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

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Date:   Mon, 18 Dec 2017 09:50:04 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Hildenbrand <david@...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 09:30, 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);
 

Thanks,

Paolo