* [PATCH v2 1/2] tty: Move sysctl setup into "core" tty logic
2022-10-15 6:45 [PATCH v2 0/2] tty: Allow TIOCSTI to be disabled Kees Cook
@ 2022-10-15 6:45 ` Kees Cook
2022-10-15 6:45 ` [PATCH v2 2/2] tty: Allow TIOCSTI to be disabled Kees Cook
1 sibling, 0 replies; 4+ messages in thread
From: Kees Cook @ 2022-10-15 6:45 UTC (permalink / raw)
To: Greg Kroah-Hartman
Cc: Kees Cook, Jiri Slaby, Simon Brand, linux-kernel, linux-hardening
In preparation for adding another sysctl to the tty subsystem, move the
tty setup code into the "core" tty code, which contains tty_init() itself.
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
drivers/tty/tty.h | 2 +-
drivers/tty/tty_io.c | 34 ++++++++++++++++++++++++++++++++--
drivers/tty/tty_ldisc.c | 38 +-------------------------------------
3 files changed, 34 insertions(+), 40 deletions(-)
diff --git a/drivers/tty/tty.h b/drivers/tty/tty.h
index f310a8274df1..b1f22259ed4c 100644
--- a/drivers/tty/tty.h
+++ b/drivers/tty/tty.h
@@ -93,7 +93,7 @@ void tty_ldisc_release(struct tty_struct *tty);
int __must_check tty_ldisc_init(struct tty_struct *tty);
void tty_ldisc_deinit(struct tty_struct *tty);
-void tty_sysctl_init(void);
+extern int tty_ldisc_autoload;
/* tty_audit.c */
#ifdef CONFIG_AUDIT
diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 82a8855981f7..b397b223eada 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -3581,13 +3581,44 @@ void console_sysfs_notify(void)
sysfs_notify(&consdev->kobj, NULL, "active");
}
+static struct ctl_table tty_table[] = {
+ {
+ .procname = "ldisc_autoload",
+ .data = &tty_ldisc_autoload,
+ .maxlen = sizeof(tty_ldisc_autoload),
+ .mode = 0644,
+ .proc_handler = proc_dointvec,
+ .extra1 = SYSCTL_ZERO,
+ .extra2 = SYSCTL_ONE,
+ },
+ { }
+};
+
+static struct ctl_table tty_dir_table[] = {
+ {
+ .procname = "tty",
+ .mode = 0555,
+ .child = tty_table,
+ },
+ { }
+};
+
+static struct ctl_table tty_root_table[] = {
+ {
+ .procname = "dev",
+ .mode = 0555,
+ .child = tty_dir_table,
+ },
+ { }
+};
+
/*
* Ok, now we can initialize the rest of the tty devices and can count
* on memory allocations, interrupts etc..
*/
int __init tty_init(void)
{
- tty_sysctl_init();
+ register_sysctl_table(tty_root_table);
cdev_init(&tty_cdev, &tty_fops);
if (cdev_add(&tty_cdev, MKDEV(TTYAUX_MAJOR, 0), 1) ||
register_chrdev_region(MKDEV(TTYAUX_MAJOR, 0), 1, "/dev/tty") < 0)
@@ -3609,4 +3640,3 @@ int __init tty_init(void)
#endif
return 0;
}
-
diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index 776d8a62f77c..e758f44729e7 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -117,7 +117,7 @@ static void put_ldops(struct tty_ldisc_ops *ldops)
raw_spin_unlock_irqrestore(&tty_ldiscs_lock, flags);
}
-static int tty_ldisc_autoload = IS_BUILTIN(CONFIG_LDISC_AUTOLOAD);
+int tty_ldisc_autoload = IS_BUILTIN(CONFIG_LDISC_AUTOLOAD);
/**
* tty_ldisc_get - take a reference to an ldisc
@@ -817,39 +817,3 @@ void tty_ldisc_deinit(struct tty_struct *tty)
tty_ldisc_put(tty->ldisc);
tty->ldisc = NULL;
}
-
-static struct ctl_table tty_table[] = {
- {
- .procname = "ldisc_autoload",
- .data = &tty_ldisc_autoload,
- .maxlen = sizeof(tty_ldisc_autoload),
- .mode = 0644,
- .proc_handler = proc_dointvec,
- .extra1 = SYSCTL_ZERO,
- .extra2 = SYSCTL_ONE,
- },
- { }
-};
-
-static struct ctl_table tty_dir_table[] = {
- {
- .procname = "tty",
- .mode = 0555,
- .child = tty_table,
- },
- { }
-};
-
-static struct ctl_table tty_root_table[] = {
- {
- .procname = "dev",
- .mode = 0555,
- .child = tty_dir_table,
- },
- { }
-};
-
-void tty_sysctl_init(void)
-{
- register_sysctl_table(tty_root_table);
-}
--
2.34.1
^ permalink raw reply related [flat|nested] 4+ messages in thread* [PATCH v2 2/2] tty: Allow TIOCSTI to be disabled
2022-10-15 6:45 [PATCH v2 0/2] tty: Allow TIOCSTI to be disabled Kees Cook
2022-10-15 6:45 ` [PATCH v2 1/2] tty: Move sysctl setup into "core" tty logic Kees Cook
@ 2022-10-15 6:45 ` Kees Cook
2022-10-20 6:05 ` Jiri Slaby
1 sibling, 1 reply; 4+ messages in thread
From: Kees Cook @ 2022-10-15 6:45 UTC (permalink / raw)
To: Greg Kroah-Hartman
Cc: Kees Cook, Jiri Slaby, Simon Brand, linux-kernel, linux-hardening
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(+)
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 b397b223eada..fa36dac7559e 100644
--- 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);
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,
--
2.34.1
^ permalink raw reply related [flat|nested] 4+ messages in thread