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Date:   Wed, 20 Dec 2017 16:48:59 +0800
From:   Lan Tianyu <tianyu.lan@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        David Hildenbrand <david@...hat.com>
Cc:     rkrcmar@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, dvyukov@...gle.com,
        kernellwp@...il.com
Subject: Re: [PATCH] KVM/Eventfd: Avoid crash when assign and deassign same
 eventfd in parallel.

On 2017年12月19日 18:41, Paolo Bonzini wrote:
> On 19/12/2017 07:35, Lan Tianyu wrote:
>> On 2017年12月18日 16:50, Paolo Bonzini wrote:
>>> On 18/12/2017 09:30, David Hildenbrand wrote:
>>>> The ugly thing in kvm_irqfd_assign() is that we access irqfd without
>>>> holding a lock. I think that should rather be fixed than working around
>>>> that issue. (e.g. lock() -> lookup again -> verify still in list ->
>>>> unlock())
>>>
>>> I wonder if it's even simpler:
>>>
>>> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
>>> index f2ac53ab8243..17ed298bd66f 100644
>>> --- a/virt/kvm/eventfd.c
>>> +++ b/virt/kvm/eventfd.c
>>> @@ -387,7 +387,6 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
>>>  
>>>  	idx = srcu_read_lock(&kvm->irq_srcu);
>>>  	irqfd_update(kvm, irqfd);
>>> -	srcu_read_unlock(&kvm->irq_srcu, idx);
>>>  
>>>  	list_add_tail(&irqfd->list, &kvm->irqfds.items);
>>>  
>>> @@ -420,10 +419,12 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
>>>  				irqfd->consumer.token, ret);
>>>  	}
>>>  #endif
>>> +	srcu_read_unlock(&kvm->irq_srcu, idx);
>>>  
>>>  	return 0;
>>>  
>>>  fail:
>>> +	/* irq_srcu is *not* held here.  */
>>>  	if (irqfd->resampler)
>>>  		irqfd_resampler_shutdown(irqfd);
>>>  
>>>
>>
>> Hi Paolo:
>> 	This patch still can't prevent from freeing struct irqfd in
>> irq_shutdown() during assign irqfd. Once irqfd is added to
>> kvm->irqfds.items list, deassign path can get irqfd and free it.
> 
> You're right, that also has to be protected by SRCU.  So a new bool is
> needed (probably "active" more than "initialized") in order to replace
> list_del_init with list_del_rcu.
> 

Adding new flag is sufficient to fix the crash issue. All list
operations for kvm->irqfds.items list are already under protection
kvm->irqfds.lock. Do you mean we should use list_add/del_rcu for
kvm->irqfds.items?

-- 
Best regards
Tianyu Lan

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