[PATCH v2 0/2] tty: Allow TIOCSTI to be disabled

[PATCH v2 0/2] tty: Allow TIOCSTI to be disabled

[Kees Cook]

Hi,

v2: add missed "static" to new sysctl var; add missed "extern" to old var. :)
v1: https://lore.kernel.org/lkml/20221015041352.never.966-kees@kernel.org/

This is so very long over-due. We just need to do this and put an end
to this legacy operation.

Repeating the commit log from patch 2:

    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/

Thanks,

-Kees

Kees Cook (2):
  tty: Move sysctl setup into "core" tty logic
  tty: Allow TIOCSTI to be disabled

 drivers/tty/Kconfig     | 19 +++++++++++++++++
 drivers/tty/tty.h       |  2 +-
 drivers/tty/tty_io.c    | 47 +++++++++++++++++++++++++++++++++++++++--
 drivers/tty/tty_ldisc.c | 38 +--------------------------------
 4 files changed, 66 insertions(+), 40 deletions(-)

-- 
2.34.1

[PATCH v2 1/2] tty: Move sysctl setup into "core" tty logic

[Kees Cook]

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

[PATCH v2 2/2] tty: Allow TIOCSTI to be disabled

[Kees Cook]

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

Re: [PATCH v2 2/2] tty: Allow TIOCSTI to be disabled

[Jiri Slaby]

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,
-- 
js
suse labs