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

[Li Zefan <lizefan@huawei.com>]

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.
> 
>>       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
> 
> .
>