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    Subject[PATCH 04/34] KVM: WARN if there are dangling MMU invalidations at VM destruction
    Date
    From: Sean Christopherson <seanjc@google.com>

    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>
    ---
    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

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