Re: [PATCH v1 1/2] KVM: s390: Fix incorrect usage of mmu_notifier_register()

["Christoph Schlameuss" <schlameuss@linux.ibm.com>]

On Tue Aug 5, 2025 at 1:14 PM CEST, Claudio Imbrenda wrote:
> If mmu_notifier_register() fails, for example because a signal was
> pending, the mmu_notifier will not be registered. But when the VM gets
> destroyed, it will get unregistered anyway and that will cause one
> extra mmdrop(), which will eventually cause the mm of the process to
> be freed too early, and cause a use-after free.
>
> This bug happens rarely, and only when secure guests are involved.
>
> The solution is to check the return value of mmu_notifier_register()
> and return it to the caller (ultimately it will be propagated all the
> way to userspace). In case of -EINTR, userspace will try again.
>
> Fixes: ca2fd0609b5d ("KVM: s390: pv: add mmu_notifier")
> Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>

Reviewed-by: Christoph Schlameuss <schlameuss@linux.ibm.com>

> ---
>  arch/s390/kvm/pv.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
>
> diff --git a/arch/s390/kvm/pv.c b/arch/s390/kvm/pv.c
> index 14c330ec8ceb..e85fb3247b0e 100644
> --- a/arch/s390/kvm/pv.c
> +++ b/arch/s390/kvm/pv.c
> @@ -622,6 +622,15 @@ int kvm_s390_pv_init_vm(struct kvm *kvm, u16 *rc, u16 *rrc)
>  	int cc, ret;
>  	u16 dummy;
>  
> +	/* Add the notifier only once. No races because we hold kvm->lock */
> +	if (kvm->arch.pv.mmu_notifier.ops != &kvm_s390_pv_mmu_notifier_ops) {
> +		ret = mmu_notifier_register(&kvm->arch.pv.mmu_notifier, kvm->mm);
> +		if (ret)
> +			return ret;
> +		/* The notifier will be unregistered when the VM is destroyed */
> +		kvm->arch.pv.mmu_notifier.ops = &kvm_s390_pv_mmu_notifier_ops;
> +	}
> +
>  	ret = kvm_s390_pv_alloc_vm(kvm);
>  	if (ret)
>  		return ret;
> @@ -657,11 +666,6 @@ int kvm_s390_pv_init_vm(struct kvm *kvm, u16 *rc, u16 *rrc)
>  		return -EIO;
>  	}
>  	kvm->arch.gmap->guest_handle = uvcb.guest_handle;
> -	/* Add the notifier only once. No races because we hold kvm->lock */
> -	if (kvm->arch.pv.mmu_notifier.ops != &kvm_s390_pv_mmu_notifier_ops) {
> -		kvm->arch.pv.mmu_notifier.ops = &kvm_s390_pv_mmu_notifier_ops;
> -		mmu_notifier_register(&kvm->arch.pv.mmu_notifier, kvm->mm);
> -	}
>  	return 0;
>  }
>