linux-kernel - Re: [PATCH] usb: gadget: f_ncm: Fix NPE in ncm

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <d37497c1-904e-4a04-a300-a60a21bcc212@linaro.org>
Date: Thu, 11 Sep 2025 10:49:45 +0200
From: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To: Greg KH <gregkh@...uxfoundation.org>, Kuen-Han Tsai <khtsai@...gle.com>
Cc: zack.rusin@...adcom.com, namcao@...utronix.de, linux-usb@...r.kernel.org,
 linux-kernel@...r.kernel.org, stable@...nel.org
Subject: Re: [PATCH] usb: gadget: f_ncm: Fix NPE in ncm_bind error path

On 11/09/2025 10:35, Greg KH wrote:
> On Thu, Sep 11, 2025 at 02:50:15PM +0800, Kuen-Han Tsai wrote:
>> Hi Greg,
>>
>> On Sat, Sep 6, 2025 at 8:15 PM Greg KH <gregkh@...uxfoundation.org> wrote:
>>>
>>> On Thu, Sep 04, 2025 at 07:46:13PM +0800, Kuen-Han Tsai wrote:
>>>> When an ncm_bind/unbind cycle occurs, the ncm->notify_req pointer is
>>>> left pointing to a stale address. If a subsequent call to ncm_bind()
>>>> fails to allocate the endpoints, the function jumps to the unified error
>>>> label. The cleanup code sees the stale ncm->notify_req pointer and calls
>>>> usb_ep_free_request().
>>>>
>>>> This results in a NPE because it attempts to access the free_request
>>>> function through the endpoint's operations table (ep->ops), which is
>>>> NULL.
>>>>
>>>> Refactor the error path to use cascading goto labels, ensuring that
>>>> resources are freed in reverse order of allocation. Besides, explicitly
>>>> set pointers to NULL after freeing them.
>>>
>>> Why must the pointers be set to NULL?  What is checking and requiring
>>> that?
>>
>> I set them to null as a standard safety measure to prevent potential
>> use-after-free issues. I can remove it if you prefer.
> 
> So either you have a use-after-free, or a NULL crash, either way it's
> bad and the real bug should be fixed if this can happen.  If it can not
> happen, then there is no need to set this to NULL.


... or there is a second (wrong) free somewhere else, which would crash
and this NULL prevents it. In that case there is a real bug which,
instead of being solved, is being hidden by this NULL assignment making
it even more difficult to find and fix later. :(

Usually that's the case I saw when people null-ify pointer after free.

Best regards,
Krzysztof