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>

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

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))
@@ -3573,6 +3577,13 @@ 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,