On systems where certain filesystem contents cannot be entirely
trusted,
it is beneficial to block mounts on symlinks. This makes sure that
malicious filesystem contents cannot trigger the over-mounting of
trusted
filesystems. (For example, a bind-mounted subdirectory of /var cannot
be
redirected to mount on /etc via a symlink: a daemon cannot elevate
privs
to uid-0.)
Signed-off-by: Kees Cook <keesc...@chromium.org>
---
security/Kconfig | 6 ++++
security/Makefile | 2 ++
security/mntrestrict/Kconfig | 13 ++++++++
security/mntrestrict/Makefile | 1 +
security/mntrestrict/mntrestrict.c | 63
++++++++++++++++++++++++++++++++++++
5 files changed, 85 insertions(+)
create mode 100644 security/mntrestrict/Kconfig
create mode 100644 security/mntrestrict/Makefile
create mode 100644 security/mntrestrict/mntrestrict.c
diff --git a/security/Kconfig b/security/Kconfig
index e9c6ac7..84d41f4 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -121,6 +121,7 @@ source security/selinux/Kconfig
source security/smack/Kconfig
source security/tomoyo/Kconfig
source security/apparmor/Kconfig
+source security/mntrestrict/Kconfig
source security/yama/Kconfig
source security/integrity/Kconfig
@@ -131,6 +132,7 @@ choice
default DEFAULT_SECURITY_SMACK if SECURITY_SMACK
default DEFAULT_SECURITY_TOMOYO if SECURITY_TOMOYO
default DEFAULT_SECURITY_APPARMOR if SECURITY_APPARMOR
+ default DEFAULT_SECURITY_MNTRESTRICT if SECURITY_MNTRESTRICT
default DEFAULT_SECURITY_YAMA if SECURITY_YAMA
default DEFAULT_SECURITY_DAC
@@ -150,6 +152,9 @@ choice
config DEFAULT_SECURITY_APPARMOR
bool "AppArmor" if SECURITY_APPARMOR=y
+ config DEFAULT_SECURITY_MNTRESTRICT
+ bool "MntRestrict" if SECURITY_MNTRESTRICT=y
+
config DEFAULT_SECURITY_YAMA
bool "Yama" if SECURITY_YAMA=y
@@ -164,6 +169,7 @@ config DEFAULT_SECURITY
default "smack" if DEFAULT_SECURITY_SMACK
default "tomoyo" if DEFAULT_SECURITY_TOMOYO
default "apparmor" if DEFAULT_SECURITY_APPARMOR
+ default "mntrestrict" if DEFAULT_SECURITY_MNTRESTRICT
default "yama" if DEFAULT_SECURITY_YAMA
default "" if DEFAULT_SECURITY_DAC
diff --git a/security/Makefile b/security/Makefile
index c26c81e..33f38b1 100644
--- a/security/Makefile
+++ b/security/Makefile
@@ -7,6 +7,7 @@ subdir-$(CONFIG_SECURITY_SELINUX) += selinux
subdir-$(CONFIG_SECURITY_SMACK) += smack
subdir-$(CONFIG_SECURITY_TOMOYO) += tomoyo
subdir-$(CONFIG_SECURITY_APPARMOR) += apparmor
+subdir-$(CONFIG_SECURITY_MNTRESTRICT) += mntrestrict
subdir-$(CONFIG_SECURITY_YAMA) += yama
# always enable default capabilities
@@ -22,6 +23,7 @@ obj-$(CONFIG_SECURITY_SMACK) += smack/built-in.o
obj-$(CONFIG_AUDIT) += lsm_audit.o
obj-$(CONFIG_SECURITY_TOMOYO) += tomoyo/built-in.o
obj-$(CONFIG_SECURITY_APPARMOR) += apparmor/built-in.o
+obj-$(CONFIG_SECURITY_MNTRESTRICT) += mntrestrict/built-in.o
obj-$(CONFIG_SECURITY_YAMA) += yama/built-in.o
obj-$(CONFIG_CGROUP_DEVICE) += device_cgroup.o
diff --git a/security/mntrestrict/Kconfig
b/security/mntrestrict/Kconfig
new file mode 100644
index 0000000..9a7310d
--- /dev/null
+++ b/security/mntrestrict/Kconfig
@@ -0,0 +1,13 @@
+config SECURITY_MNTRESTRICT
+ bool "Restrict mounts on symlinks system-wide"
+ depends on SECURITY
+ help
+ Enforces that rule that symlinks cannot be used as mount
+ destinations across the entire system. This is to make sure
+ it is never possible to redirect locations using symlinks
+ when the process doing the mount may not be able to strictly
+ trust the contents of the filesystem where it is happening. For
+ example, making sure a bind mounted subdirectory in /var is not
+ redirected into /etc: stops a daemon without full privileges
+ from elevating to uid-0 by tricking the system init filesystem
+ mounting infrastructure.
diff --git a/security/mntrestrict/Makefile
b/security/mntrestrict/Makefile
new file mode 100644
index 0000000..340be1f
--- /dev/null
+++ b/security/mntrestrict/Makefile
@@ -0,0 +1 @@
+obj-$(CONFIG_SECURITY_MNTRESTRICT) += mntrestrict.o
diff --git a/security/mntrestrict/mntrestrict.c
b/security/mntrestrict/mntrestrict.c
new file mode 100644
index 0000000..257ae69
--- /dev/null
+++ b/security/mntrestrict/mntrestrict.c
@@ -0,0 +1,63 @@
+/*
+ * Mount Restriction Security Module
+ *
+ * Copyright 2011-2013 Google Inc.
+ *
+ * Authors:
+ * Stephan Uphoff <u...@google.com>
+ * Kees Cook <keesc...@chromium.org>
+ *
+ * This software is licensed under the terms of the GNU General
Public
+ * License version 2, as published by the Free Software Foundation,
and
+ * may be copied, distributed, and modified under those terms.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ */
+
+#define pr_fmt(fmt) "MntRestrict LSM: " fmt
+
+#include <linux/module.h>
+#include <linux/security.h>
+#include <linux/sched.h>
+#include <linux/fs.h>
+#include <linux/fs_struct.h>
+#include <linux/mount.h>
+#include <linux/path.h>
+#include <linux/root_dev.h>
+
+static int mntrestrict_sb_mount(const char *dev_name, struct path
*path,
+ const char *type, unsigned long flags,
+ void *data)
+{
+ int error = current->total_link_count ? -ELOOP : 0;
+
+ if (error) {
+ pr_notice("Mount path with symlinks prohibited - pid=%d\n",
+ task_pid_nr(current));
+ }
+
+ return error;
+}
+
+static struct security_operations mntrestrict_ops = {
+ .name = "mntrestrict",
+ .sb_mount = mntrestrict_sb_mount,
+};
+
+static int __init mntrestrict_init(void)
+{
+ int error;
+
+ error = register_security(&mntrestrict_ops);
+
+ if (error)
+ panic("Could not register MntRestrict security module");
+
+ pr_info("symlink destinations will be blocked.\n");
+
+ return error;
+}
+security_initcall(mntrestrict_init);
--
1.7.9.5
--
Kees Cook
Chrome OS Security
--
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