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

[Geert Uytterhoeven]

Hi Kees,

On Sat, Oct 22, 2022 at 9:14 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> 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.
>
> Provide a simple CONFIG and global sysctl to disable this for the system
> builders who have wanted this functionality for literally decades now,
> much like the ldisc_autoload CONFIG and sysctl.
>
> [1] https://lore.kernel.org/linux-hardening/Y0m9l52AKmw6Yxi1@hostpad
> [2] https://undeadly.org/cgi?action=article;sid=20170701132619
> [3] https://lore.kernel.org/lkml/CAFJ0LnFGRuEEn1tCLhoki8ZyWrKfktbF+rwwN7WzyC_kBFoQVA@xxxxxxxxxxxxxx/
>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: Jiri Slaby <jirislaby@xxxxxxxxxx>
> Cc: Simon Brand <simon.brand@xxxxxxxxxxxxxxxx>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>

Thanks for your patch, which is now commit 83efeeeb3d04b22a ("tty:
Allow TIOCSTI to be disabled") in tty/tty-next.

> --- a/drivers/tty/Kconfig
> +++ b/drivers/tty/Kconfig
> @@ -149,6 +149,25 @@ config LEGACY_PTY_COUNT
>           When not in use, each legacy PTY occupies 12 bytes on 32-bit
>           architectures and 24 bytes on 64-bit architectures.
>
> +config LEGACY_TIOCSTI
> +       bool "Allow legacy TIOCSTI usage"
> +       default y

Obviously this should either default to n, ...

> +       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. Its use is considered
> +         a dangerous legacy operation, and can be disabled on most
> +         systems.
> +
> +         Say 'Y here only if you have confirmed that your system's
> +         userspace depends on this functionality to continue operating
> +         normally.

... or the help text should be made less scary.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds