netdev - Re: use-after-free in usbnet

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Date:	Sat, 21 Apr 2012 15:06:24 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Huajun Li <huajun.li.lee@...il.com>
Cc:	Oliver Neukum <oneukum@...e.de>,
	Alan Stern <stern@...land.harvard.edu>,
	Dave Jones <davej@...hat.com>, netdev@...r.kernel.org,
	linux-usb@...r.kernel.org,
	Fedora Kernel Team <kernel-team@...oraproject.org>
Subject: Re: use-after-free in usbnet

Hi Huajun,

On Sat, Apr 21, 2012 at 2:39 PM, Huajun Li <huajun.li.lee@...il.com> wrote:

> Hi Ming,
>     That's why my patch uses skb_queue_walk() to traverse the queue,
> it guarantees the skb available in each loop.  Is this what you
> expected?
>
>     The main idea of my patch(it is based on current mainline: 3.4.0-rc3) is:
>     1. If the skb in txq/rxq, then it must be available,
> unlink_urbs() can refer to it safely while it holds  q->lock;
>     2. If the skb in txq/rxq and its state is
> rx_done/tx_done/rx_cleanup, that means the skb's URB is complete, then
> don't need to unlink it again;

As I said before, at least current code is not mistaken, since unlink
can handle the case correctly.

>     3. Before releasing  q->lock in unlink_urbs(), it will increase
> the URB's refercount, so even the related skb is freed in future, the
> URB is still available.

No, increasing URB's reference count does not prevent the referenced skb
from being freed, see usbnet_bh(), so 'tmp = skb->next' in skb_queue_walk_safe
still may reference a freed pointer.

>> As far as I can think of, we can hold lock of done queue to forbid skb free
>> during unlinking. The below patch may fix the problem, are you OK
>> with it?
>
> Just skip trying this per your following email's comments.

I will prepare a new patch later, if you'd like to try it.


Thanks
-- 
Ming Lei
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