linux-kernel - Re: [PATCH v3 2/2] tty: Allow TIOCSTI to be disabled

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Date:   Wed, 28 Dec 2022 21:57:26 +0100
From:   Samuel Thibault <samuel.thibault@...ilenet.fr>
To:     Kees Cook <kees@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Simon Brand <simon.brand@...tadigitale.de>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v3 2/2] tty: Allow TIOCSTI to be disabled

Hello,

Kees Cook, le mar. 27 déc. 2022 19:32:55 -0800, a ecrit:
> On December 27, 2022 3:40:00 PM PST, Samuel Thibault <samuel.thibault@...ilenet.fr> wrote:
> >Kees Cook, le sam. 22 oct. 2022 11:29:49 -0700, a ecrit:
> >> TIOCSTI continues its long history of being used in privilege escalation
> >> attacks[1]. Prior attempts to provide a mechanism to disable this have
> >> devolved into discussions around creating full-blown LSMs to provide
> >> arbitrary ioctl filtering, which is hugely over-engineered -- only
> >> TIOCSTI is being used this way. 3 years ago OpenBSD entirely removed
> >> TIOCSTI[2], Android has had it filtered for longer[3], and the tools that
> >> had historically used TIOCSTI either do not need it, are not commonly
> >> built with it, or have had its use removed.
> >
> >No. The Brltty screen reader entirely relies on TIOCSTI to be able to
> >support input from various Braille devices. Please make sure to keep
> >TIOCSTI enabled by default, otherwise some people would just completely
> >lose their usual way of simply typing on Linux.
> 
> Yup, it remains default enabled:

Yes, but thining of it, very soon people in various security-sensitive
distributions will disable it, as they should indeed. And people who
need to use their Braille device on such distributions will get stuck.

Can we perhaps just introduce a CAP_TIOCSTI that the brltty daemon would
be able to use? We could even make it only allow TIOCSTI on the linux
console (tty->ops == con_ops).

> Also, what does FreeBSD use for screen readers?

FreeBSD provides poor support for that, people have to use a patched
screen tool to somehow access the console (but only after login).

Samuel