KVM: WARN if there are dangling MMU invalidations at VM destruction

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@linux.intel.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>
Message-Id: <20231027182217.3615211-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 9cc57b23ec81fb8bad00f03c39f3d148e19c477d..5422ce20dcbace265f2b512305b4feebda914743 100644 (file)
--- 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