On Fri, Oct 15, 2021 at 01:14:20PM +0200, Johan Hovold wrote:

Commit 868f3ee6e452 ("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.

Nice catch! Thanks, Johan, for fixing this! Acked-by: Andy Shevchenko andriy.shevchenko@linux.intel.com

Fixes: 868f3ee6e452 ("serial: 8250: Add 8250 port clock update method") Fixes: c8dff3aa8241 ("serial: 8250: Skip uninitialized TTY port baud rate update") Cc: stable@vger.kernel.org # 5.9 Cc: Serge Semin Sergey.Semin@baikalelectronics.ru Signed-off-by: Johan Hovold johan@kernel.org

---

drivers/tty/serial/8250/8250_port.c | 21 +++++++++++++++++---- 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c index 66374704747e..e4dd82fd7c2a 100644 --- 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); -- 2.32.0