linux-kernel - Re: [PATCH v2 2/2] usb: gadget: u_serial: Fix race condition in TTY wakeup

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Message-ID: <CAKzKK0p0fx4bsqVVPWjJQxG0sEHee0b0OPE7dqCb7cbW7+XkgA@mail.gmail.com>
Date: Wed, 18 Jun 2025 22:44:40 +0800
From: Kuen-Han Tsai <khtsai@...gle.com>
To: Prashanth K <prashanth.k@....qualcomm.com>
Cc: gregkh@...uxfoundation.org, 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 v2 2/2] usb: gadget: u_serial: Fix race condition in TTY wakeup

On Wed, Jun 18, 2025 at 8:26 PM Prashanth K
<prashanth.k@....qualcomm.com> wrote:
>
>
>
> On 6/17/2025 10:37 AM, 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.
> >
> > Example
> >   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()      // NPE
> >
> > Fixes: 35f95fd7f234 ("TTY: usb/u_serial, use tty from tty_port")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Kuen-Han Tsai <khtsai@...gle.com>
> > ---
>
> Reviewed-by: Prashanth K <prashanth.k@....qualcomm.com>
>
> > v2:
> > - Move the example up to the changelog
> >
> > 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
> > ---
> >  drivers/usb/gadget/function/u_serial.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/usb/gadget/function/u_serial.c b/drivers/usb/gadget/function/u_serial.c
> > index c043bdc30d8a..540dc5ab96fc 100644
> > --- a/drivers/usb/gadget/function/u_serial.c
> > +++ b/drivers/usb/gadget/function/u_serial.c
> > @@ -295,8 +295,8 @@ __acquires(&port->port_lock)
> >                       break;
> >       }
> >
> > -     if (do_tty_wake && port->port.tty)
> > -             tty_wakeup(port->port.tty);
> > +     if (do_tty_wake)
> > +             tty_port_tty_wakeup(&port->port);
> >       return status;
> >  }
> >
> > @@ -574,7 +574,7 @@ static int gs_start_io(struct gs_port *port)
> >               gs_start_tx(port);
> >               /* Unblock any pending writes into our circular buffer, in case
> >                * we didn't in gs_start_tx() */
> > -             tty_wakeup(port->port.tty);
>
> Just curious, since this is already under lock, checking for
> port->port.tty would have also helped, right? Anyways looks like
> tty_port_tty_wakeup is better.

You're right, adding a null check for port->port.tty also solves the
problem. I actually submitted that exact solution last year, but it
was rejected.

Link: https://lore.kernel.org/linux-usb/20240116141801.396398-1-khtsai@google.com/

>
> > +             tty_port_tty_wakeup(&port->port);
> >       } else {
> >               /* Free reqs only if we are still connected */
> >               if (port->port_usb) {
> > --
> > 2.50.0.rc2.692.g299adb8693-goog
> >