serial: 8250: Don't touch RTS modem control while in rs485 mode

serial8250_do_set_mctrl() currently allows modifying the RTS modem
control line even when RTS is used as an rs485 Transmit Enable signal.
It is thus possible for user space to interfere with rs485 communication
by invoking a TIOCMSET ioctl().

Ignore such change requests and retain the current RTS polarity when in
rs485 mode.  Note that serial8250_set_mctrl() is always called with
port->lock held, so there's no risk that RTS is changed concurrently.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: Matwey V. Kornilov <matwey@sai.msu.ru>
Link: https://lore.kernel.org/r/b1ce34ca9bc4d7bdc6e9852fcf30b1f4e37c8a80.1582895077.git.lukas@wunner.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index f398f162a1fd45b1c83df734af82e34ea3842ca8..a8f4cedde4dcf0d246034ab145563a6c68476f47 100644 (file)
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -1924,6 +1924,13 @@ void serial8250_do_set_mctrl(struct uart_port *port, unsigned int mctrl)
        struct uart_8250_port *up = up_to_u8250p(port);
        unsigned char mcr;
 
+       if (port->rs485.flags & SER_RS485_ENABLED) {
+               if (serial8250_in_MCR(up) & UART_MCR_RTS)
+                       mctrl |= TIOCM_RTS;
+               else
+                       mctrl &= ~TIOCM_RTS;
+       }
+
        mcr = serial8250_TIOCM_to_MCR(mctrl);
 
        mcr = (mcr & up->mcr_mask) | up->mcr_force | up->mcr;