netdev - Re: use-after-free in usbnet

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Message-ID: <CACVXFVP+U1k7JFTmbabF-k8F3bO9zc58c3tLG6=1nQPcrR9p1g@mail.gmail.com>
Date:	Wed, 21 Mar 2012 23:02:18 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	netdev@...r.kernel.org, linux-usb@...r.kernel.org,
	Fedora Kernel Team <kernel-team@...oraproject.org>,
	Dave Jones <davej@...hat.com>
Subject: Re: use-after-free in usbnet

On Wed, Mar 21, 2012 at 10:35 PM, Alan Stern <stern@...land.harvard.edu> wrote:
> On Wed, 21 Mar 2012, Ming Lei wrote:
>
>> Looks it is a general issue about usb_hcd_unlink_urb.
>>
>> Alan, how about increasing URB reference count before calling unlink1
>> inside usb_hcd_unlink_urb to fix this kind of problem?
>
> No, that won't fix the problem.  The URB could complete and be
> deallocated even before usb_hcd_unlink_urb() is called, so nothing that
> function can do will prevent the error.

IMO, driver should not call usb_hcd_unlink_urb after urb is freed from
the driver,
but this problem is that URB may be freed during usb_hcd_unlink_urb.

In fact, it is allowed that usb_free_urb is called inside .complete handler,
at least as said in Documentation/URB.txt:

         "You may free an urb that you've submitted,..."

So looks reasonable to increase the URB reference count before calling
unlink1(), just like that done inside usb_hcd_flush_endpoint().  And I
think it is a general solution for avoiding this kind of problem.

>
> It is the caller's responsibility to make sure that the URB does not
> get freed before usb_unlink_urb() or usb_kill_urb() returns.  We
> probably should mention this in the kerneldoc...

If so, looks it is a bit contrary with Documentation/URB.txt, also
this may add extra constraint(maybe unnecessary) to the driver.

Thanks,
-- 
Ming Lei
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