staging: greybus: uart: Fix atomicity violation in get_serial_info()

Our static checker found a bug where set_serial_info() uses a mutex, but
get_serial_info() does not. Fortunately, the impact of this is relatively
minor. It doesn't cause a crash or any other serious issues. However, if a
race condition occurs between set_serial_info() and get_serial_info(),
there is a chance that the data returned by get_serial_info() will be
meaningless.

Signed-off-by: Qiu-ji Chen <[email protected]>
Fixes: 0aad5ad ("greybus/uart: switch to ->[sg]et_serial()")
Reviewed-by: Johan Hovold <[email protected]>
Reviewed-by: Dan Carpenter <[email protected]>
Reviewed-by: Alex Elder <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: TomRita999

drivers/staging/greybus/uart.c

@@ -596,11 +596,13 @@ static int get_serial_info(struct tty_struct *tty,
 	struct gb_tty *gb_tty = tty->driver_data;
 
 	ss->line = gb_tty->minor;
+	mutex_lock(&gb_tty->port.mutex);
 	ss->close_delay = jiffies_to_msecs(gb_tty->port.close_delay) / 10;
 	ss->closing_wait =
 		gb_tty->port.closing_wait == ASYNC_CLOSING_WAIT_NONE ?
 		ASYNC_CLOSING_WAIT_NONE :
 		jiffies_to_msecs(gb_tty->port.closing_wait) / 10;
+	mutex_unlock(&gb_tty->port.mutex);
 
 	return 0;
 }