Re: [RFC][PATCH] kvm: fix a race when closing irq eventfd

[Li Zefan]

On 2013/2/18 12:09, Li Zefan wrote:
> On 2013/2/18 12:02, Alex Williamson wrote:
>> 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
>>
> 
> I'm aware of this.
> 
> As I said, kvm_irqfd_deassign() and kvm_irqfd_release() are not acquiring
> wqh->lock.
> 
>>>     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.
>>

Oh you're right, this is not obvious. Thanks for the explanation.

Now I'll go to see how to fix cgroup.

>>>       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.
>>
> 
> It doesn't matter if the list is empty or not.
> 
> The point is, irqfd has been kfreed, so the if statement is simply not safe!
> 
>>> 						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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