From 83efeeeb3d04b22aaed1df99bc70a48fe9d22c4d Mon Sep 17 00:00:00 2001 From: Kees Cook Date: Sat, 22 Oct 2022 11:29:49 -0700 Subject: 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 Cc: Jiri Slaby Cc: Simon Brand Signed-off-by: Kees Cook Link: https://lore.kernel.org/r/20221022182949.2684794-2-keescook@chromium.org Signed-off-by: Greg Kroah-Hartman --- drivers/tty/Kconfig | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) (limited to 'drivers/tty/Kconfig') diff --git a/drivers/tty/Kconfig b/drivers/tty/Kconfig index cc30ff93e2e4..d35fc068da74 100644 --- 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 + 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 -- cgit v1.2.3