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

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Date:   Tue, 27 Jun 2023 19:48:45 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Samuel Thibault <samuel.thibault@...ilenet.fr>,
        Kees Cook <kees@...nel.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,
        Dave@...lke.cc
Subject: Re: [PATCH v3 2/2] tty: Allow TIOCSTI to be disabled

On Sun, Jun 25, 2023 at 05:56:25PM +0200, Samuel Thibault wrote:
> > 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).

Does brltty run with CAP_SYS_ADMIN? That seems a sensible exception to
be made.

> *Please* comment on this so we can progress. ATM people are
> advising each other to set dev.tty.legacy_tiocsti=1, which is just
> counter-productive in terms of security...

So is there really no solution for brltty and TIOCSTI being disabled?
What is FreeBSD doing? I imagine it's the same situation there too,
though maybe there is just no support?

https://www.mail-archive.com/brltty@brltty.app/msg02892.html

> Really, this a serious regression for the people affected by this.

Can you send a patch adding a CAP_SYS_ADMIN exception?

-Kees

-- 
Kees Cook