linux-kernel - Re: [PATCH] udc: Memory leak on error path and use after free

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Date:   Tue, 22 Aug 2017 18:45:29 +0300
From:   Anton Vasilyev <vasilyev@...ras.ru>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Felipe Balbi <balbi@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jussi Kivilinna <jussi.kivilinna@...tian.com>,
        Peter Senna Tschudin <peter.senna@...labora.com>,
        Raz Manor <Raz.Manor@...ens.com>,
        Romain Perier <romain.perier@...labora.com>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        ldv-project@...uxtesting.org
Subject: Re: [PATCH] udc: Memory leak on error path and use after free


Sorry for delayed reply.

On 16.08.2017 19:35, Alan Stern wrote:
> On Wed, 16 Aug 2017, Anton Vasilyev wrote:
> 
>> On 16.08.2017 18:29, Alan Stern wrote:
>>> On Wed, 16 Aug 2017, Anton Vasilyev wrote:
>>>
>>>> gadget_release() is responsible for cleanup dev memory.
>>>> But if net2280_probe() fails after dev allocation, then
>>>> gadget_release() become unregistered and dev memory leaks.
>>>
>>> This isn't needed if usb_add_gadget_udc_release() is fixed, right?
>>>
>>
>> No, this situation could appear before call
>> usb_add_gadget_udc_release().
>>
>>>> Also net2280_remove() calls usb_del_gadget_udc() which
>>>> perform schedule_delayed_work() with gadget_release(), so
>>>> it is possible that dev will be deallocated exactly after
>>>> this call and leads to use after free.
>>>
>>> Where is there a possible use after free?
>>>
>>
>> net2280_remove() continue work with struct net2280 *dev after call
>> usb_del_gadget_udc(&dev->gadget), but this net2280 *dev could be
>> deallocated by gadget_release()
>>
>>>> The patch moves deallocation from gadget_release() to
>>>> net2280_remove().
>>>
>>> Alan Stern
> 
> Okay, now I understand what you were saying.  Yes, I agree, the
> existing code isn't right.
> 
> But a better solution would be to move the usb_del_gadget_udc() call
> from the beginning of net2280_remove() to the end.  And make the call
> conditional, depending on whether usb_add_gadget_udc_release() has
> already been called successfully.

If allow gadget_release() to deallocate net2280 *dev then it will be 
called on fail of usb_add_gadget_udc_release() and it will be unsafe to 
perform clean-up.
My point is that gadget shouldn't deallocate its parent memory at all.

> 
> The point is that the device core does not allow drivers to deallocate
> memory containing a struct device before the ->release callback has
> been invoked.  Your patch might do that, if the release was delayed for
> some reason.

I don't see possibility for parent device to be removed before its child 
was released. Please point if I'm wrong.

Alternative way to move allocation under devm interface.

> 
> Alan Stern
> 

-- 
Anton Vasilyev
Linux Verification Center, ISPRAS
web: http://linuxtesting.org
e-mail: vasilyev@...ras.ru