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@xxxxxxxxxxxxx>
Reviewed-by: Christoph Schlameuss <schlameuss@xxxxxxxxxxxxx>
> ---
> 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;
> }
>
