usbnet: fix memory leak in error case

usbnet_write_cmd_async() mixed up which buffers
need to be freed in which error case.

v2: add Fixes tag
v3: fix uninitialized buf pointer

Fixes: 877bd862 ("usbnet: introduce usbnet 3 command helpers")
Signed-off-by: Oliver Neukum <oneukum@suse.com>
Link: https://lore.kernel.org/r/20220705125351.17309-1-oneukum@suse.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/usb/usbnet.c

@@ -2137,7 +2137,7 @@ static void usbnet_async_cmd_cb(struct urb *urb)
 int usbnet_write_cmd_async(struct usbnet *dev, u8 cmd, u8 reqtype,
 			   u16 value, u16 index, const void *data, u16 size)
 {
-	struct usb_ctrlrequest *req = NULL;
+	struct usb_ctrlrequest *req;
 	struct urb *urb;
 	int err = -ENOMEM;
 	void *buf = NULL;
@@ -2155,7 +2155,7 @@ int usbnet_write_cmd_async(struct usbnet *dev, u8 cmd, u8 reqtype,
 		if (!buf) {
 			netdev_err(dev->net, "Error allocating buffer"
 				   " in %s!\n", __func__);
-			goto fail_free;
+			goto fail_free_urb;
 		}
 	}
 
@@ -2179,14 +2179,21 @@ int usbnet_write_cmd_async(struct usbnet *dev, u8 cmd, u8 reqtype,
 	if (err < 0) {
 		netdev_err(dev->net, "Error submitting the control"
 			   " message: status=%d\n", err);
-		goto fail_free;
+		goto fail_free_all;
 	}
 	return 0;
 
+fail_free_all:
+	kfree(req);
 fail_free_buf:
 	kfree(buf);
-fail_free:
-	kfree(req);
+	/*
+	 * avoid a double free
+	 * needed because the flag can be set only
+	 * after filling the URB
+	 */
+	urb->transfer_flags = 0;
+fail_free_urb:
 	usb_free_urb(urb);
 fail:
 	return err;