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

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Message-ID: <20220425082804.209e3676@kernel.org>
Date:   Mon, 25 Apr 2022 08:28:04 -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 08:13:40 -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.

> 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.