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Subject[PATCH] LSM: ModPin LSM for module loading restrictions
This LSM enforces that modules must all come from the same filesystem,
with the expectation that such a filesystem is backed by a read-only
device such as dm-verity or CDROM. This allows systems that have a
verified or unchanging filesystem to enforce module loading restrictions
without needing to sign the modules individually.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
security/Kconfig | 6 ++
security/Makefile | 2 +
security/modpin/Kconfig | 9 +++
security/modpin/Makefile | 1 +
security/modpin/modpin.c | 197 ++++++++++++++++++++++++++++++++++++++++++++++
5 files changed, 215 insertions(+)
create mode 100644 security/modpin/Kconfig
create mode 100644 security/modpin/Makefile
create mode 100644 security/modpin/modpin.c

diff --git a/security/Kconfig b/security/Kconfig
index e9c6ac7..80172fd 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/modpin/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_MODPIN if SECURITY_MODPIN
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_MODPIN
+ bool "ModPin" if SECURITY_MODPIN=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 "modpin" if DEFAULT_SECURITY_MODPIN
default "yama" if DEFAULT_SECURITY_YAMA
default "" if DEFAULT_SECURITY_DAC

diff --git a/security/Makefile b/security/Makefile
index c26c81e..73d0a05 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-$(CONFIG_SECURITY_MODPIN) += modpin
subdir-$(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-$(CONFIG_SECURITY_MODPIN) += modpin/built-in.o
obj-$(CONFIG_SECURITY_YAMA) += yama/built-in.o
obj-$(CONFIG_CGROUP_DEVICE) += device_cgroup.o

diff --git a/security/modpin/Kconfig b/security/modpin/Kconfig
new file mode 100644
index 0000000..5be9dd5
--- /dev/null
+++ b/security/modpin/Kconfig
@@ -0,0 +1,9 @@
+config SECURITY_MODPIN
+ bool "Module filesystem origin pinning"
+ depends on SECURITY
+ help
+ Module loading will be pinned to the first filesystem used for
+ loading. Any modules that come from other filesystems will be
+ rejected. This is best used on systems without an initrd that
+ have a root filesystem backed by a read-only device such as
+ dm-verity or a CDROM.
diff --git a/security/modpin/Makefile b/security/modpin/Makefile
new file mode 100644
index 0000000..9080b29
--- /dev/null
+++ b/security/modpin/Makefile
@@ -0,0 +1 @@
+obj-$(CONFIG_SECURITY_MODPIN) += modpin.o
diff --git a/security/modpin/modpin.c b/security/modpin/modpin.c
new file mode 100644
index 0000000..107b4d8
--- /dev/null
+++ b/security/modpin/modpin.c
@@ -0,0 +1,197 @@
+/*
+ * Module Pinning Security Module
+ *
+ * Copyright 2011-2013 Google Inc.
+ *
+ * Authors:
+ * Kees Cook <keescook@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) "ModPin 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 void report_load_module(struct path *path, char *operation)
+{
+ char *alloced = NULL;
+ char *pathname; /* Pointer to either static string or "alloced". */
+
+ if (!path)
+ pathname = "<unknown>";
+ else {
+ /* We will allow 11 spaces for ' (deleted)' to be appended */
+ alloced = pathname = kmalloc(PATH_MAX+11, GFP_KERNEL);
+ if (!pathname)
+ pathname = "<no_memory>";
+ else {
+ pathname = d_path(path, pathname, PATH_MAX+11);
+ if (IS_ERR(pathname))
+ pathname = "<too_long>";
+ }
+ }
+
+ pr_notice("init_module %s module=%s pid=%d\n",
+ operation, pathname, task_pid_nr(current));
+
+ kfree(alloced);
+}
+
+static int modpin_enforced = 1;
+static struct dentry *pinned_root;
+static DEFINE_SPINLOCK(pinned_root_spinlock);
+
+#ifdef CONFIG_SYSCTL
+static int zero;
+static int one = 1;
+
+static struct ctl_path modpin_sysctl_path[] = {
+ { .procname = "kernel", },
+ { .procname = "modpin", },
+ { }
+};
+
+static struct ctl_table modpin_sysctl_table[] = {
+ {
+ .procname = "enforced",
+ .data = &modpin_enforced,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = proc_dointvec_minmax,
+ .extra1 = &zero,
+ .extra2 = &one,
+ },
+ { }
+};
+
+/*
+ * Check if the root device is read-only (e.g. dm-verity is enabled).
+ * This must be called after early kernel init, since only then is the
+ * rootdev available.
+ */
+static bool rootdev_readonly(void)
+{
+ bool rc;
+ struct block_device *bdev;
+ const fmode_t mode = FMODE_WRITE;
+
+ bdev = blkdev_get_by_dev(ROOT_DEV, mode, NULL);
+ if (IS_ERR(bdev)) {
+ /* In this weird case, assume it is read-only. */
+ pr_info("dev(%u,%u): FMODE_WRITE disallowed?!\n",
+ MAJOR(ROOT_DEV), MINOR(ROOT_DEV));
+ return true;
+ }
+
+ rc = bdev_read_only(bdev);
+ blkdev_put(bdev, mode);
+
+ pr_info("dev(%u,%u): %s\n", MAJOR(ROOT_DEV), MINOR(ROOT_DEV),
+ rc ? "read-only" : "writable");
+
+ return rc;
+}
+
+static void check_pinning_enforcement(void)
+{
+ /*
+ * If module pinning is not being enforced, allow sysctl to change
+ * modes for testing.
+ */
+ if (!rootdev_readonly()) {
+ if (!register_sysctl_paths(modpin_sysctl_path,
+ modpin_sysctl_table))
+ pr_notice("sysctl registration failed!\n");
+ else
+ pr_info("module pinning can be disabled.\n");
+ } else
+ pr_info("module pinning engaged.\n");
+}
+#else
+static void check_pinning_enforcement(void) { }
+#endif
+
+
+static int modpin_load_module(struct file *file)
+{
+ struct dentry *module_root;
+
+ if (!file) {
+ if (!modpin_enforced) {
+ report_load_module(NULL, "old-api-pinning-ignored");
+ return 0;
+ }
+
+ report_load_module(NULL, "old-api-denied");
+ return -EPERM;
+ }
+
+ module_root = file->f_path.mnt->mnt_root;
+
+ /* First loaded module defines the root for all others. */
+ spin_lock(&pinned_root_spinlock);
+ if (!pinned_root) {
+ pinned_root = dget(module_root);
+ /*
+ * Unlock now since it's only pinned_root we care about.
+ * In the worst case, we will (correctly) report pinning
+ * failures before we have announced that pinning is
+ * enabled. This would be purely cosmetic.
+ */
+ spin_unlock(&pinned_root_spinlock);
+ check_pinning_enforcement();
+ report_load_module(&file->f_path, "pinned");
+ return 0;
+ }
+ spin_unlock(&pinned_root_spinlock);
+
+ if (module_root != pinned_root) {
+ if (unlikely(!modpin_enforced)) {
+ report_load_module(&file->f_path, "pinning-ignored");
+ return 0;
+ }
+
+ report_load_module(&file->f_path, "denied");
+ return -EPERM;
+ }
+
+ return 0;
+}
+
+static struct security_operations modpin_ops = {
+ .name = "modpin",
+ .kernel_module_from_file = modpin_load_module,
+};
+
+static int __init modpin_init(void)
+{
+ int error;
+
+ error = register_security(&modpin_ops);
+
+ if (error)
+ panic("Could not register ModPin security module");
+
+ pr_info("ready to pin.\n");
+
+ return error;
+}
+security_initcall(modpin_init);
+
+module_param(modpin_enforced, int, S_IRUSR);
+MODULE_PARM_DESC(modpin_enforced, "Module pinning enforced (default: true)");
--
1.7.9.5

--
Kees Cook
Chrome OS Security


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