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

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Message-ID: <C95AF535-7A95-48BA-8921-1932C15A1931@kernel.org>
Date:   Tue, 27 Dec 2022 19:32:55 -0800
From:   Kees Cook <kees@...nel.org>
To:     Samuel Thibault <samuel.thibault@...ilenet.fr>,
        Kees Cook <keescook@...omium.org>
CC:     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

On December 27, 2022 3:40:00 PM PST, Samuel Thibault <samuel.thibault@...ilenet.fr> wrote:
>Hello,
>
>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:

> [...]
>> +config LEGACY_TIOCSTI
>> +	bool "Allow legacy TIOCSTI usage"
>> +	default y
>> +	help
>> +	  Historically the kernel has allowed TIOCSTI, which will push
>> +	  characters into a controlling TTY. This continues to be used
>> +	  as a malicious privilege escalation mechanism, and provides no
>> +	  meaningful real-world utility any more.
>
>Yes it does.

Can you send a patch to adjust this language?

Also, what does FreeBSD use for screen readers?

-Kees


-- 
Kees Cook