serial: 8250: fix racy uartclk update

Commit 663afba4cf5d ("serial: 8250: Add 8250 port clock update method")
added a hack to support SoCs where the UART reference clock can
change behind the back of the driver but failed to add the proper
locking.

First, make sure to take a reference to the tty struct to avoid
dereferencing a NULL pointer if the clock change races with a hangup.

Second, the termios semaphore must be held during the update to prevent
a racing termios change.

Fixes: 663afba4cf5d ("serial: 8250: Add 8250 port clock update method")
Fixes: cc629609b85a ("serial: 8250: Skip uninitialized TTY port baud rate update")
Cc: stable@vger.kernel.org      # 5.9
Cc: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Tested-by: Serge Semin <fancer.lancer@gmail.com>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20211015111422.1027-2-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 66374704747ec4d455dc6806be99f444592df31a..e4dd82fd7c2a57e6475668b6879465b038834dc5 100644 (file)
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2696,21 +2696,32 @@ static unsigned int serial8250_get_baud_rate(struct uart_port *port,
 void serial8250_update_uartclk(struct uart_port *port, unsigned int uartclk)
 {
        struct uart_8250_port *up = up_to_u8250p(port);
+       struct tty_port *tport = &port->state->port;
        unsigned int baud, quot, frac = 0;
        struct ktermios *termios;
+       struct tty_struct *tty;
        unsigned long flags;
 
-       mutex_lock(&port->state->port.mutex);
+       tty = tty_port_tty_get(tport);
+       if (!tty) {
+               mutex_lock(&tport->mutex);
+               port->uartclk = uartclk;
+               mutex_unlock(&tport->mutex);
+               return;
+       }
+
+       down_write(&tty->termios_rwsem);
+       mutex_lock(&tport->mutex);
 
        if (port->uartclk == uartclk)
                goto out_lock;
 
        port->uartclk = uartclk;
 
-       if (!tty_port_initialized(&port->state->port))
+       if (!tty_port_initialized(tport))
                goto out_lock;
 
-       termios = &port->state->port.tty->termios;
+       termios = &tty->termios;
 
        baud = serial8250_get_baud_rate(port, termios, NULL);
        quot = serial8250_get_divisor(port, baud, &frac);
@@ -2727,7 +2738,9 @@ void serial8250_update_uartclk(struct uart_port *port, unsigned int uartclk)
        serial8250_rpm_put(up);
 
 out_lock:
-       mutex_unlock(&port->state->port.mutex);
+       mutex_unlock(&tport->mutex);
+       up_write(&tty->termios_rwsem);
+       tty_kref_put(tty);
 }
 EXPORT_SYMBOL_GPL(serial8250_update_uartclk);