Re: [PATCH] driver: usb: nullify dangling pointer in cdc_ncm_free

From: Oleksij Rempel <o.rempel@pengutronix.de>
To: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Johan Hovold <johan@kernel.org>,
	Oleksij Rempel <linux@rempel-privat.de>,
	Dongliang Mu <dzm91@hust.edu.cn>,
	Oliver Neukum <oliver@neukum.org>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	Dongliang Mu <mudongliangabcd@gmail.com>,
	syzbot+eabbf2aaa999cc507108@syzkaller.appspotmail.com,
	USB <linux-usb@vger.kernel.org>, netdev <netdev@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] driver: usb: nullify dangling pointer in cdc_ncm_free
Date: Fri, 15 Apr 2022 09:19:39 +0200	[thread overview]
Message-ID: <20220415071939.GB27951@pengutronix.de> (raw)
In-Reply-To: <CAHp75VeTqmdLhavZ+VbBYSFMDHr0FG4iKFGdbzE-wo5MCNikAA@mail.gmail.com>

On Thu, Apr 14, 2022 at 06:01:57PM +0300, Andy Shevchenko wrote:
> On Mon, Apr 11, 2022 at 9:33 PM Johan Hovold <johan@kernel.org> wrote:
> > On Sat, Apr 09, 2022 at 08:09:00PM +0800, Dongliang Mu wrote:
> > > From: Dongliang Mu <mudongliangabcd@gmail.com>
> > >
> > > cdc_ncm_bind calls cdc_ncm_bind_common and sets dev->data[0]
> > > with ctx. However, in the unbind function - cdc_ncm_unbind,
> > > it calls cdc_ncm_free and frees ctx, leaving dev->data[0] as
> > > a dangling pointer. The following ioctl operation will trigger
> > > the UAF in the function cdc_ncm_set_dgram_size.
> > >
> > > Fix this by setting dev->data[0] as zero.
> >
> > This sounds like a poor band-aid. Please explain how this prevent the
> > ioctl() from racing with unbind().
> 
> Good question. Isn't it the commit 2c9d6c2b871d ("usbnet: run unbind()
> before unregister_netdev()") which changed the ordering of the
> interface shutdown and basically makes this race happen? I don't see
> how we can guarantee that IOCTL won't be called until we quiescence
> the network device — my understanding that on device surprise removal
> we have to first shutdown what it created and then unbind the device.
> If I understand the original issue correctly then the problem is in
> usbnet->unbind and it should actually be split to two hooks, otherwise
> it seems every possible IOCTL callback must have some kind of
> reference counting and keep an eye on the surprise removal.
> 
> Johan, can you correct me if my understanding is wrong?

Hi,

the possible fix for this issue is under discussion here:
https://lore.kernel.org/netdev/d13e3a34-7e85-92dd-d0c0-5efb3fb08182@suse.com/T/

Regards,
Oleksij
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