RE: [PATCH v4] vfio: fix potential deadlock on vfio group lock

[Tian, Kevin]

> From: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>
> Sent: Wednesday, January 18, 2023 10:56 PM
> 
> On 1/18/23 4:03 AM, Tian, Kevin wrote:
> >> From: Alex Williamson
> >> Sent: Wednesday, January 18, 2023 5:23 AM
> >>
> >> On Fri, 13 Jan 2023 19:03:51 -0500
> >> Matthew Rosato <mjrosato@xxxxxxxxxxxxx> wrote:
> >>
> >>>  void vfio_device_group_close(struct vfio_device *device)
> >>>  {
> >>> +	void (*put_kvm)(struct kvm *kvm);
> >>> +	struct kvm *kvm;
> >>> +
> >>>  	mutex_lock(&device->group->group_lock);
> >>> +	kvm = device->kvm;
> >>> +	put_kvm = device->put_kvm;
> >>>  	vfio_device_close(device, device->group->iommufd);
> >>> +	if (kvm == device->kvm)
> >>> +		kvm = NULL;
> >>
> >> Hmm, so we're using whether the device->kvm pointer gets cleared in
> >> last_close to detect whether we should put the kvm reference.  That's a
> >> bit obscure.  Our get and put is also asymmetric.
> >>
> >> Did we decide that we couldn't do this via a schedule_work() from the
> >> last_close function, ie. implementing our own version of an async put?
> >> It seems like that potentially has a cleaner implementation, symmetric
> >> call points, handling all the storing and clearing of kvm related
> >> pointers within the get/put wrappers, passing only a vfio_device to the
> >> put wrapper, using the "vfio_device_" prefix for both.  Potentially
> >> we'd just want an unconditional flush outside of lock here for
> >> deterministic release.
> >>
> >> What's the downside?  Thanks,
> >>
> >
> > btw I guess this can be also fixed by Yi's work here:
> >
> > https://lore.kernel.org/kvm/20230117134942.101112-6-yi.l.liu@xxxxxxxxx/
> >
> > with set_kvm(NULL) moved to the release callback of kvm_vfio device,
> > such circular lock dependency can be avoided too.
> 
> Oh, interesting...  It seems to me that this would eliminate the reported call
> chain altogether:
> 
> kvm_put_kvm
>  -> kvm_destroy_vm
>   -> kvm_destroy_devices
>    -> kvm_vfio_destroy (starting here -- this would no longer be executed)
>     -> kvm_vfio_file_set_kvm
>      -> vfio_file_set_kvm
>       -> group->group_lock/group_rwsem
> 
> because kvm_destroy_devices now can't end up calling kvm_vfio_destroy
> and friends, it won't try and acquire the group lock a 2nd time making a
> kvm_put_kvm while the group lock is held OK to do.  The vfio_file_set_kvm
> call will now always come from a separate thread of execution,
> kvm_vfio_group_add, kvm_vfio_group_del or the release thread:
> 
> kvm_device_release (where the group->group_lock would not be held since
> vfio does not trigger closing of the kvm fd)
>  -> kvm_vfio_destroy (or, kvm_vfio_release)
>   -> kvm_vfio_file_set_kvm
>    -> vfio_file_set_kvm
>     -> group->group_lock/group_rwsem

Yes, that's my point. If Alex/Jason are also OK with it probably Yi can
send that patch separately as a fix to this issue. It's much simpler. 😊