Date:	Sun, 14 Jun 2009 23:48:18 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, avi@...hat.com,
	Davide Libenzi <davidel@...ilserver.org>,
	paulmck@...ux.vnet.ibm.com, akpm@...ux-foundation.org
Subject: Re: [KVM PATCH v2 2/2] kvm: use POLLHUP to close an irqfd instead
 of an explicit ioctl

[ restoring poor Davide's proper email address.  Sorry for the constant
fat-fingering of your addr, Davide! ]

Michael S. Tsirkin wrote:
> On Sun, Jun 14, 2009 at 08:53:11AM -0400, Gregory Haskins wrote:
>   
>> Michael S. Tsirkin wrote:
>>     
>>> On Thu, Jun 04, 2009 at 08:48:12AM -0400, Gregory Haskins wrote:
>>>   
>>>       
>>>> +static void
>>>> +irqfd_disconnect(struct _irqfd *irqfd)
>>>> +{
>>>> +	struct kvm *kvm;
>>>> +
>>>> +	mutex_lock(&irqfd->lock);
>>>> +
>>>> +	kvm = rcu_dereference(irqfd->kvm);
>>>> +	rcu_assign_pointer(irqfd->kvm, NULL);
>>>> +
>>>> +	mutex_unlock(&irqfd->lock);
>>>> +
>>>> +	if (!kvm)
>>>> +		return;
>>>>  
>>>>  	mutex_lock(&kvm->lock);
>>>> -	kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 1);
>>>> -	kvm_set_irq(kvm, KVM_USERSPACE_IRQ_SOURCE_ID, irqfd->gsi, 0);
>>>> +	list_del(&irqfd->list);
>>>>  	mutex_unlock(&kvm->lock);
>>>> +
>>>> +	/*
>>>> +	 * It is important to not drop the kvm reference until the next grace
>>>> +	 * period because there might be lockless references in flight up
>>>> +	 * until then
>>>> +	 */
>>>> +	synchronize_srcu(&irqfd->srcu);
>>>> +	kvm_put_kvm(kvm);
>>>>  }
>>>>     
>>>>         
>>> So irqfd object will persist after kvm goes away, until eventfd is closed?
>>>   
>>>       
>> Yep, by design.  It becomes part of the eventfd and is thus associated
>> with its lifetime.  Consider it as if we made our own anon-fd
>> implementation for irqfd and the lifetime looks similar.  The difference
>> is that we are reusing eventfd and its interface semantics.
>>     
>>>   
>>>       
>>>>  
>>>>  static int
>>>>  irqfd_wakeup(wait_queue_t *wait, unsigned mode, int sync, void *key)
>>>>  {
>>>>  	struct _irqfd *irqfd = container_of(wait, struct _irqfd, wait);
>>>> +	unsigned long flags = (unsigned long)key;
>>>>  
>>>> -	/*
>>>> -	 * The wake_up is called with interrupts disabled.  Therefore we need
>>>> -	 * to defer the IRQ injection until later since we need to acquire the
>>>> -	 * kvm->lock to do so.
>>>> -	 */
>>>> -	schedule_work(&irqfd->work);
>>>> +	if (flags & POLLIN)
>>>> +		/*
>>>> +		 * The POLLIN wake_up is called with interrupts disabled.
>>>> +		 * Therefore we need to defer the IRQ injection until later
>>>> +		 * since we need to acquire the kvm->lock to do so.
>>>> +		 */
>>>> +		schedule_work(&irqfd->inject);
>>>> +
>>>> +	if (flags & POLLHUP) {
>>>> +		/*
>>>> +		 * The POLLHUP is called unlocked, so it theoretically should
>>>> +		 * be safe to remove ourselves from the wqh using the locked
>>>> +		 * variant of remove_wait_queue()
>>>> +		 */
>>>> +		remove_wait_queue(irqfd->wqh, &irqfd->wait);
>>>> +		flush_work(&irqfd->inject);
>>>> +		irqfd_disconnect(irqfd);
>>>> +
>>>> +		cleanup_srcu_struct(&irqfd->srcu);
>>>> +		kfree(irqfd);
>>>> +	}
>>>>  
>>>>  	return 0;
>>>>  }
>>>>     
>>>>         
>>> And it is removed by this function when eventfd is closed.
>>> But what prevents the kvm module from going away, meanwhile?
>>>   
>>>       
>> Well, we hold a reference to struct kvm until we call
>> irqfd_disconnect().  If kvm closes first, we disconnect and disassociate
>> all references to kvm leaving irqfd->kvm = NULL.  Likewise, if irqfd
>> closes first, we disassociate with kvm with the above quoted logic.  In
>> either case, we are holding a kvm reference up until that "disconnect"
>> point.  Therefore kvm should not be able to disappear before that
>> disconnect, and after that point we do not care.
>>     
>
> Yes, we do care.
>
> Here's the scenario in more detail:
>
> - kvm is closed
> - irq disconnect is called
> - kvm is put
> - kvm module is removed: all irqs are disconnected
> - eventfd closes and triggers callback into removed kvm module
> - crash
>   

[ lightbulb turns on]

Ah, now I see the point you were making.  I thought you were talking
about the .text in kvm_set_irq() (which would be protected by my
kvm_get_kvm() reference afaict).  But you are actually talking about the
irqfd .text itself.  Indeed, you are correct that is this currently a
race.  Good catch!

>   
>> If that is not sufficient to prevent kvm.ko from going away in the
>> middle, then IMO kvm_get_kvm() has a bug, not irqfd. ;) However, I
>> believe everything is actually ok here.
>>
>> -Greg
>>
>>     
>
>
> BTW, why can't we remove irqfds in kvm_release?
>   

Well, this would be ideal but we run into that bi-directional reference
thing that we talked about earlier and we both agree is non-trivial to
solve.  Solving this locking problem would incidentally also pave the
way for restoring the DEASSIGN feature, so patches welcome!  In the
meantime, I think we can close the hole you found with the following
patch (build-tested only):

commit f3a8dccc9e815599438e9feb0ea53e8eb10ad2b3
Author: Gregory Haskins <ghaskins@...ell.com>
Date:   Sun Jun 14 23:37:49 2009 -0400

    KVM: make irqfd take kvm.ko module reference

    Michael Tsirkin pointed out that we currently have a race between
someone
    holding an irqfd reference and an rmmod against kvm.ko.  This patch
closes
    that hole by making sure that irqfd holds a kvm.ko reference for its
lifetime.

    Found-by: Michael S. Tsirkin <mst@...hat.com>
    Signed-off-by: Gregory Haskins <ghaskins@...ell.com>

diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
index 2c8028c..67e4eca 100644
--- a/virt/kvm/eventfd.c
+++ b/virt/kvm/eventfd.c
@@ -29,6 +29,7 @@
 #include <linux/list.h>
 #include <linux/eventfd.h>
 #include <linux/srcu.h>
+#include <linux/module.h>

 /*
  * --------------------------------------------------------------------
@@ -123,6 +124,7 @@ irqfd_wakeup(wait_queue_t *wait, unsigned mode, int
sync, void
 *key)

                cleanup_srcu_struct(&irqfd->srcu);
                kfree(irqfd);
+               module_put(THIS_MODULE);
        }

        return 0;
@@ -176,6 +178,7 @@ kvm_irqfd(struct kvm *kvm, int fd, int gsi, int flags)
        if (ret < 0)
                goto fail;

+       __module_get(THIS_MODULE);
        kvm_get_kvm(kvm);

        mutex_lock(&kvm->lock);





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