tty: Fix unsafe ldisc reference via ioctl(TIOCGETD)

ioctl(TIOCGETD) retrieves the line discipline id directly from the
ldisc because the line discipline id (c_line) in termios is untrustworthy;
userspace may have set termios via ioctl(TCSETS*) without actually
changing the line discipline via ioctl(TIOCSETD).

However, directly accessing the current ldisc via tty->ldisc is
unsafe; the ldisc ptr dereferenced may be stale if the line discipline
is changing via ioctl(TIOCSETD) or hangup.

Wait for the line discipline reference (just like read() or write())
to retrieve the "current" line discipline id.

Cc: <stable@vger.kernel.org>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
static ssize_t show_cons_active(struct device *dev,
				struct device_attribute *attr, char *buf)
{
	struct console *cs[16];
	int i = 0;
	struct console *c;
	ssize_t count = 0;

	console_lock();
	for_each_console(c) {
		if (!c->device)
			continue;
		if (!c->write)
			continue;
		if ((c->flags & CON_ENABLED) == 0)
			continue;
		cs[i++] = c;
		if (i >= ARRAY_SIZE(cs))
			break;
	}
	while (i--) {
		int index = cs[i]->index;
		struct tty_driver *drv = cs[i]->device(cs[i], &index);

		/* don't resolve tty0 as some programs depend on it */
		if (drv && (cs[i]->index > 0 || drv->major != TTY_MAJOR))
			count += tty_line_name(drv, index, buf + count);
		else
			count += sprintf(buf + count, "%s%d",
					 cs[i]->name, cs[i]->index);

		count += sprintf(buf + count, "%c", i ? ' ':'\n');
	}
	console_unlock();

	return count;
}

static ssize_t show_cons_active(struct device *dev,
				struct device_attribute *attr, char *buf)
{
	struct console *cs[16];
	int i = 0;
	struct console *c;
	ssize_t count = 0;

	console_lock();
	for_each_console(c) {
		if (!c->device)
			continue;
		if (!c->write)
			continue;
		if ((c->flags & CON_ENABLED) == 0)
			continue;
		cs[i++] = c;
		if (i >= ARRAY_SIZE(cs))
			break;
	}
	while (i--) {
		int index = cs[i]->index;
		struct tty_driver *drv = cs[i]->device(cs[i], &index);

		/* don't resolve tty0 as some programs depend on it */
		if (drv && (cs[i]->index > 0 || drv->major != TTY_MAJOR))
			count += tty_line_name(drv, index, buf + count);
		else
			count += sprintf(buf + count, "%s%d",
					 cs[i]->name, cs[i]->index);

		count += sprintf(buf + count, "%c", i ? ' ':'\n');
	}
	console_unlock();

	return count;
}
