Re: [PATCH] tty: plug a use-after-free in TIOCGETD ioctl

Greg Kroah-Hartman
2016-01-07T15:33:13Z

On Thu, Jan 07, 2016 at 03:58:00PM +0100, Mateusz Guzik wrote:
> When the line discipline is being changed, the old one is freed.
> However, the handler for TIOCGETD would dereference it without taking
> any locks, in effect possibly reading freed memory.
>
> Line discipline changes are protected with tty lock. Use it on reader
> side as well.
>
> CVE: CVE-2016-0723

Why a cve tag?

> Found-by: Milos Vyletel <milos@redhat.com>
> Signed-off-by: Mateusz Guzik <mguzik@redhat.com>
> ---
> drivers/tty/tty_io.c | 23 ++++++++++++++++++++++-
> 1 file changed, 22 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> index 892c923..1b10469 100644
> --- a/drivers/tty/tty_io.c
> +++ b/drivers/tty/tty_io.c
> @@ -2626,6 +2626,27 @@ static int tiocgsid(struct tty_struct *tty, struct tty_struct *real_tty, pid_t _
> }
>
> /**
> + * tiocgetd - get line discipline
> + * @tty: tty device
> + * @p: pointer to returned line discipline
> + *
> + * Get the line discipline associated with the tty.
> + *
> + * Locking: none
> + */
> +
> +static int tiocgetd(struct tty_struct *tty, int __user *p)
> +{
> + int ldisc;
> +
> + tty_lock(tty);
> + ldisc = tty->ldisc->ops->num;
> + tty_unlock(tty);
> +
> + return put_user(ldisc, p);

Does this really protect anything? What is preventing ldisc from going
away right after the tty_unlock call?

And how are you able to trigger the tty to go away while the file is
still held open and this ioctl is being called?

thanks,

greg k-h