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

[Oleksij Rempel]

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@xxxxxxxxxx> wrote:
> > On Sat, Apr 09, 2022 at 08:09:00PM +0800, Dongliang Mu wrote:
> > > From: Dongliang Mu <mudongliangabcd@xxxxxxxxx>
> > >
> > > 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@xxxxxxxx/T/

Regards,
Oleksij
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