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 int tiocspgrp(struct tty_struct *tty, struct tty_struct *real_tty, pid_t __user *p)
{
	struct pid *pgrp;
	pid_t pgrp_nr;
	int retval = tty_check_change(real_tty);

	if (retval == -EIO)
		return -ENOTTY;
	if (retval)
		return retval;
	if (!current->signal->tty ||
	    (current->signal->tty != real_tty) ||
	    (real_tty->session != task_session(current)))
		return -ENOTTY;
	if (get_user(pgrp_nr, p))
		return -EFAULT;
	if (pgrp_nr < 0)
		return -EINVAL;
	rcu_read_lock();
	pgrp = find_vpid(pgrp_nr);
	retval = -ESRCH;
	if (!pgrp)
		goto out_unlock;
	retval = -EPERM;
	if (session_of_pgrp(pgrp) != task_session(current))
		goto out_unlock;
	retval = 0;
	spin_lock_irq(&tty->ctrl_lock);
	put_pid(real_tty->pgrp);
	real_tty->pgrp = get_pid(pgrp);
	spin_unlock_irq(&tty->ctrl_lock);
out_unlock:
	rcu_read_unlock();
	return retval;
}

static int tiocspgrp(struct tty_struct *tty, struct tty_struct *real_tty, pid_t __user *p)
{
	struct pid *pgrp;
	pid_t pgrp_nr;
	int retval = tty_check_change(real_tty);

	if (retval == -EIO)
		return -ENOTTY;
	if (retval)
		return retval;
	if (!current->signal->tty ||
	    (current->signal->tty != real_tty) ||
	    (real_tty->session != task_session(current)))
		return -ENOTTY;
	if (get_user(pgrp_nr, p))
		return -EFAULT;
	if (pgrp_nr < 0)
		return -EINVAL;
	rcu_read_lock();
	pgrp = find_vpid(pgrp_nr);
	retval = -ESRCH;
	if (!pgrp)
		goto out_unlock;
	retval = -EPERM;
	if (session_of_pgrp(pgrp) != task_session(current))
		goto out_unlock;
	retval = 0;
	spin_lock_irq(&tty->ctrl_lock);
	put_pid(real_tty->pgrp);
	real_tty->pgrp = get_pid(pgrp);
	spin_unlock_irq(&tty->ctrl_lock);
out_unlock:
	rcu_read_unlock();
	return retval;
}
