Re: [PATCH v13 02/35] KVM: Assert that mmu_invalidate_in_progress *never* goes negative

[Fuad Tabba]

On Fri, Oct 27, 2023 at 7:22 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> Move the assertion on the in-progress invalidation count from the primary
> MMU's notifier path to KVM's common notification path, i.e. assert that
> the count doesn't go negative even when the invalidation is coming from
> KVM itself.
>
> Opportunistically convert the assertion to a KVM_BUG_ON(), i.e. kill only
> the affected VM, not the entire kernel.  A corrupted count is fatal to the
> VM, e.g. the non-zero (negative) count will cause mmu_invalidate_retry()
> to block any and all attempts to install new mappings.  But it's far from
> guaranteed that an end() without a start() is fatal or even problematic to
> anything other than the target VM, e.g. the underlying bug could simply be
> a duplicate call to end().  And it's much more likely that a missed
> invalidation, i.e. a potential use-after-free, would manifest as no
> notification whatsoever, not an end() without a start().
>
> 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 | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 0524933856d4..5a97e6c7d9c2 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -833,6 +833,7 @@ void kvm_mmu_invalidate_end(struct kvm *kvm, unsigned long start,
>          * in conjunction with the smp_rmb in mmu_invalidate_retry().
>          */
>         kvm->mmu_invalidate_in_progress--;
> +       KVM_BUG_ON(kvm->mmu_invalidate_in_progress < 0, kvm);
>  }
>
>  static void kvm_mmu_notifier_invalidate_range_end(struct mmu_notifier *mn,
> @@ -863,8 +864,6 @@ static void kvm_mmu_notifier_invalidate_range_end(struct mmu_notifier *mn,
>          */
>         if (wake)
>                 rcuwait_wake_up(&kvm->mn_memslots_update_rcuwait);
> -
> -       BUG_ON(kvm->mmu_invalidate_in_progress < 0);
>  }
>
>  static int kvm_mmu_notifier_clear_flush_young(struct mmu_notifier *mn,
> --
> 2.42.0.820.g83a721a137-goog
>