Re: [PATCH 1/2] KVM: async kvm_destroy_vm for vfio devices

[Anthony Krowiak]

LGTM

Reviewed-by: Tony Krowiak <akrowiak@xxxxxxxxxxxxx>

On 1/9/23 3:10 PM, Matthew Rosato wrote:
> 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:
>
> kvm_put_kvm
>   -> kvm_destroy_vm
>    -> kvm_destroy_devices
>     -> kvm_vfio_destroy
>      -> kvm_vfio_file_set_kvm
>       -> vfio_file_set_kvm
>        -> group->group_lock/group_rwsem
>
> Avoid this scenario by adding kvm_put_kvm_async which will perform the
> kvm_destroy_vm asynchronously if the refcount reaches 0.
>
> Fixes: 421cfe6596f6 ("vfio: remove VFIO_GROUP_NOTIFY_SET_KVM")
> Reported-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
> Signed-off-by: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>
> ---
>   drivers/gpu/drm/i915/gvt/kvmgt.c  |  6 +++++-
>   drivers/s390/crypto/vfio_ap_ops.c |  7 ++++++-
>   include/linux/kvm_host.h          |  3 +++
>   virt/kvm/kvm_main.c               | 22 ++++++++++++++++++++++
>   4 files changed, 36 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gvt/kvmgt.c b/drivers/gpu/drm/i915/gvt/kvmgt.c
> index 8ae7039b3683..24511c877572 100644
> --- a/drivers/gpu/drm/i915/gvt/kvmgt.c
> +++ b/drivers/gpu/drm/i915/gvt/kvmgt.c
> @@ -703,7 +703,11 @@ static void intel_vgpu_close_device(struct vfio_device *vfio_dev)
>   
>   	kvm_page_track_unregister_notifier(vgpu->vfio_device.kvm,
>   					   &vgpu->track_node);
> -	kvm_put_kvm(vgpu->vfio_device.kvm);
> +	/*
> +	 * Avoid possible deadlock on any currently-held vfio lock by
> +	 * ensuring the potential kvm_destroy_vm call is done asynchronously
> +	 */
> +	kvm_put_kvm_async(vgpu->vfio_device.kvm);
>   
>   	kvmgt_protect_table_destroy(vgpu);
>   	gvt_cache_destroy(vgpu);
> diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
> index e93bb9c468ce..a37b2baefb36 100644
> --- a/drivers/s390/crypto/vfio_ap_ops.c
> +++ b/drivers/s390/crypto/vfio_ap_ops.c
> @@ -1574,7 +1574,12 @@ static void vfio_ap_mdev_unset_kvm(struct ap_matrix_mdev *matrix_mdev)
>   
>   		kvm_arch_crypto_clear_masks(kvm);
>   		vfio_ap_mdev_reset_queues(&matrix_mdev->qtable);
> -		kvm_put_kvm(kvm);
> +		/*
> +		 * Avoid possible deadlock on any currently-held vfio lock by
> +		 * ensuring the potential kvm_destroy_vm call is done
> +		 * asynchronously
> +		 */
> +		kvm_put_kvm_async(kvm);
>   		matrix_mdev->kvm = NULL;
>   
>   		release_update_locks_for_kvm(kvm);
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 4f26b244f6d0..2ef6a5102265 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -34,6 +34,7 @@
>   #include <linux/instrumentation.h>
>   #include <linux/interval_tree.h>
>   #include <linux/rbtree.h>
> +#include <linux/workqueue.h>
>   #include <linux/xarray.h>
>   #include <asm/signal.h>
>   
> @@ -793,6 +794,7 @@ struct kvm {
>   	struct kvm_stat_data **debugfs_stat_data;
>   	struct srcu_struct srcu;
>   	struct srcu_struct irq_srcu;
> +	struct work_struct async_work;
>   	pid_t userspace_pid;
>   	bool override_halt_poll_ns;
>   	unsigned int max_halt_poll_ns;
> @@ -963,6 +965,7 @@ void kvm_exit(void);
>   void kvm_get_kvm(struct kvm *kvm);
>   bool kvm_get_kvm_safe(struct kvm *kvm);
>   void kvm_put_kvm(struct kvm *kvm);
> +void kvm_put_kvm_async(struct kvm *kvm);
>   bool file_is_kvm(struct file *file);
>   void kvm_put_kvm_no_destroy(struct kvm *kvm);
>   
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 13e88297f999..fbe8d127028b 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1353,6 +1353,28 @@ void kvm_put_kvm(struct kvm *kvm)
>   }
>   EXPORT_SYMBOL_GPL(kvm_put_kvm);
>   
> +static void kvm_put_async_fn(struct work_struct *work)
> +{
> +	struct kvm *kvm = container_of(work, struct kvm,
> +				       async_work);
> +
> +	kvm_destroy_vm(kvm);
> +}
> +
> +/*
> + * Put a reference but only destroy the vm asynchronously.  Can be used in
> + * cases where the caller holds a mutex that could cause deadlock if
> + * kvm_destroy_vm is triggered
> + */
> +void kvm_put_kvm_async(struct kvm *kvm)
> +{
> +	if (refcount_dec_and_test(&kvm->users_count)) {
> +		INIT_WORK(&kvm->async_work, kvm_put_async_fn);
> +		schedule_work(&kvm->async_work);
> +	}
> +}
> +EXPORT_SYMBOL_GPL(kvm_put_kvm_async);
> +
>   /*
>    * Used to put a reference that was taken on behalf of an object associated
>    * with a user-visible file descriptor, e.g. a vcpu or device, if installation