Re: [KVM PATCH v2 2/2] kvm: use POLLHUP to close an irqfd instead of an explicit ioctl

Gregory Haskins
2009-06-15T03:48:18Z

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[ 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@novell.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@redhat.com>
Signed-off-by: Gregory Haskins <ghaskins@novell.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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