linux-kernel - Re: [PATCH] TIOCSTI: always enable for CAP_SYS

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Message-ID: <202307031241.C7286A7B2@keescook>
Date:   Mon, 3 Jul 2023 12:43:15 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Samuel Thibault <samuel.thibault@...-lyon.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        Simon Brand <simon.brand@...tadigitale.de>,
        stable@...r.kernel.org, linux-kernel@...r.kernel.org,
        Dave@...lke.cc
Subject: Re: [PATCH] TIOCSTI: always enable for CAP_SYS_ADMIN

On Sun, Jul 02, 2023 at 01:59:18AM +0200, Samuel Thibault wrote:
> 83efeeeb3d04 ("tty: Allow TIOCSTI to be disabled") broke BRLTTY's
> ability to simulate keypresses on the console, thus effectively breaking
> braille keyboards of blind users.
> 
> This restores the TIOCSTI feature for CAP_SYS_ADMIN processes, which
> BRLTTY is, thus fixing braille keyboards without re-opening the security
> issue.
> 
> Signed-off-by: Samuel Thibault <samuel.thibault@...-lyon.org>
> Fixes: 83efeeeb3d04 ("tty: Allow TIOCSTI to be disabled")

Based on the design of brltty, this appears to be the only solution. I
remain surprised that FreeBSD had no brltty support, which is why they
didn't run into this problem.

Acked-by: Kees Cook <keescook@...omium.org>

-Kees

> 
> Index: linux-6.4/drivers/tty/tty_io.c
> ===================================================================
> --- linux-6.4.orig/drivers/tty/tty_io.c
> +++ linux-6.4/drivers/tty/tty_io.c
> @@ -2276,7 +2276,7 @@ static int tiocsti(struct tty_struct *tt
>  	char ch, mbz = 0;
>  	struct tty_ldisc *ld;
>  
> -	if (!tty_legacy_tiocsti)
> +	if (!tty_legacy_tiocsti && !capable(CAP_SYS_ADMIN))
>  		return -EIO;
>  
>  	if ((current->signal->tty != tty) && !capable(CAP_SYS_ADMIN))

-- 
Kees Cook