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

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

[Lan Tianyu]

Syzroot reports crash in kvm_irqfd_assign() is caused by use-after-free.
Because kvm_irqfd_assign() and kvm_irqfd_deassign() can't run in parallel
for same eventfd. When assign path hasn't been finished after irqfd
has been added to kvm->irqfds.items list, another thead may deassign the
eventfd and free struct kvm_kernel_irqfd(). This causes assign path still
uses struct kvm_kernel_irqfd freed by deassign path. To avoid such issue,
add "initialized" flag in the struct kvm_kernel_irqfd and check the flag before
deactivating irqfd. If irqfd is still in initialization, deassign path
return fault.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Wanpeng Li <kernellwp@gmail.com>
Signed-off-by: Tianyu Lan <tianyu.lan@intel.com>
---
 include/linux/kvm_irqfd.h |  1 +
 virt/kvm/eventfd.c        | 11 +++++++++--
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/include/linux/kvm_irqfd.h b/include/linux/kvm_irqfd.h
index 76c2fbc..be6b254 100644
--- a/include/linux/kvm_irqfd.h
+++ b/include/linux/kvm_irqfd.h
@@ -66,6 +66,7 @@ struct kvm_kernel_irqfd {
 	struct work_struct shutdown;
 	struct irq_bypass_consumer consumer;
 	struct irq_bypass_producer *producer;
+	u8 initialized:1;
 };
 
 #endif /* __LINUX_KVM_IRQFD_H */
diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
index a334399..80f06e6 100644
--- a/virt/kvm/eventfd.c
+++ b/virt/kvm/eventfd.c
@@ -421,6 +421,7 @@ int  __attribute__((weak)) kvm_arch_update_irqfd_routing(
 	}
 #endif
 
+	irqfd->initialized = 1;
 	return 0;
 
 fail:
@@ -525,6 +526,7 @@ void kvm_unregister_irq_ack_notifier(struct kvm *kvm,
 {
 	struct kvm_kernel_irqfd *irqfd, *tmp;
 	struct eventfd_ctx *eventfd;
+	int ret = 0;
 
 	eventfd = eventfd_ctx_fdget(args->fd);
 	if (IS_ERR(eventfd))
@@ -543,7 +545,12 @@ void kvm_unregister_irq_ack_notifier(struct kvm *kvm,
 			write_seqcount_begin(&irqfd->irq_entry_sc);
 			irqfd->irq_entry.type = 0;
 			write_seqcount_end(&irqfd->irq_entry_sc);
-			irqfd_deactivate(irqfd);
+
+			if (irqfd->initialized)
+				irqfd_deactivate(irqfd);
+			else
+				ret = -EFAULT;
+
 		}
 	}
 
@@ -557,7 +564,7 @@ void kvm_unregister_irq_ack_notifier(struct kvm *kvm,
 	 */
 	flush_workqueue(irqfd_cleanup_wq);
 
-	return 0;
+	return ret;
 }
 
 int
-- 
1.8.3.1

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

[David Hildenbrand]

On 18.12.2017 00:40, Lan Tianyu wrote:
> Syzroot reports crash in kvm_irqfd_assign() is caused by use-after-free.
> Because kvm_irqfd_assign() and kvm_irqfd_deassign() can't run in parallel
> for same eventfd. When assign path hasn't been finished after irqfd
> has been added to kvm->irqfds.items list, another thead may deassign the
> eventfd and free struct kvm_kernel_irqfd(). This causes assign path still
> uses struct kvm_kernel_irqfd freed by deassign path. To avoid such issue,
> add "initialized" flag in the struct kvm_kernel_irqfd and check the flag before
> deactivating irqfd. If irqfd is still in initialization, deassign path
> return fault.>
> Reported-by: Dmitry Vyukov <dvyukov@google.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: Wanpeng Li <kernellwp@gmail.com>
> Signed-off-by: Tianyu Lan <tianyu.lan@intel.com>
> ---
>  include/linux/kvm_irqfd.h |  1 +
>  virt/kvm/eventfd.c        | 11 +++++++++--
>  2 files changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/kvm_irqfd.h b/include/linux/kvm_irqfd.h
> index 76c2fbc..be6b254 100644
> --- a/include/linux/kvm_irqfd.h
> +++ b/include/linux/kvm_irqfd.h
> @@ -66,6 +66,7 @@ struct kvm_kernel_irqfd {
>  	struct work_struct shutdown;
>  	struct irq_bypass_consumer consumer;
>  	struct irq_bypass_producer *producer;
> +	u8 initialized:1;
>  };
>  
>  #endif /* __LINUX_KVM_IRQFD_H */
> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
> index a334399..80f06e6 100644
> --- a/virt/kvm/eventfd.c
> +++ b/virt/kvm/eventfd.c
> @@ -421,6 +421,7 @@ int  __attribute__((weak)) kvm_arch_update_irqfd_routing(
>  	}
>  #endif
>  
> +	irqfd->initialized = 1;

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())

>  	return 0;
>  
>  fail:
> @@ -525,6 +526,7 @@ void kvm_unregister_irq_ack_notifier(struct kvm *kvm,
>  {

Which tool are you using to generate diffs? git format-patch?

Mentioning, because the indicated function here .... (kvm_irqfd_deassign)

>  	struct kvm_kernel_irqfd *irqfd, *tmp;
>  	struct eventfd_ctx *eventfd;
> +	int ret = 0;
>  
>  	eventfd = eventfd_ctx_fdget(args->fd);
>  	if (IS_ERR(eventfd))
> @@ -543,7 +545,12 @@ void kvm_unregister_irq_ack_notifier(struct kvm *kvm,
>  			write_seqcount_begin(&irqfd->irq_entry_sc);
>  			irqfd->irq_entry.type = 0;
>  			write_seqcount_end(&irqfd->irq_entry_sc);
> -			irqfd_deactivate(irqfd);
> +
> +			if (irqfd->initialized)
> +				irqfd_deactivate(irqfd);
> +			else
> +				ret = -EFAULT;
> +
>  		}
>  	}
>  
> @@ -557,7 +564,7 @@ void kvm_unregister_irq_ack_notifier(struct kvm *kvm,
>  	 */

and here are wrong and misleading (kvm_irqfd_deassig). (and just noticed
also in the second hunk)

>  	flush_workqueue(irqfd_cleanup_wq);
>  
> -	return 0;
> +	return ret;
>  }
>  
>  int
> 


-- 

Thanks,

David / dhildenb

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

[Paolo Bonzini]

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

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

[David Hildenbrand]

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

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

[Paolo Bonzini]

On 18/12/2017 10:08, David Hildenbrand wrote:
> 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.

Good point.

The poll() call is effectively a callback to irqfd_ptable_queue_proc.
So, after the above change,  rqfd_wakeup takes irq_srcu inside
wqh->lock, while kvm_irqfd_assign would take them in the opposite order.

However, this is a read-side critical section so this doesn't cause a
deadlock directly.  The effect is only that synchronize_srcu would now
wait for wqh->lock to be released.  The opposite, which *would* cause a
deadlock, would be a call to synchronize_srcu while wqh->lock is held.

However, this cannot happen because wqh->lock is a spinlock and
synchronize_srcu, which sleeps, cannot be called at all while wqh->lock
is held.  So I think it's okay.

Thanks,

Paolo

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

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

[Lan Tianyu]

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

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

[Paolo Bonzini]

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.

Paolo

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

[Lan Tianyu]

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

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

[Peter Xu]

On Mon, Dec 18, 2017 at 09:50:04AM +0100, 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);

Sorry if I miss anything important, but... should we extend the unlock
of kvm->irqfds.lock instead of kvm->irq_srcu here? AFAIU kvm->irq_srcu
is protecting accesses to kvm->irq_routing, while kvm->irqfds.lock is
the one that really protects kvm->irqfds?  Thanks,

-- 
Peter Xu

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

[Lan Tianyu]

Hi David:
	Thanks for your review.

On 2017年12月18日 16:30, David Hildenbrand wrote:
> On 18.12.2017 00:40, Lan Tianyu wrote:
>> Syzroot reports crash in kvm_irqfd_assign() is caused by use-after-free.
>> Because kvm_irqfd_assign() and kvm_irqfd_deassign() can't run in parallel
>> for same eventfd. When assign path hasn't been finished after irqfd
>> has been added to kvm->irqfds.items list, another thead may deassign the
>> eventfd and free struct kvm_kernel_irqfd(). This causes assign path still
>> uses struct kvm_kernel_irqfd freed by deassign path. To avoid such issue,
>> add "initialized" flag in the struct kvm_kernel_irqfd and check the flag before
>> deactivating irqfd. If irqfd is still in initialization, deassign path
>> return fault.>
>> Reported-by: Dmitry Vyukov <dvyukov@google.com>
>> Cc: Paolo Bonzini <pbonzini@redhat.com>
>> Cc: Radim Krčmář <rkrcmar@redhat.com>
>> Cc: Dmitry Vyukov <dvyukov@google.com>
>> Cc: Wanpeng Li <kernellwp@gmail.com>
>> Signed-off-by: Tianyu Lan <tianyu.lan@intel.com>
>> ---
>>  include/linux/kvm_irqfd.h |  1 +
>>  virt/kvm/eventfd.c        | 11 +++++++++--
>>  2 files changed, 10 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/kvm_irqfd.h b/include/linux/kvm_irqfd.h
>> index 76c2fbc..be6b254 100644
>> --- a/include/linux/kvm_irqfd.h
>> +++ b/include/linux/kvm_irqfd.h
>> @@ -66,6 +66,7 @@ struct kvm_kernel_irqfd {
>>  	struct work_struct shutdown;
>>  	struct irq_bypass_consumer consumer;
>>  	struct irq_bypass_producer *producer;
>> +	u8 initialized:1;
>>  };
>>  
>>  #endif /* __LINUX_KVM_IRQFD_H */
>> diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
>> index a334399..80f06e6 100644
>> --- a/virt/kvm/eventfd.c
>> +++ b/virt/kvm/eventfd.c
>> @@ -421,6 +421,7 @@ int  __attribute__((weak)) kvm_arch_update_irqfd_routing(
>>  	}
>>  #endif
>>  
>> +	irqfd->initialized = 1;
> 
> 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())

The new lock should be always held in assign path otherwise we need to
lookup irqfds list frequently, right?

At first, I tried to use a mutex lock between assign and deassign path
but assign path already involves some locks and add new lock maybe
introduce dead lock. So I used flag check to replace with new lock.

> 
>>  	return 0;
>>  
>>  fail:
>> @@ -525,6 +526,7 @@ void kvm_unregister_irq_ack_notifier(struct kvm *kvm,
>>  {
> 
> Which tool are you using to generate diffs? git format-patch?

Yes, I used git version 1.8.3.1 :)
This also confused me. I will try newer version.

> 
> Mentioning, because the indicated function here .... (kvm_irqfd_deassign)
> 
>>  	struct kvm_kernel_irqfd *irqfd, *tmp;
>>  	struct eventfd_ctx *eventfd;
>> +	int ret = 0;
>>  
>>  	eventfd = eventfd_ctx_fdget(args->fd);
>>  	if (IS_ERR(eventfd))
>> @@ -543,7 +545,12 @@ void kvm_unregister_irq_ack_notifier(struct kvm *kvm,
>>  			write_seqcount_begin(&irqfd->irq_entry_sc);
>>  			irqfd->irq_entry.type = 0;
>>  			write_seqcount_end(&irqfd->irq_entry_sc);
>> -			irqfd_deactivate(irqfd);
>> +
>> +			if (irqfd->initialized)
>> +				irqfd_deactivate(irqfd);
>> +			else
>> +				ret = -EFAULT;
>> +
>>  		}
>>  	}
>>  
>> @@ -557,7 +564,7 @@ void kvm_unregister_irq_ack_notifier(struct kvm *kvm,
>>  	 */
> 
> and here are wrong and misleading (kvm_irqfd_deassig). (and just noticed
> also in the second hunk)
> 
>>  	flush_workqueue(irqfd_cleanup_wq);
>>  
>> -	return 0;
>> +	return ret;
>>  }
>>  
>>  int
>>
> 
> 


-- 
Best regards
Tianyu Lan