vhost_vdpa: assign irq bypass producer token correctly

We used to call irq_bypass_unregister_producer() in
vhost_vdpa_setup_vq_irq() which is problematic as we don't know if the
token pointer is still valid or not.

Actually, we use the eventfd_ctx as the token so the life cycle of the
token should be bound to the VHOST_SET_VRING_CALL instead of
vhost_vdpa_setup_vq_irq() which could be called by set_status().

Fixing this by setting up irq bypass producer's token when handling
VHOST_SET_VRING_CALL and un-registering the producer before calling
vhost_vring_ioctl() to prevent a possible use after free as eventfd
could have been released in vhost_vring_ioctl(). And such registering
and unregistering will only be done if DRIVER_OK is set.

Reported-by: Dragos Tatulea <[e-mail]>
Tested-by: Dragos Tatulea <[e-mail]>
Reviewed-by: Dragos Tatulea <[e-mail]>
Fixes: 2cf1ba9a4d15 ("vhost_vdpa: implement IRQ offloading in vhost_vdpa")
Signed-off-by: Jason Wang <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Michael S. Tsirkin <[e-mail]>

drivers/vhost/vdpa.c

···
 	if (irq < 0)
 		return;
 
-	irq_bypass_unregister_producer(&vq->call_ctx.producer);
 	if (!vq->call_ctx.ctx)
 		return;
 
-	vq->call_ctx.producer.token = vq->call_ctx.ctx;
 	vq->call_ctx.producer.irq = irq;
 	ret = irq_bypass_register_producer(&vq->call_ctx.producer);
 	if (unlikely(ret))
···
 			vq->last_avail_idx = vq_state.split.avail_index;
 		}
 		break;
+	case VHOST_SET_VRING_CALL:
+		if (vq->call_ctx.ctx) {
+			if (ops->get_status(vdpa) &
+			    VIRTIO_CONFIG_S_DRIVER_OK)
+				vhost_vdpa_unsetup_vq_irq(v, idx);
+			vq->call_ctx.producer.token = NULL;
+		}
+		break;
 	}
 
 	r = vhost_vring_ioctl(&v->vdev, cmd, argp);
···
 			cb.callback = vhost_vdpa_virtqueue_cb;
 			cb.private = vq;
 			cb.trigger = vq->call_ctx.ctx;
+			vq->call_ctx.producer.token = vq->call_ctx.ctx;
+			if (ops->get_status(vdpa) &
+			    VIRTIO_CONFIG_S_DRIVER_OK)
+				vhost_vdpa_setup_vq_irq(v, idx);
 		} else {
 			cb.callback = NULL;
 			cb.private = NULL;
 			cb.trigger = NULL;
 		}
 		ops->set_vq_cb(vdpa, idx, &cb);
-		vhost_vdpa_setup_vq_irq(v, idx);
 		break;
 
 	case VHOST_SET_VRING_NUM:
···
 	for (i = 0; i < nvqs; i++) {
 		vqs[i] = &v->vqs[i];
 		vqs[i]->handle_kick = handle_vq_kick;
+		vqs[i]->call_ctx.ctx = NULL;
 	}
 	vhost_dev_init(dev, vqs, nvqs, 0, 0, 0, false,
 		       vhost_vdpa_process_iotlb_msg);