[v2,2/2] tty: Allow TIOCSTI to be disabled
Commit Message
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>
---
drivers/tty/Kconfig | 19 +++++++++++++++++++
drivers/tty/tty_io.c | 13 +++++++++++++
2 files changed, 32 insertions(+)
Comments
Hi,
On 15. 10. 22, 8:45, Kees Cook wrote:
> 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>
...
> --- a/drivers/tty/tty_io.c
> +++ b/drivers/tty/tty_io.c
> @@ -2275,11 +2275,15 @@ 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 int tty_legacy_tiocsti __read_mostly = IS_BUILTIN(CONFIG_LEGACY_TIOCSTI);
This can be bool, right? And IS_ENABLED() sounds more appropriate here.
> 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))
> @@ -3582,6 +3586,15 @@ 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_dointvec,
> + .extra1 = SYSCTL_ZERO,
> + .extra2 = SYSCTL_ONE,
Then this becomes just proc_dobool without extras.
Or we can leave it as int, allow 0, 1, and 2. 2 would log_limited the
caller's comm before EIO. Just thinking loudly. Maybe the EIO is enough
for users to notice. Likely…
thanks,
@@ -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
@@ -2275,11 +2275,15 @@ 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 int tty_legacy_tiocsti __read_mostly = IS_BUILTIN(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))
@@ -3582,6 +3586,15 @@ 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_dointvec,
+ .extra1 = SYSCTL_ZERO,
+ .extra2 = SYSCTL_ONE,
+ },
{
.procname = "ldisc_autoload",
.data = &tty_ldisc_autoload,