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

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Message-ID: <CAG48ez3ibQjhs9Qxb0AAKE4-UZiZ5UdXG1JWcPWHAWBoO-1fVw@mail.gmail.com>
Date:   Mon, 25 Apr 2022 16:49:34 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     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>,
        Eric Dumazet <edumazet@...gle.com>, netdev@...r.kernel.org,
        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, Apr 25, 2022 at 4:41 PM Jakub Kicinski <kuba@...nel.org> wrote:
> On Sat, 23 Apr 2022 18:07:23 +0200 Lukas Wunner wrote:
> > > Looking at the original report it looks like the issue could be
> > > resolved with a more usb-specific change: e.g. it looks like
> > > usbnet_defer_kevent() is not acquiring a dev reference as it should.
> > >
> > > Have you considered that path?
> >
> > First of all, the diffstat of the patch shows this is an opportunity
> > to reduce LoC as well as simplify and speed up device teardown.
> >
> > Second, the approach you're proposing won't work if a driver calls
> > netif_carrier_on/off() after unregister_netdev().
> >
> > It seems prudent to prevent such a misbehavior in *any* driver,
> > not just usbnet.  usbnet may not be the only one doing it wrong.
> > Jann pointed out that there are more syzbot reports related
> > to a UAF in linkwatch:
> >
> > https://lore.kernel.org/netdev/?q=__linkwatch_run_queue+syzbot
> >
> > Third, I think an API which schedules work, invisibly to the driver,
> > is dangerous and misguided.  If it is illegal to call
> > netif_carrier_on/off() for an unregistered but not yet freed netdev,
> > catch that in core networking code and don't expect drivers to respect
> > a rule which isn't even documented.
>
> Doesn't mean we should make it legal. We can add a warning to catch
> abuses.

That was the idea with
https://lore.kernel.org/netdev/20220128014303.2334568-1-jannh@google.com/,
but I didn't get any replies when I asked what the precise semantics
of dev_hold() are supposed to be
(https://lore.kernel.org/netdev/CAG48ez1-OyZETvrYAfaHicYW1LbrQUVp=C0EukSWqZrYMej73w@mail.gmail.com/),
so I don't know how to proceed...