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

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Date:   Mon, 25 Apr 2022 14:39:18 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
        Oliver Neukum <oneukum@...e.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jann Horn <jannh@...gle.com>,
        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, Apr 25, 2022 at 2:18 PM Lukas Wunner <lukas@...ner.de> wrote:
>
> On Mon, Apr 25, 2022 at 07:41:46AM -0700, Jakub Kicinski 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 would be inconsequent, considering that netif_carrier_on/off()
> do not warn for a reg_state of NETREG_UNINITIALIZED.
>

Yes, only 1500 calls to audit ;)

I guess we could start adding WARN_ON_ONCE(), then wait for a few
syzbot/users reports to fix offenders...

commit b47300168e770b60ab96c8924854c3b0eb4260eb
Author: David S. Miller <davem@...emloft.net>
Date:   Wed Nov 19 15:33:54 2008 -0800

    net: Do not fire linkwatch events until the device is registered.

    Several device drivers try to do things like netif_carrier_off()
    before register_netdev() is invoked.  This is bogus, but too many
    drivers do this to fix them all up in one go.

    Reported-by: Folkert van Heusden <folkert@...heusden.com>
    Signed-off-by: David S. Miller <davem@...emloft.net>