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>
diff --git a/drivers/tty/Kconfig b/drivers/tty/Kconfig
index cc30ff9..d35fc06 100644
--- a/drivers/tty/Kconfig
+++ b/drivers/tty/Kconfig
@@ -149,6 +149,25 @@
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
+ 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.
+
+ This functionality can be changed at runtime with the
+ dev.tty.legacy_tiocsti sysctl. This configuration option sets
+ the default value of the sysctl.
+
config LDISC_AUTOLOAD
bool "Automatically load TTY Line Disciplines"
default y
