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SubjectRe: [RFC][PATCH] kvm: fix a race when closing irq eventfd
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On Mon, 2013-02-18 at 11:13 +0800, Li Zefan wrote:
> While trying to fix a race when closing cgroup eventfd, I took a look
> at how kvm deals with this problem, and I found it doesn't.
>
> I may be wrong, as I don't know kvm code, so correct me if I'm.
>
> /*
> * Race-free decouple logic (ordering is critical)
> */
> static void
> irqfd_shutdown(struct work_struct *work)
>
> I don't think it's race-free!
>
> static int
> irqfd_wakeup(wait_queue_t *wait, unsigned mode, int sync, void *key)
> {
> ...
> * We cannot race against the irqfd going away since the
> * other side is required to acquire wqh->lock, which we hold
> */
> if (irqfd_is_active(irqfd))
> irqfd_deactivate(irqfd);
> }
>
> In kvm_irqfd_deassign() and kvm_irqfd_release() where irqfds are freed,
> wqh->lock is not acquired!
>
> So here is the race:
>
> CPU0 CPU1
> ----------------------------------- ---------------------------------
> kvm_irqfd_release()
> spin_lock(kvm->irqfds.lock);
> ...
> irqfd_deactivate(irqfd);
> list_del_init(&irqfd->list);
> spin_unlock(kvm->irqfd.lock);
> ...
> close(eventfd)
> irqfd_wakeup();

irqfd_wakeup is assumed to be called with wqh->lock held

> irqfd_shutdown();

eventfd_ctx_remove_wait_queue has to acquire wqh->lock to complete or
else irqfd_shutdown never makes it to the kfree. So in your scenario
this cpu0 spins here until cpu1 completes.

> remove_waitqueue(irqfd->wait);
> kfree(irqfd);
> spin_lock(kvm->irqfd.lock);
> if (!list_empty(&irqfd->list))

We don't take this branch because we already did list_del_init above,
which makes irqfd->list empty.

> irqfd_deactivate(irqfd);
> list_del_init(&irqfd->list);
> spin_unlock(kvm->irqfd.lock);
>
> Look, we're accessing irqfd though it has already been freed!

Unless the irqfd_wakeup path isn't acquiring wqh->lock, it looks
race-free to me. Thanks,

Alex



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