From c235ccc1c4d6fd8b7d48b976b87416230ffd5149 Mon Sep 17 00:00:00 2001 From: Peter Hurley Date: Tue, 2 Sep 2014 17:39:13 -0400 Subject: [PATCH] serial: core: Fix x_char race The UART driver is expected to clear port->x_char after transmission while holding the port->lock. However, the serial core fails to take the port->lock before assigning port->xchar. This allows for the following race CPU 0 | CPU 1 | | serial8250_handle_irq | ... | serial8250_tx_chars | if (port->x_char) | serial_out(up, UART_TX, port->x_char) uart_send_xchar | port->x_char = ch | | port->x_char = 0 port->ops->start_tx() | | The x_char on CPU 0 will never be sent. Take the port->lock in uart_send_xchar() before assigning port->x_char. Signed-off-by: Peter Hurley Signed-off-by: Greg Kroah-Hartman --- drivers/tty/serial/serial_core.c | 7 +++---- 1 file changed, 3 insertions(+), 4 deletions(-) diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c index 0742f77ac410..bd20cf51e912 100644 --- a/drivers/tty/serial/serial_core.c +++ b/drivers/tty/serial/serial_core.c @@ -600,12 +600,11 @@ static void uart_send_xchar(struct tty_struct *tty, char ch) if (port->ops->send_xchar) port->ops->send_xchar(port, ch); else { + spin_lock_irqsave(&port->lock, flags); port->x_char = ch; - if (ch) { - spin_lock_irqsave(&port->lock, flags); + if (ch) port->ops->start_tx(port); - spin_unlock_irqrestore(&port->lock, flags); - } + spin_unlock_irqrestore(&port->lock, flags); } } -- 2.20.1