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tty: Allow TIOCSTI to be disabled

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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>
This commit is contained in:
Kees Cook 2022-10-22 11:29:49 -07:00 • committed by Greg Kroah-Hartman
parent 5fd8c2d3de
commit 83efeeeb3d
2 changed files with 30 additions and 0 deletions
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19
drivers/tty/Kconfig
View file

@ -149,6 +149,25 @@ config LEGACY_PTY_COUNT
When not in use, each legacy PTY occupies 12 bytes on 32-bit When not in use, each legacy PTY occupies 12 bytes on 32-bit
architectures and 24 bytes on 64-bit architectures. 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 config LDISC_AUTOLOAD
bool "Automatically load TTY Line Disciplines" bool "Automatically load TTY Line Disciplines"
default y default y

11
drivers/tty/tty_io.c
View file

@ -2268,11 +2268,15 @@ static int tty_fasync(int fd, struct file *filp, int on)
* * Called functions take tty_ldiscs_lock * * Called functions take tty_ldiscs_lock
* * current->signal->tty check is safe without locks * * 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) static int tiocsti(struct tty_struct *tty, char __user *p)
{ {
char ch, mbz = 0; char ch, mbz = 0;
struct tty_ldisc *ld; struct tty_ldisc *ld;
if (!tty_legacy_tiocsti)
return -EIO;
if ((current->signal->tty != tty) && !capable(CAP_SYS_ADMIN)) if ((current->signal->tty != tty) && !capable(CAP_SYS_ADMIN))
return -EPERM; return -EPERM;
if (get_user(ch, p)) if (get_user(ch, p))
@ -3573,6 +3577,13 @@ void console_sysfs_notify(void)
} }
static struct ctl_table tty_table[] = { 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", .procname = "ldisc_autoload",
.data = &tty_ldisc_autoload, .data = &tty_ldisc_autoload,