linux-kernel - Re: [PATCH 2/2] usb: gadget: u_serial: Fix race condition in TTY wakeup

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Message-ID: <CAKzKK0oB83B3EwX75NTFd55E0qQ=QwNpKv3hUf9Oe3ZF3AAzrQ@mail.gmail.com>
Date: Tue, 17 Jun 2025 11:31:17 +0800
From: Kuen-Han Tsai <khtsai@...gle.com>
To: Greg KH <gregkh@...uxfoundation.org>
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 2/2] usb: gadget: u_serial: Fix race condition in TTY wakeup

On Mon, Jun 16, 2025 at 10:17 PM Greg KH <gregkh@...uxfoundation.org> wrote:
>
> On Mon, Jun 16, 2025 at 09:21:47PM +0800, Kuen-Han Tsai wrote:
> > A race condition occurs when gs_start_io() calls either gs_start_rx() or
> > gs_start_tx(), as those functions briefly drop the port_lock for
> > usb_ep_queue(). This allows gs_close() and gserial_disconnect() to clear
> > port.tty and port_usb, respectively.
> >
> > Use the null-safe TTY Port helper function to wake up TTY.
> >
> > Cc: stable@...r.kernel.org
> > Fixes: 35f95fd7f234 ("TTY: usb/u_serial, use tty from tty_port")
> > Signed-off-by: Kuen-Han Tsai <khtsai@...gle.com>
> > ---
> > Explanation:
> >     CPU1:                            CPU2:
> >     gserial_connect() // lock
> >                                      gs_close() // await lock
> >     gs_start_rx()     // unlock
> >     usb_ep_queue()
> >                                      gs_close() // lock, reset port_tty and unlock
> >     gs_start_rx()     // lock
> >     tty_wakeup()      // dereference
>
> Why isn't this up in the changelog?

Thanks for the suggestion. I'll move this part up in the changelog.

>
> >
> > Stack traces:
> > [   51.494375][  T278] ttyGS1: shutdown
> > [   51.494817][  T269] android_work: sent uevent USB_STATE=DISCONNECTED
> > [   52.115792][ T1508] usb: [dm_bind] generic ttyGS1: super speed IN/ep1in OUT/ep1out
> > [   52.516288][ T1026] android_work: sent uevent USB_STATE=CONNECTED
> > [   52.551667][ T1533] gserial_connect: start ttyGS1
> > [   52.565634][ T1533] [khtsai] enter gs_start_io, ttyGS1, port->port.tty=0000000046bd4060
> > [   52.565671][ T1533] [khtsai] gs_start_rx, unlock port ttyGS1
> > [   52.591552][ T1533] [khtsai] gs_start_rx, lock port ttyGS1
> > [   52.619901][ T1533] [khtsai] gs_start_rx, unlock port ttyGS1
> > [   52.638659][ T1325] [khtsai] gs_close, lock port ttyGS1
> > [   52.656842][ T1325] gs_close: ttyGS1 (0000000046bd4060,00000000be9750a5) ...
> > [   52.683005][ T1325] [khtsai] gs_close, clear ttyGS1
> > [   52.683007][ T1325] gs_close: ttyGS1 (0000000046bd4060,00000000be9750a5) done!
> > [   52.708643][ T1325] [khtsai] gs_close, unlock port ttyGS1
> > [   52.747592][ T1533] [khtsai] gs_start_rx, lock port ttyGS1
> > [   52.747616][ T1533] [khtsai] gs_start_io, ttyGS1, going to call tty_wakeup(), port->port.tty=0000000000000000
> > [   52.747629][ T1533] Unable to handle kernel NULL pointer dereference at virtual address 00000000000001f8
>
> What is [khtsai] from?
>
> thanks,
>
> greg k-h

I added the "[khtsai]" logs for debugging and left them in the traces
because they clearly reveal the sequence of events that led to the
problem.

Regards,
Kuen-Han