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

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Message-ID: <CAKzKK0q5xRpR9hW=j-Hj6LNjogPK6jELRqB0Ob+VF6TbSve4bw@mail.gmail.com>
Date: Thu, 11 Sep 2025 17:09:13 +0800
From: Kuen-Han Tsai <khtsai@...gle.com>
To: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc: Greg KH <gregkh@...uxfoundation.org>, 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

Hi Greg, Krzysztof,

On Thu, Sep 11, 2025 at 4:49 PM Krzysztof Kozlowski
<krzysztof.kozlowski@...aro.org> wrote:
>
> 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.

I really appreciate you taking the time to explain this. I see your
point clearly now: my change could potentially mask the real bug
rather than fixing it.

I'll rework the patch to use the __free() helpers for automatic cleanup.

Regards,
Kuen-Han