serial: omap: Prevent DoS using unprivileged ioctl(TIOCSRS485)

commit 308bbc9a upstream.

The omap-serial driver emulates RS485 delays using software timers,
but neglects to clamp the input values from the unprivileged
ioctl(TIOCSRS485). Because the software implementation busy-waits,
malicious userspace could stall the cpu for ~49 days.

Clamp the input values to < 100ms.

Fixes: 4a0ac0f5 ("OMAP: add RS485 support")
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[ luis: backported to 3.16: adjusted context ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

drivers/tty/serial/omap-serial.c

@@ -1365,7 +1365,7 @@ static inline void serial_omap_add_console_port(struct uart_omap_port *up)
 
 /* Enable or disable the rs485 support */
 static void
-serial_omap_config_rs485(struct uart_port *port, struct serial_rs485 *rs485conf)
+serial_omap_config_rs485(struct uart_port *port, struct serial_rs485 *rs485)
 {
 	struct uart_omap_port *up = to_uart_omap_port(port);
 	unsigned long flags;
@@ -1380,8 +1380,12 @@ serial_omap_config_rs485(struct uart_port *port, struct serial_rs485 *rs485conf)
 	up->ier = 0;
 	serial_out(up, UART_IER, 0);
 
+	/* Clamp the delays to [0, 100ms] */
+	rs485->delay_rts_before_send = min(rs485->delay_rts_before_send, 100U);
+	rs485->delay_rts_after_send  = min(rs485->delay_rts_after_send, 100U);
+
 	/* store new config */
-	up->rs485 = *rs485conf;
+	up->rs485 = *rs485;
 
 	/*
 	 * Just as a precaution, only allow rs485