diff options
| author | Kees Cook <keescook@chromium.org> | 2022-10-22 11:29:49 -0700 |
|---|---|---|
| committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2022-11-03 01:58:03 +0100 |
| commit | 83efeeeb3d04b22aaed1df99bc70a48fe9d22c4d (patch) | |
| tree | 9a7adfeb695312c548f895d3ba6d8792eb41922b | |
| parent | 5fd8c2d3de3dd3cc6d36a0c7a08e44cd5bf173e6 (diff) | |
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>
| -rw-r--r-- | drivers/tty/Kconfig | 19 | ||||
| -rw-r--r-- | drivers/tty/tty_io.c | 11 |
2 files changed, 30 insertions, 0 deletions
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 diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c index fe77a3d41326..a6a16cf986b7 100644 --- a/drivers/tty/tty_io.c +++ b/drivers/tty/tty_io.c @@ -2268,11 +2268,15 @@ static int tty_fasync(int fd, struct file *filp, int on) * * Called functions take tty_ldiscs_lock * * current->signal->tty check is safe without locks */ +static bool tty_legacy_tiocsti __read_mostly = IS_ENABLED(CONFIG_LEGACY_TIOCSTI); static int tiocsti(struct tty_struct *tty, char __user *p) { char ch, mbz = 0; struct tty_ldisc *ld; + if (!tty_legacy_tiocsti) + return -EIO; + if ((current->signal->tty != tty) && !capable(CAP_SYS_ADMIN)) return -EPERM; if (get_user(ch, p)) @@ -3574,6 +3578,13 @@ void console_sysfs_notify(void) static struct ctl_table tty_table[] = { { + .procname = "legacy_tiocsti", + .data = &tty_legacy_tiocsti, + .maxlen = sizeof(tty_legacy_tiocsti), + .mode = 0644, + .proc_handler = proc_dobool, + }, + { .procname = "ldisc_autoload", .data = &tty_ldisc_autoload, .maxlen = sizeof(tty_ldisc_autoload), |