4.17-stable review patch. If anyone has any objections, please let me know. ------------------ From: Paolo Bonzini <pbonzini@xxxxxxxxxx> commit 9432a3175770e06cb83eada2d91fac90c977cb99 upstream. A comment warning against this bug is there, but the code is not doing what the comment says. Therefore it is possible that an EPOLLHUP races against irq_bypass_register_consumer. The EPOLLHUP handler schedules irqfd_shutdown, and if that runs soon enough, you get a use-after-free. Reported-by: syzbot <syzkaller@xxxxxxxxxxxxxxxx> Cc: stable@xxxxxxxxxxxxxxx Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> Reviewed-by: David Hildenbrand <david@xxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- virt/kvm/eventfd.c | 11 ++++++----- 1 file changed, 6 insertions(+), 5 deletions(-) --- a/virt/kvm/eventfd.c +++ b/virt/kvm/eventfd.c @@ -405,11 +405,6 @@ kvm_irqfd_assign(struct kvm *kvm, struct if (events & EPOLLIN) schedule_work(&irqfd->inject); - /* - * do not drop the file until the irqfd is fully initialized, otherwise - * we might race against the EPOLLHUP - */ - fdput(f); #ifdef CONFIG_HAVE_KVM_IRQ_BYPASS if (kvm_arch_has_irq_bypass()) { irqfd->consumer.token = (void *)irqfd->eventfd; @@ -425,6 +420,12 @@ kvm_irqfd_assign(struct kvm *kvm, struct #endif srcu_read_unlock(&kvm->irq_srcu, idx); + + /* + * do not drop the file until the irqfd is fully initialized, otherwise + * we might race against the EPOLLHUP + */ + fdput(f); return 0; fail: