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

[Greg Kroah-Hartman]

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@xxxxxxxxxx>
> Signed-off-by: Mateusz Guzik <mguzik@xxxxxxxxxx>
> ---
>  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
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