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serial: 8250: fix racy uartclk update

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commit 211cde4f5817dc88ef7f8f2fa286e57fbf14c8ee upstream.

Commit 868f3ee6e4 ("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: 868f3ee6e4 ("serial: 8250: Add 8250 port clock update method")
Fixes: c8dff3aa82 ("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>
This commit is contained in:
Johan Hovold 2021-10-15 13:14:20 +02:00 committed by Greg Kroah-Hartman
parent 5f31af4e78
commit edc5466254
1 changed files with 17 additions and 4 deletions
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21
drivers/tty/serial/8250/8250_port.c
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@ -2675,21 +2675,32 @@ 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);
@ -2706,7 +2717,9 @@ 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);