RE: [PATCH] kvm/vfio: Fix potential deadlock on vfio group_lock

Liu, Yi L
2023-01-20T15:09:06Z

> From: Liu, Yi L <yi.l.liu@intel.com>
> Sent: Friday, January 20, 2023 11:05 PM
>
> Currently it is possible that the final put of a KVM reference comes from
> vfio during its device close operation. This occurs while the vfio group
> lock is held; however, if the vfio device is still in the kvm device list,
> then the following call chain could result in a deadlock:
>
> VFIO holds group->group_lock/group_rwsem
> -> kvm_put_kvm
> -> kvm_destroy_vm
> -> kvm_destroy_devices
> -> kvm_vfio_destroy
> -> kvm_vfio_file_set_kvm
> -> vfio_file_set_kvm
> -> try to hold group->group_lock/group_rwsem
>
> The key function is the kvm_destroy_devices() which triggers destroy cb
> of kvm_device_ops. It calls back to vfio and try to hold group_lock. So
> if this path doesn't call back to vfio, this dead lock would be fixed.
> Actually, there is a way for it. KVM provides another point to free the
> kvm-vfio device which is the point when the device file descriptor is
> closed. This can be achieved by providing the release cb instead of the
> destroy cb. Also rename kvm_vfio_destroy() to be kvm_vfio_release().
>
> /*
> * Destroy is responsible for freeing dev.
> *
> * Destroy may be called before or after destructors are called
> * on emulated I/O regions, depending on whether a reference is
> * held by a vcpu or other kvm component that gets destroyed
> * after the emulated I/O.
> */
> void (*destroy)(struct kvm_device *dev);
>
> /*
> * Release is an alternative method to free the device. It is
> * called when the device file descriptor is closed. Once
> * release is called, the destroy method will not be called
> * anymore as the device is removed from the device list of
> * the VM. kvm->lock is held.
> */
> void (*release)(struct kvm_device *dev);
>
> Fixes: 421cfe6596f6 ("vfio: remove VFIO_GROUP_NOTIFY_SET_KVM")
> Reported-by: Alex Williamson <alex.williamson@redhat.com>
> Suggested-by: Kevin Tian <kevin.tian@intel.com>
> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> Signed-off-by: Yi Liu <yi.l.liu@intel.com>

More background can be found in Mathew's work.
https://lore.kernel.org/kvm/20230114000351.115444-1-mjrosato@linux.ibm.com/T/#u

Regards,
Yi Liu