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

Matthew Rosato
2023-01-18T15:00:59Z

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@linux.ibm.com> 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@intel.com/
>
> 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