netdev - Re: [PATCH] usbnet: Fix use-after-free on disconnect

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Date:   Thu, 14 Apr 2022 12:58:58 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     Oliver Neukum <oneukum@...e.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, Jann Horn <jannh@...gle.com>,
        Oleksij Rempel <o.rempel@...gutronix.de>,
        netdev@...r.kernel.org, linux-usb@...r.kernel.org,
        Andrew Lunn <andrew@...n.ch>,
        Eric Dumazet <edumazet@...gle.com>,
        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>
Subject: Re: [PATCH] usbnet: Fix use-after-free on disconnect

On Wed, Apr 13, 2022 at 08:59:48PM +0200, Oliver Neukum wrote:
> On 13.04.22 16:16, Lukas Wunner wrote:
> > Jann Horn reports a use-after-free on disconnect of a USB Ethernet
> > (ax88179_178a.c).  Oleksij Rempel has witnessed the same issue with a
> > different driver (ax88172a.c).
> 
> I see. Very good catch
> 
> > --- a/drivers/net/usb/usbnet.c
> > +++ b/drivers/net/usb/usbnet.c
> > @@ -469,6 +469,9 @@ static enum skb_state defer_bh(struct usbnet *dev, struct sk_buff *skb,
> >   */
> >  void usbnet_defer_kevent (struct usbnet *dev, int work)
> >  {
> > +	if (dev->intf->condition == USB_INTERFACE_UNBINDING)
> > +		return;
> 
> But, no, you cannot do this. This is a very blatant layering violation.
> You cannot use states internal to usb core like that in a driver.

Why do you think it's internal?

enum usb_interface_condition is defined in include/linux/usb.h
for everyone to see and use.  If it was meant to be private,
I'd expect it to be marked as such or live in drivers/usb/core/usb.h.

Adding Greg to clarify.


> I see two options.
> 1. A dedicated flag in usbnet (then please with the correct smp barriers)
> 2. You introduce an API to usb core to query this.

I'd definitely prefer option 2 as I'd hate to duplicate functionality.

What do you have in mind?  A simple accessor to return intf->condition
or something like usb_interface_unbinding() which returns a bool?

Thanks,

Lukas