Re: [PATCH] net: linkwatch: ignore events for unregistered netdevs

[Eric Dumazet <edumazet@google.com>]

On Mon, Apr 25, 2022 at 8:28 AM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Mon, 25 Apr 2022 08:13:40 -0700 Eric Dumazet wrote:
> > dev_hold() has been an increment of a refcount, and dev_put() a decrement.
> >
> > Not sure why it is fundamentally broken.
>
> Jann described a case where someone does
>
>     CPU 0      CPU 1     CPU 2
>
>   dev_hold()
>    ------  #unregister -------
>              dev_hold()
>                          dev_put()
>
> Our check for refcount == 0 goes over the CPUs one by one,
> so if it sums up CPUs 0 and 1 at the "unregister" point above
> and CPU2 after the CPU1 hold and CPU2 release it will "miss"
> one refcount.
>
> That's a problem unless doing a dev_hold() on a netdev we only have
> a reference on is illegal.

What is 'illegal' is trying to keep using the device after #unregister.

We have barriers to prevent that.

Somehow a layer does not care about the barriers and pretends the
device is still good to use.

It is of course perfectly fine to stack multiple dev_hold() from one
path (if these do not leak, but this is a different issue)

>
> > There are specific steps at device dismantles making sure no more
> > users can dev_hold()
> >
> > It is a contract. Any buggy layer can overwrite any piece of memory,
> > including a refcount_t.
> >
> > Traditionally we could not add a test in dev_hold() to prevent an
> > increment if the device is in dismantle phase.
> > Maybe the situation is better nowadays.


>