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

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Date:   Mon, 25 Apr 2022 08:36:09 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Jann Horn <jannh@...gle.com>, Lukas Wunner <lukas@...ner.de>,
        Paolo Abeni <pabeni@...hat.com>,
        Oliver Neukum <oneukum@...e.com>,
        "David S. Miller" <davem@...emloft.net>,
        Oleksij Rempel <o.rempel@...gutronix.de>,
        netdev <netdev@...r.kernel.org>,
        USB list <linux-usb@...r.kernel.org>,
        Andrew Lunn <andrew@...n.ch>,
        Jacky Chou <jackychou@...x.com.tw>, Willy Tarreau <w@....eu>,
        Lino Sanfilippo <LinoSanfilippo@....de>,
        Philipp Rosenberger <p.rosenberger@...bus.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] net: linkwatch: ignore events for unregistered netdevs

On Mon, 25 Apr 2022 08:31:23 -0700 Eric Dumazet wrote:
> > 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)

So we'd need something like

WARN_ON(dev->reg_state != NETREG_REGISTERED && !rtnl_held())

in dev_hold()?