linux-kernel - Re: [PATCH 1/2] Revert "usb: gadget: u_serial: Add null pointer check in gs_start

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Message-ID: <2025061642-likeness-heaving-dd75@gregkh>
Date: Mon, 16 Jun 2025 16:18:13 +0200
From: Greg KH <gregkh@...uxfoundation.org>
To: Kuen-Han Tsai <khtsai@...gle.com>
Cc: prashanth.k@....qualcomm.com, hulianqin@...o.com,
	krzysztof.kozlowski@...aro.org, mwalle@...nel.org,
	jirislaby@...nel.org, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 1/2] Revert "usb: gadget: u_serial: Add null pointer
 check in gs_start_io"

On Mon, Jun 16, 2025 at 09:21:46PM +0800, Kuen-Han Tsai wrote:
> This reverts commit ffd603f214237e250271162a5b325c6199a65382.
> 
> Commit ffd603f21423 ("usb: gadget: u_serial: Add null pointer check in
> gs_start_io") adds null pointer checks at the beginning of the
> gs_start_io() function to prevent a null pointer dereference. However,
> these checks are redundant because the function's comment already
> requires callers to hold the port_lock and ensure port.tty and port_usb
> are not null. All existing callers already follow these rules.
> 
> The true cause of the null pointer dereference is a race condition. When
> gs_start_io() calls either gs_start_rx() or gs_start_tx(), the port_lock
> is temporarily released for usb_ep_queue(). This allows port.tty and
> port_usb to be cleared.
> 
> Cc: stable@...r.kernel.org
> Fixes: ffd603f21423 ("usb: gadget: u_serial: Add null pointer check in gs_start_io")

As this is removing unneeded checks, why is it cc: stable?  What bug is
being resolved here?

confused,

greg k-h