tty: Allow TIOCSTI to be disabled

tty: Allow TIOCSTI to be disabled

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@mail.gmail.com/

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: Simon Brand <simon.brand@postadigitale.de>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20221022182949.2684794-2-keescook@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>