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: 6fd9c53f7186 ("net: seeq: Convert timers to use timer_setup()")
Signed-off-by: Kaixin Wang <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

drivers/net/ethernet/seeq/ether3.c

···
 {
 	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);
 }