Re: [PATCH v13 04/35] KVM: WARN if there are dangling MMU invalidations at VM destruction

[Fuad Tabba]

On Fri, Oct 27, 2023 at 7:22 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> 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: Fuad Tabba <tabba@xxxxxxxxxx>
Tested-by: Fuad Tabba <tabba@xxxxxxxxxx>

Cheers,
/fuad

>  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 1a577a25de47..4dba682586ee 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1356,9 +1356,16 @@ 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
> --
> 2.42.0.820.g83a721a137-goog
>