[PATCH 1/3] serial: 8250: fix racy uartclk update

[Johan Hovold <johan@xxxxxxxxxx>]

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.

Fixes: 868f3ee6e452 ("serial: 8250: Add 8250 port clock update method")
Fixes: c8dff3aa8241 ("serial: 8250: Skip uninitialized TTY port baud rate update")
Cc: stable@xxxxxxxxxxxxxxx      # 5.9
Cc: Serge Semin <Sergey.Semin@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Johan Hovold <johan@xxxxxxxxxx>
---
 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