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

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Message-ID: <fc20c4ec-acb2-00d3-ee78-a3eec2220288@redhat.com>
Date:   Mon, 18 Dec 2017 10:08:26 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Lan Tianyu <tianyu.lan@...el.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 18.12.2017 09:50, 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);
>  

Was worried about the poll() call. But if that works, it would be very nice.

>  	return 0;
>  
>  fail:
> +	/* irq_srcu is *not* held here.  */
>  	if (irqfd->resampler)
>  		irqfd_resampler_shutdown(irqfd);
>  
> 
> Thanks,
> 
> Paolo
> 


-- 

Thanks,

David / dhildenb