tty: Only allow slave pty as controlling tty

A master pty should never be a controlling tty in Linux; if the
master pty is specified to ioctl(TIOCSCTTY), silently substitute the slave
pty as the controlling tty.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_io.c

@@ -2866,7 +2866,7 @@ long tty_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 		no_tty();
 		return 0;
 	case TIOCSCTTY:
-		return tiocsctty(tty, file, arg);
+		return tiocsctty(real_tty, file, arg);
 	case TIOCGPGRP:
 		return tiocgpgrp(tty, real_tty, p);
 	case TIOCSPGRP: