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Message-Id: <20170423235417.12809-3-matt@nmatt.com>
Date: Sun, 23 Apr 2017 19:54:17 -0400
From: Matt Brown <matt@...tt.com>
To: serge@...lyn.com,
jmorris@...ei.org,
gregkh@...uxfoundation.org,
jslaby@...e.com,
corbet@....net,
keescook@...omium.org
Cc: akpm@...ux-foundation.org,
jannh@...gle.com,
kernel-hardening@...ts.openwall.com,
linux-security-module@...r.kernel.org,
linux-kernel@...r.kernel.org,
linux-doc@...r.kernel.org,
Matt Brown <matt@...tt.com>
Subject: [PATCH v3 2/2] tiocsti-restrict : make TIOCSTI ioctl require CAP_SYS_ADMIN
This introduces the tiocsti_restrict sysctl, whose default is controlled via
CONFIG_SECURITY_TIOCSTI_RESTRICT. When activated, this control restricts
all TIOCSTI ioctl calls from non CAP_SYS_ADMIN users.
This patch was inspired from GRKERNSEC_HARDEN_TTY.
This patch would have prevented
https://bugzilla.redhat.com/show_bug.cgi?id=1411256 under the following
conditions:
* non-privileged container
* container run inside new user namespace
Possible effects on userland:
There could be a few user programs that would be effected by this
change.
See: <https://codesearch.debian.net/search?q=ioctl%5C%28.*TIOCSTI>
notable programs are: agetty, csh, xemacs and tcsh
However, I still believe that this change is worth it given that the
Kconfig defaults to n. This will be a feature that is turned on for the
same reason that people activate it when using grsecurity. Users of this
opt-in feature will realize that they are choosing security over some OS
features like unprivileged TIOCSTI ioctls, as should be clear in the
Kconfig help message.
Threat Model/Patch Rational:
>From grsecurity's config for GRKERNSEC_HARDEN_TTY.
| There are very few legitimate uses for this functionality and it
| has made vulnerabilities in several 'su'-like programs possible in
| the past. Even without these vulnerabilities, it provides an
| attacker with an easy mechanism to move laterally among other
| processes within the same user's compromised session.
So if one process within a tty session becomes compromised it can follow
that additional processes, that are thought to be in different security
boundaries, can be compromised as a result. When using a program like su
or sudo, these additional processes could be in a tty session where TTY file
descriptors are indeed shared over privilege boundaries.
This is also an excellent writeup about the issue:
<http://www.halfdog.net/Security/2012/TtyPushbackPrivilegeEscalation/>
When user namespaces are in use, the check for the capability
CAP_SYS_ADMIN is done against the user namespace that originally opened
the tty.
Signed-off-by: Matt Brown <matt@...tt.com>
---
Documentation/sysctl/kernel.txt | 21 +++++++++++++++++++++
drivers/tty/tty_io.c | 6 ++++++
include/linux/tty.h | 2 ++
kernel/sysctl.c | 12 ++++++++++++
security/Kconfig | 13 +++++++++++++
5 files changed, 54 insertions(+)
diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
index bac23c1..c15c660 100644
--- a/Documentation/sysctl/kernel.txt
+++ b/Documentation/sysctl/kernel.txt
@@ -89,6 +89,7 @@ show up in /proc/sys/kernel:
- sysctl_writes_strict
- tainted
- threads-max
+- tiocsti_restrict
- unknown_nmi_panic
- watchdog
- watchdog_thresh
@@ -987,6 +988,26 @@ available RAM pages threads-max is reduced accordingly.
==============================================================
+tiocsti_restrict:
+
+This toggle indicates whether unprivileged users are prevented
+from using the TIOCSTI ioctl to inject commands into otherprocesses
+which share a tty session.
+
+When tiocsti_restrict is set to (0) there are no restrictions(accept
+the default restriction of only being able to injection commands into
+one's own tty). When tiocsti_restrict is set to (1), users must
+have CAP_SYS_ADMIN to use the TIOCSTI ioctl.
+
+When user namespaces are in use, the check for the capability
+CAP_SYS_ADMIN is done against the user namespace that originally
+opened the tty.
+
+The kernel config option CONFIG_SECURITY_TIOCSTI_RESTRICT sets the
+default value of tiocsti_restrict.
+
+==============================================================
+
unknown_nmi_panic:
The value in this file affects behavior of handling NMI. When the
diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 03d5ea2..5667a5e 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -2298,11 +2298,17 @@ static int tty_fasync(int fd, struct file *filp, int on)
* FIXME: may race normal receive processing
*/
+int tiocsti_restrict = IS_ENABLED(CONFIG_SECURITY_TIOCSTI_RESTRICT);
+
static int tiocsti(struct tty_struct *tty, char __user *p)
{
char ch, mbz = 0;
struct tty_ldisc *ld;
+ if (tiocsti_restrict && !ns_capable(tty->owner_user_ns,CAP_SYS_ADMIN)) {
+ pr_warn_ratelimited("TIOCSTI ioctl call blocked for non-privileged process\n");
+ return -EPERM;
+ }
if ((current->signal->tty != tty) && !capable(CAP_SYS_ADMIN))
return -EPERM;
if (get_user(ch, p))
diff --git a/include/linux/tty.h b/include/linux/tty.h
index d902d42..2fd7f49 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -344,6 +344,8 @@ struct tty_file_private {
struct list_head list;
};
+extern int tiocsti_restrict;
+
/* tty magic number */
#define TTY_MAGIC 0x5401
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index acf0a5a..68d1363 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -67,6 +67,7 @@
#include <linux/kexec.h>
#include <linux/bpf.h>
#include <linux/mount.h>
+#include <linux/tty.h>
#include <linux/uaccess.h>
#include <asm/processor.h>
@@ -833,6 +834,17 @@ static struct ctl_table kern_table[] = {
.extra2 = &two,
},
#endif
+#if defined CONFIG_TTY
+ {
+ .procname = "tiocsti_restrict",
+ .data = &tiocsti_restrict,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = proc_dointvec_minmax_sysadmin,
+ .extra1 = &zero,
+ .extra2 = &one,
+ },
+#endif
{
.procname = "ngroups_max",
.data = &ngroups_max,
diff --git a/security/Kconfig b/security/Kconfig
index 3ff1bf9..7d13331 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -18,6 +18,19 @@ config SECURITY_DMESG_RESTRICT
If you are unsure how to answer this question, answer N.
+config SECURITY_TIOCSTI_RESTRICT
+ bool "Restrict unprivileged use of tiocsti command injection"
+ default n
+ help
+ This enforces restrictions on unprivileged users injecting commands
+ into other processes which share a tty session using the TIOCSTI
+ ioctl. This option makes TIOCSTI use require CAP_SYS_ADMIN.
+
+ If this option is not selected, no restrictions will be enforced
+ unless the tiocsti_restrict sysctl is explicitly set to (1).
+
+ If you are unsure how to answer this question, answer N.
+
config SECURITY
bool "Enable different security models"
depends on SYSFS
--
2.10.2
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