Re: [PATCH] TIOCSTI: always enable for CAP_SYS_ADMIN

[Kees Cook <keescook@chromium.org>]

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@ens-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@chromium.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