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

[Prashanth K <prashanth.k@oss.qualcomm.com>]

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@vger.kernel.org
> Signed-off-by: Kuen-Han Tsai <khtsai@google.com>
> ---

Reviewed-by: Prashanth K <prashanth.k@oss.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.

> +		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
>