mctp: serial: Cancel pending work from ndo_uninit handler

We cannot do the cancel_work_sync from after the unregister_netdev, as
the dev pointer is no longer valid, causing a uaf on ldisc unregister
(or device close).

Instead, do the cancel_work_sync from the ndo_uninit op, where the dev
still exists, but the queue has stopped.

Fixes: 7bd9890f ("mctp: serial: cancel tx work on ldisc close")
Reported-by: Luo Likang <luolikang@nsfocus.com>
Tested-by: Luo Likang <luolikang@nsfocus.com>
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Link: https://lore.kernel.org/r/20220211011552.1861886-1-jk@codeconstruct.com.auSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/mctp/mctp-serial.c

@@ -403,8 +403,16 @@ static void mctp_serial_tty_receive_buf(struct tty_struct *tty,
 		mctp_serial_push(dev, c[i]);
 }
 
+static void mctp_serial_uninit(struct net_device *ndev)
+{
+	struct mctp_serial *dev = netdev_priv(ndev);
+
+	cancel_work_sync(&dev->tx_work);
+}
+
 static const struct net_device_ops mctp_serial_netdev_ops = {
 	.ndo_start_xmit = mctp_serial_tx,
+	.ndo_uninit = mctp_serial_uninit,
 };
 
 static void mctp_serial_setup(struct net_device *ndev)
@@ -483,7 +491,6 @@ static void mctp_serial_close(struct tty_struct *tty)
 	int idx = dev->idx;
 
 	unregister_netdev(dev->netdev);
-	cancel_work_sync(&dev->tx_work);
 	ida_free(&mctp_serial_ida, idx);
 }