linux-kernel - Re: [PATCH] KVM/Eventfd: Avoid crash when assign and deassign same eventfd in parallel.

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Date:   Tue, 19 Dec 2017 14:35:09 +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.
-- 
Best regards
Tianyu Lan