The root cause of this issue are:
1. When probing the usbnet device, executing usbnet_link_change(dev, 0, 0);
put the kevent work in global workqueue. However, the kevent has not yet
been scheduled when the usbnet device is unregistered. Therefore, executing
free_netdev() results in the "free active object (kevent)" error reported
here.

2. Another factor is that when calling usbnet_disconnect()->unregister_netdev(),
if the usbnet device is up, ndo_stop() is executed to cancel the kevent.
However, because the device is not up, ndo_stop() is not executed.

The solution to this problem is to cancel the kevent before executing
free_netdev(), which also deletes the delay timer.

Fixes: a69e617e533e ("usbnet: Fix linkwatch use-after-free on disconnect")
Reported-by: Sam Sun <samsun1006219@gmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=8bfd7bcc98f7300afb84
Signed-off-by: Lizhi Xu <lizhi.xu@windriver.com>
---
V1 -> V2: update comments for typos
V2 -> V3: add fixes tag

 drivers/net/usb/usbnet.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
index bf01f2728531..f0294f0e6612 100644
--- a/drivers/net/usb/usbnet.c
+++ b/drivers/net/usb/usbnet.c
@@ -1672,6 +1672,9 @@ void usbnet_disconnect (struct usb_interface *intf)
 	usb_free_urb(dev->interrupt);
 	kfree(dev->padding_pkt);
 
+	cancel_work_sync(&dev->kevent);
+	timer_delete_sync(&dev->delay);
+
 	free_netdev(net);
 }
 EXPORT_SYMBOL_GPL(usbnet_disconnect);
-- 
2.43.0

On Sat, 18 Oct 2025 22:49:40 +0800 Lizhi Xu wrote:
> --- a/drivers/net/usb/usbnet.c
> +++ b/drivers/net/usb/usbnet.c
> @@ -1672,6 +1672,9 @@ void usbnet_disconnect (struct usb_interface *intf)
>  	usb_free_urb(dev->interrupt);
>  	kfree(dev->padding_pkt);
>  
> +	cancel_work_sync(&dev->kevent);
> +	timer_delete_sync(&dev->delay);
> +
>  	free_netdev(net);

Is this the best spot to place the cancel?
I think it may be better right after unregister_netdev().
I haven't analyze this driver too closely but for example since
kevent may call the sub-driver having it running after we already
called dev->driver_info->unbind() seems risky.
-- 
pw-bot: cr

On Tue, 21 Oct 2025 18:32:52 -0700, Jakub Kicinski wrote:
> > --- a/drivers/net/usb/usbnet.c
> > +++ b/drivers/net/usb/usbnet.c
> > @@ -1672,6 +1672,9 @@ void usbnet_disconnect (struct usb_interface *intf)
> >  	usb_free_urb(dev->interrupt);
> >  	kfree(dev->padding_pkt);
> >  
> > +	cancel_work_sync(&dev->kevent);
> > +	timer_delete_sync(&dev->delay);
> > +
> >  	free_netdev(net);
> 
> Is this the best spot to place the cancel?
> I think it may be better right after unregister_netdev().
> I haven't analyze this driver too closely but for example since
> kevent may call the sub-driver having it running after we already
> called dev->driver_info->unbind() seems risky.
I agree with your analysis. I'll update the patch and release a new version.

BR,
Lizhi