Re: [syzbot] [kvm?] [net?] [virt?] general protection fault in vhost_work_queue

[Stefano Garzarella <sgarzare@xxxxxxxxxx>]

On Wed, May 31, 2023 at 11:27:12AM -0500, Mike Christie wrote:
> On 5/31/23 10:15 AM, Mike Christie wrote:
> > > > rcu would work for your case and for what Jason had requested.
> > > Yeah, so you already have some patches?
> > >
> > > Do you want to send it to solve this problem?
> > Yeah, I'll break them out and send them later today when I can retest
> > rebased patches.
>
> Just one question. Do you core vhost developers consider RCU more complex
> or switching to READ_ONCE/WRITE_ONCE? I am asking because for this immediate
> regression fix we could just switch to the latter like below to just fix
> the crash if we think that is more simple.
>
> I think RCU is just a little more complex/invasive because it will have the
> extra synchronize_rcu calls.

Yes, you may be right, in this case we should just need
READ_ONCE/WRITE_ONCE if dev->worker is no longer a pointer.

> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index a92af08e7864..03fd47a22a73 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -235,7 +235,7 @@ void vhost_dev_flush(struct vhost_dev *dev)
> {
> 	struct vhost_flush_struct flush;
>
> -	if (dev->worker) {
> +	if (READ_ONCE(dev->worker.vtsk)) {
> 		init_completion(&flush.wait_event);
> 		vhost_work_init(&flush.work, vhost_flush_work);
>
> @@ -247,7 +247,9 @@ EXPORT_SYMBOL_GPL(vhost_dev_flush);
>
> void vhost_work_queue(struct vhost_dev *dev, struct vhost_work *work)
> {
> -	if (!dev->worker)
> +	struct vhost_task *vtsk = READ_ONCE(dev->worker.vtsk);
> +
> +	if (!vtsk)
> 		return;
>
> 	if (!test_and_set_bit(VHOST_WORK_QUEUED, &work->flags)) {
> @@ -255,8 +257,8 @@ void vhost_work_queue(struct vhost_dev *dev, struct vhost_work *work)
> 		 * sure it was not in the list.
> 		 * test_and_set_bit() implies a memory barrier.
> 		 */
> -		llist_add(&work->node, &dev->worker->work_list);
> -		wake_up_process(dev->worker->vtsk->task);
> +		llist_add(&work->node, &dev->worker.work_list);
> +		wake_up_process(vtsk->task);
> 	}
> }
> EXPORT_SYMBOL_GPL(vhost_work_queue);
> @@ -264,7 +266,7 @@ EXPORT_SYMBOL_GPL(vhost_work_queue);
> /* A lockless hint for busy polling code to exit the loop */
> bool vhost_has_work(struct vhost_dev *dev)
> {
> -	return dev->worker && !llist_empty(&dev->worker->work_list);
> +	return !llist_empty(&dev->worker.work_list);
> }
> EXPORT_SYMBOL_GPL(vhost_has_work);
>
> @@ -468,7 +470,7 @@ void vhost_dev_init(struct vhost_dev *dev,
> 	dev->umem = NULL;
> 	dev->iotlb = NULL;
> 	dev->mm = NULL;
> -	dev->worker = NULL;
> +	memset(&dev->worker, 0, sizeof(dev->worker));
> 	dev->iov_limit = iov_limit;
> 	dev->weight = weight;
> 	dev->byte_weight = byte_weight;
> @@ -542,46 +544,38 @@ static void vhost_detach_mm(struct vhost_dev *dev)
>
> static void vhost_worker_free(struct vhost_dev *dev)
> {
> -	struct vhost_worker *worker = dev->worker;
> +	struct vhost_task *vtsk = READ_ONCE(dev->worker.vtsk);
>
> -	if (!worker)
> +	if (!vtsk)
> 		return;
>
> -	dev->worker = NULL;
> -	WARN_ON(!llist_empty(&worker->work_list));
> -	vhost_task_stop(worker->vtsk);
> -	kfree(worker);
> +	vhost_task_stop(vtsk);
> +	WARN_ON(!llist_empty(&dev->worker.work_list));
> +	WRITE_ONCE(dev->worker.vtsk, NULL);

The patch LGTM, I just wonder if we should set dev->worker to zero here,
but maybe we don't need to.

Thanks,
Stefano

> }
>
> static int vhost_worker_create(struct vhost_dev *dev)
> {
> -	struct vhost_worker *worker;
> 	struct vhost_task *vtsk;
> 	char name[TASK_COMM_LEN];
> 	int ret;
>
> -	worker = kzalloc(sizeof(*worker), GFP_KERNEL_ACCOUNT);
> -	if (!worker)
> -		return -ENOMEM;
> -
> -	dev->worker = worker;
> -	worker->kcov_handle = kcov_common_handle();
> -	init_llist_head(&worker->work_list);
> +	dev->worker.kcov_handle = kcov_common_handle();
> +	init_llist_head(&dev->worker.work_list);
> 	snprintf(name, sizeof(name), "vhost-%d", current->pid);
>
> -	vtsk = vhost_task_create(vhost_worker, worker, name);
> +	vtsk = vhost_task_create(vhost_worker, &dev->worker, name);
> 	if (!vtsk) {
> 		ret = -ENOMEM;
> 		goto free_worker;
> 	}
>
> -	worker->vtsk = vtsk;
> +	WRITE_ONCE(dev->worker.vtsk, vtsk);
> 	vhost_task_start(vtsk);
> 	return 0;
>
> free_worker:
> -	kfree(worker);
> -	dev->worker = NULL;
> +	WRITE_ONCE(dev->worker.vtsk, NULL);
> 	return ret;
> }
>
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index 0308638cdeee..305ec8593d46 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -154,7 +154,7 @@ struct vhost_dev {
> 	struct vhost_virtqueue **vqs;
> 	int nvqs;
> 	struct eventfd_ctx *log_ctx;
> -	struct vhost_worker *worker;
> +	struct vhost_worker worker;
> 	struct vhost_iotlb *umem;
> 	struct vhost_iotlb *iotlb;
> 	spinlock_t iotlb_lock;