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

[Paolo Bonzini]

On 10/27/23 20:21, Sean Christopherson 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().

Reviewed-by: Paolo Bonzini <pbonzini at redhat.com>

> Signed-off-by: Sean Christopherson <seanjc at google.com>
> ---
>   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,