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Date:	Fri, 4 Mar 2016 14:43:38 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andrey Konovalov <andreyknvl@...il.com>
Cc:	Oliver Neukum <oneukum@...e.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Kostya Serebryany <kcc@...gle.com>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	USB list <linux-usb@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>
Subject: Re: Possible double-free in the usbnet driver

On Fri, Mar 4, 2016 at 2:26 PM, Andrey Konovalov <andreyknvl@...il.com> wrote:
>
> and when I run the vm and connect the device I get:
>
> [   23.672662] cdc_ncm 1-1:1.6: bind() failure
> [   23.673447] usbnet_probe(): freeing netdev: ffff88006ab48000
> [   23.675822] usbnet_probe(): freeing netdev: ffff88006ab48000
>
> So this seems to be a double-free (or at least a double free_netdev()
> call), but the object gets freed twice from usbnet_probe() and not
> from usbnet_disconnect(), so you're right that the latter doesn't get
> called. I'm not sure how usbnet_probe() ends up being called twice.

I still don't think it's a double free. I think the probe thing is
called twice, and that's why to different allocations get free'd twice
(and the reason it's the same pointer is that the same allocation got
reused)

But I don't know that driver, really.

>> Which particular usbnet bind routine is this? There are multiple
>> sub-drivers for usbnet that all do different things.
>
> cdc_ncm_bind()

Ok, so that matches my theory.

Does this attached stupid patch make the warning go away? Because from
what I can tell, usbnet_link_change() really will start using that
"dev" that simply isn't valid - and will be free'd - if the bind
fails.

So you have usbnet_defer_kevent() getting triggered, which in turn
ends up using "usbnet->kevent"

But somebody like Oliver is really the right person to check this. For
example, it's entirely possible that we should just instead do

        cancel_work_sync(&dev->kevent);

before the "free_netdev(net)" in the "out1" label.

And there might be other things that bind() can have touched than the
kevent workqueue.

               Linus

View attachment "patch.diff" of type "text/plain" (851 bytes)

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