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    Subject[PATCH 5.14 199/849] serial: 8250: fix racy uartclk update
    Date
    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);


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