net: seeq: Fix use after free vulnerability in ether3 Driver Due to Race Condition

In the ether3_probe function, a timer is initialized with a callback
function ether3_ledoff, bound to &prev(dev)->timer. Once the timer is
started, there is a risk of a race condition if the module or device
is removed, triggering the ether3_remove function to perform cleanup.
The sequence of operations that may lead to a UAF bug is as follows:

CPU0                                    CPU1

                      |  ether3_ledoff
ether3_remove         |
  free_netdev(dev);   |
  put_devic           |
  kfree(dev);         |
 |  ether3_outw(priv(dev)->regs.config2 |= CFG2_CTRLO, REG_CONFIG2);
                      | // use dev

Fix it by ensuring that the timer is canceled before proceeding with
the cleanup in ether3_remove.

Fixes: 6fd9c53f ("net: seeq: Convert timers to use timer_setup()")
Signed-off-by: Kaixin Wang <kxwang23@m.fudan.edu.cn>
Link: https://patch.msgid.link/20240915144045.451-1-kxwang23@m.fudan.edu.cnSigned-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/ethernet/seeq/ether3.c

@@ -847,9 +847,11 @@ static void ether3_remove(struct expansion_card *ec)
 {
 	struct net_device *dev = ecard_get_drvdata(ec);
 
+	ether3_outw(priv(dev)->regs.config2 |= CFG2_CTRLO, REG_CONFIG2);
 	ecard_set_drvdata(ec, NULL);
 
 	unregister_netdev(dev);
+	del_timer_sync(&priv(dev)->timer);
 	free_netdev(dev);
 	ecard_release_resources(ec);
 }