Re: [BUG] net_device UAF: linkwatch_fire_event() calls dev_hold() after netdev_wait_allrefs() is done

[Jakub Kicinski]

Interesting..

I don't know what link_reset does, but since it turns the carrier on it
seems like something that should be flushed/canceled when the device
goes down. unregister brings the device down under rtnl_lock.

On Fri, 28 Jan 2022 02:51:24 +0100 Jann Horn wrote:
> Is the bug that usbnet_disconnect() should be stopping &dev->kevent
> before calling unregister_netdev()?

I'd say not this one, I think the generally agreed on semantics are that
the netdev is under users control between register and unregister, we
should not cripple it before unregister.

> Or is the bug that ax88179_link_reset() doesn't take some kind of lock
> and re-check that the netdev is still alive?

That'd not be an uncommon way to fix this.. taking rtnl_lock, not even 
a driver lock in similar.

> Or should netif_carrier_on() be doing that?
> Or is it the responsibility of the linkwatch code to check whether the
> netdev is already going away?

Possibly, although we don't do much in the way of defensive programming
in networking.