vhost: create worker at end of vhost_dev_set_owner

vsock can start queueing work after VHOST_VSOCK_SET_GUEST_CID, so
after we have called vhost_worker_create it can be calling
vhost_work_queue and trying to access the vhost worker/task. If
vhost_dev_alloc_iovecs fails, then vhost_worker_free could free
the worker/task from under vsock.

This moves vhost_worker_create to the end of vhost_dev_set_owner
where we know we can no longer fail in that path. If it fails
after the VHOST_SET_OWNER and userspace closes the device, then
the normal vsock release handling will do the right thing.

Signed-off-by: Mike Christie <michael.christie@oracle.com>
Message-Id: <20230626232307.97930-2-michael.christie@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vhost/vhost.c

@@ -572,20 +572,27 @@ long vhost_dev_set_owner(struct vhost_dev *dev)
 
 	vhost_attach_mm(dev);
 
+	err = vhost_dev_alloc_iovecs(dev);
+	if (err)
+		goto err_iovecs;
+
 	if (dev->use_worker) {
+		/*
+		 * This should be done last, because vsock can queue work
+		 * before VHOST_SET_OWNER so it simplifies the failure path
+		 * below since we don't have to worry about vsock queueing
+		 * while we free the worker.
+		 */
 		err = vhost_worker_create(dev);
 		if (err)
 			goto err_worker;
 	}
 
-	err = vhost_dev_alloc_iovecs(dev);
-	if (err)
-		goto err_iovecs;
-
 	return 0;
-err_iovecs:
-	vhost_worker_free(dev);
+
 err_worker:
+	vhost_dev_free_iovecs(dev);
+err_iovecs:
 	vhost_detach_mm(dev);
 err_mm:
 	return err;