vhost: fix error handling in RESET_OWNER ioctl

RESET_OWNER ioctl would leave the fd in a bad state if
memory allocation failed: device is stopped
but owner is not reset. Make state changes
after allocating memory, such that a failed
ioctl has no effect.

Signed-off-by: Michael S. Tsirkin <[e-mail]>

drivers/vhost/net.c

···
 	struct socket *tx_sock = NULL;
 	struct socket *rx_sock = NULL;
 	long err;
+	struct vhost_memory *memory;
 
 	mutex_lock(&n->dev.mutex);
 	err = vhost_dev_check_owner(&n->dev);
 	if (err)
 		goto done;
+	memory = vhost_dev_reset_owner_prepare();
+	if (!memory) {
+		err = -ENOMEM;
+		goto done;
+	}
 	vhost_net_stop(n, &tx_sock, &rx_sock);
 	vhost_net_flush(n);
-	err = vhost_dev_reset_owner(&n->dev);
+	vhost_dev_reset_owner(&n->dev, memory);
 	vhost_net_vq_reset(n);
 done:
 	mutex_unlock(&n->dev.mutex);

drivers/vhost/test.c

···
 {
 	void *priv = NULL;
 	long err;
+	struct vhost_memory *memory;
+
 	mutex_lock(&n->dev.mutex);
 	err = vhost_dev_check_owner(&n->dev);
 	if (err)
 		goto done;
+	memory = vhost_dev_reset_owner_prepare();
+	if (!memory) {
+		err = -ENOMEM;
+		goto done;
+	}
 	vhost_test_stop(n, &priv);
 	vhost_test_flush(n);
-	err = vhost_dev_reset_owner(&n->dev);
+	vhost_dev_reset_owner(&n->dev, memory);
 done:
 	mutex_unlock(&n->dev.mutex);
 	return err;

drivers/vhost/vhost.c

···
 	return err;
 }
 
-/* Caller should have device mutex */
-long vhost_dev_reset_owner(struct vhost_dev *dev)
+struct vhost_memory *vhost_dev_reset_owner_prepare(void)
 {
-	struct vhost_memory *memory;
+	return kmalloc(offsetof(struct vhost_memory, regions), GFP_KERNEL);
+}
 
-	/* Restore memory to default empty mapping. */
-	memory = kmalloc(offsetof(struct vhost_memory, regions), GFP_KERNEL);
-	if (!memory)
-		return -ENOMEM;
-
+/* Caller should have device mutex */
+void vhost_dev_reset_owner(struct vhost_dev *dev, struct vhost_memory *memory)
+{
 	vhost_dev_cleanup(dev, true);
 
+	/* Restore memory to default empty mapping. */
 	memory->nregions = 0;
 	RCU_INIT_POINTER(dev->memory, memory);
-	return 0;
 }
 
 void vhost_dev_stop(struct vhost_dev *dev)

drivers/vhost/vhost.h

···
 
 long vhost_dev_init(struct vhost_dev *, struct vhost_virtqueue **vqs, int nvqs);
 long vhost_dev_check_owner(struct vhost_dev *);
-long vhost_dev_reset_owner(struct vhost_dev *);
+struct vhost_memory *vhost_dev_reset_owner_prepare(void);
+void vhost_dev_reset_owner(struct vhost_dev *, struct vhost_memory *);
 void vhost_dev_cleanup(struct vhost_dev *, bool locked);
 void vhost_dev_stop(struct vhost_dev *);
 long vhost_dev_ioctl(struct vhost_dev *, unsigned int ioctl, void __user *argp);