.
 	 */
 	WARN_ON(rcuwait_active(&kvm->mn_memslots_update_rcuwait));
-	kvm->mn_active_invalidate_count = 0;
+	if (kvm->mn_active_invalidate_count)
+		kvm->mn_active_invalidate_count = 0;
+	else
+		WARN_ON(kvm->mmu_invalidate_in_progress);
 #else
 	kvm_flush_shadow_all(kvm);
 #endif
-- 
2.39.1