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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

Adding Peter and linux-serial to the list here, as Peter has been doing
a ton of work in this area...

Peter, does this seem sane with the tty locking rules?

thanks,

greg k-h

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
> 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);
> +}
> +
> +/**
>   *	tiocsetd	-	set line discipline
>   *	@tty: tty device
>   *	@p: pointer to user data
> @@ -2874,7 +2895,7 @@ long tty_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
>  	case TIOCGSID:
>  		return tiocgsid(tty, real_tty, p);
>  	case TIOCGETD:
> -		return put_user(tty->ldisc->ops->num, (int __user *)p);
> +		return tiocgetd(tty, p);
>  	case TIOCSETD:
>  		return tiocsetd(tty, p);
>  	case TIOCVHANGUP:
> -- 
> 1.8.3.1