[PATCH 5.14 199/849] serial: 8250: fix racy uartclk update

Greg Kroah-Hartman
2021-11-15T22:09:11Z

From: Johan Hovold <johan@kernel.org>

commit 211cde4f5817dc88ef7f8f2fa286e57fbf14c8ee upstream.

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

--- 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_
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 ua
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);