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

[Jann Horn]

On Fri, Jan 28, 2022 at 3:19 AM Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
> 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.

Ah, I found a comment with a bit of explanation on how this is
supposed to work... usbnet_stop() explains:

    /* deferred work (task, timer, softirq) must also stop.
     * can't flush_scheduled_work() until we drop rtnl (later),
     * else workers could deadlock; so make workers a NOP.
     */

And usbnet_stop() is ->ndo_stop(), which indeed runs under RTNL.

I wonder what the work items can do that'd conflict with RTNL... or is
the comment just talking about potential issues if a bunch of *other*
work items need RTNL and clog up the system_wq so that
flush_scheduled_work() blocks forever?

If it's the latter case, I guess we could instead do cancel_work_sync() and
then maybe re-run the work function's handler one more time
synchronously?