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usb: gadget: u_serial: Fix race condition in TTY wakeup

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commit c529c3730bd09115684644e26bf01ecbd7e2c2c9 upstream.

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: 35f95fd7f2 ("TTY: usb/u_serial, use tty from tty_port")
Cc: stable <stable@kernel.org>
Signed-off-by: Kuen-Han Tsai <khtsai@google.com>
Reviewed-by: Prashanth K <prashanth.k@oss.qualcomm.com>
Link: https://lore.kernel.org/linux-usb/20240116141801.396398-1-khtsai@google.com/
Link: https://lore.kernel.org/r/20250617050844.1848232-2-khtsai@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Kuen-Han Tsai 2025-06-17 13:07:12 +08:00 • committed by Greg Kroah-Hartman
commit 18d58a467c
1 changed files with 3 additions and 3 deletions
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6
drivers/usb/gadget/function/u_serial.c
View file

@ -286,8 +286,8 @@ __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;
}
@ -564,7 +564,7 @@ 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);
tty_port_tty_wakeup(&port->port);
} else {
/* Free reqs only if we are still connected */
if (port->port_usb) {