[PATCH v4 6/6] LSM: LoadPin for kernel file loading restrictions
From: Kees Cook
Date: Tue Apr 12 2016 - 12:56:25 EST
This LSM enforces that kernel-loaded files (modules, firmware, etc)
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 and/or unchangeable
filesystem to enforce module and firmware loading restrictions without
needing to sign the files individually.
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
Documentation/security/LoadPin.txt | 17 ++++
MAINTAINERS | 6 ++
include/linux/lsm_hooks.h | 5 +
security/Kconfig | 1 +
security/Makefile | 2 +
security/loadpin/Kconfig | 10 ++
security/loadpin/Makefile | 1 +
security/loadpin/loadpin.c | 190 +++++++++++++++++++++++++++++++++++++
security/security.c | 1 +
9 files changed, 233 insertions(+)
create mode 100644 Documentation/security/LoadPin.txt
create mode 100644 security/loadpin/Kconfig
create mode 100644 security/loadpin/Makefile
create mode 100644 security/loadpin/loadpin.c
diff --git a/Documentation/security/LoadPin.txt b/Documentation/security/LoadPin.txt
new file mode 100644
index 000000000000..e11877f5d3d4
--- /dev/null
+++ b/Documentation/security/LoadPin.txt
@@ -0,0 +1,17 @@
+LoadPin is a Linux Security Module that ensures all kernel-loaded files
+(modules, firmware, etc) all originate 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
+and/or unchangeable filesystem to enforce module and firmware loading
+restrictions without needing to sign the files individually.
+
+The LSM is selectable at build-time with CONFIG_SECURITY_LOADPIN, and
+can be controlled at boot-time with the kernel command line option
+"loadpin.enabled". By default, it is enabled, but can be disabled at
+boot ("loadpin.enabled=0").
+
+LoadPin starts pinning when it sees the first file loaded. If the
+block device backing the filesystem is not read-only, a sysctl is
+created to toggle pinning: /proc/sys/kernel/loadpin/enabled. (Having
+a mutable filesystem means pinning is mutable too, but having the
+sysctl allows for easy testing on systems with a mutable filesystem.)
diff --git a/MAINTAINERS b/MAINTAINERS
index 40eb1dbe2ae5..de4cf8e9247e 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -9964,6 +9964,12 @@ T: git git://git.kernel.org/pub/scm/linux/kernel/git/jj/apparmor-dev.git
S: Supported
F: security/apparmor/
+LOADPIN SECURITY MODULE
+M: Kees Cook <keescook@xxxxxxxxxxxx>
+T: git git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git lsm/loadpin
+S: Supported
+F: security/loadpin/
+
YAMA SECURITY MODULE
M: Kees Cook <keescook@xxxxxxxxxxxx>
T: git git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git yama/tip
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index cdee11cbcdf1..f3402aab1927 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -1893,5 +1893,10 @@ extern void __init yama_add_hooks(void);
#else
static inline void __init yama_add_hooks(void) { }
#endif
+#ifdef CONFIG_SECURITY_LOADPIN
+void __init loadpin_add_hooks(void);
+#else
+static inline void loadpin_add_hooks(void) { };
+#endif
#endif /* ! __LINUX_LSM_HOOKS_H */
diff --git a/security/Kconfig b/security/Kconfig
index e45237897b43..176758cdfa57 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -122,6 +122,7 @@ source security/selinux/Kconfig
source security/smack/Kconfig
source security/tomoyo/Kconfig
source security/apparmor/Kconfig
+source security/loadpin/Kconfig
source security/yama/Kconfig
source security/integrity/Kconfig
diff --git a/security/Makefile b/security/Makefile
index c9bfbc84ff50..f2d71cdb8e19 100644
--- a/security/Makefile
+++ b/security/Makefile
@@ -8,6 +8,7 @@ subdir-$(CONFIG_SECURITY_SMACK) += smack
subdir-$(CONFIG_SECURITY_TOMOYO) += tomoyo
subdir-$(CONFIG_SECURITY_APPARMOR) += apparmor
subdir-$(CONFIG_SECURITY_YAMA) += yama
+subdir-$(CONFIG_SECURITY_LOADPIN) += loadpin
# always enable default capabilities
obj-y += commoncap.o
@@ -22,6 +23,7 @@ obj-$(CONFIG_AUDIT) += lsm_audit.o
obj-$(CONFIG_SECURITY_TOMOYO) += tomoyo/
obj-$(CONFIG_SECURITY_APPARMOR) += apparmor/
obj-$(CONFIG_SECURITY_YAMA) += yama/
+obj-$(CONFIG_SECURITY_LOADPIN) += loadpin/
obj-$(CONFIG_CGROUP_DEVICE) += device_cgroup.o
# Object integrity file lists
diff --git a/security/loadpin/Kconfig b/security/loadpin/Kconfig
new file mode 100644
index 000000000000..c668ac4eda65
--- /dev/null
+++ b/security/loadpin/Kconfig
@@ -0,0 +1,10 @@
+config SECURITY_LOADPIN
+ bool "Pin load of kernel files (modules, fw, etc) to one filesystem"
+ depends on SECURITY && BLOCK
+ help
+ Any files read through the kernel file reading interface
+ (kernel modules, firmware, kexec images, security policy) will
+ be pinned to the first filesystem used for loading. Any files
+ that come from other filesystems will be rejected. This is best
+ used on systems without an initrd that have a root filesystem
+ backed by a read-only device such as dm-verity or a CDROM.
diff --git a/security/loadpin/Makefile b/security/loadpin/Makefile
new file mode 100644
index 000000000000..c2d77f83037b
--- /dev/null
+++ b/security/loadpin/Makefile
@@ -0,0 +1 @@
+obj-$(CONFIG_SECURITY_LOADPIN) += loadpin.o
diff --git a/security/loadpin/loadpin.c b/security/loadpin/loadpin.c
new file mode 100644
index 000000000000..e4debae3c4d6
--- /dev/null
+++ b/security/loadpin/loadpin.c
@@ -0,0 +1,190 @@
+/*
+ * Module and Firmware Pinning Security Module
+ *
+ * Copyright 2011-2016 Google Inc.
+ *
+ * Author: Kees Cook <keescook@xxxxxxxxxxxx>
+ *
+ * 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) "LoadPin: " fmt
+
+#include <linux/module.h>
+#include <linux/fs.h>
+#include <linux/fs_struct.h>
+#include <linux/lsm_hooks.h>
+#include <linux/mount.h>
+#include <linux/path.h>
+#include <linux/sched.h> /* current */
+#include <linux/string_helpers.h>
+
+static void report_load(const char *origin, struct file *file, char *operation)
+{
+ char *cmdline, *pathname;
+
+ pathname = kstrdup_quotable_file(file, GFP_KERNEL);
+ cmdline = kstrdup_quotable_cmdline(current, GFP_KERNEL);
+
+ pr_notice("%s %s obj=%s%s%s pid=%d cmdline=%s%s%s\n",
+ origin, operation,
+ (pathname && pathname[0] != '<') ? "\"" : "",
+ pathname,
+ (pathname && pathname[0] != '<') ? "\"" : "",
+ task_pid_nr(current),
+ cmdline ? "\"" : "", cmdline, cmdline ? "\"" : "");
+
+ kfree(cmdline);
+ kfree(pathname);
+}
+
+static int enabled = 1;
+static struct super_block *pinned_root;
+static DEFINE_SPINLOCK(pinned_root_spinlock);
+
+#ifdef CONFIG_SYSCTL
+static int zero;
+static int one = 1;
+
+static struct ctl_path loadpin_sysctl_path[] = {
+ { .procname = "kernel", },
+ { .procname = "loadpin", },
+ { }
+};
+
+static struct ctl_table loadpin_sysctl_table[] = {
+ {
+ .procname = "enabled",
+ .data = &enabled,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = proc_dointvec_minmax,
+ .extra1 = &zero,
+ .extra2 = &one,
+ },
+ { }
+};
+
+/*
+ * This must be called after early kernel init, since then the rootdev
+ * is available.
+ */
+static void check_pinning_enforcement(struct super_block *mnt_sb)
+{
+ bool ro = false;
+
+ /*
+ * If load pinning is not enforced via a read-only block
+ * device, allow sysctl to change modes for testing.
+ */
+ if (mnt_sb->s_bdev) {
+ ro = bdev_read_only(mnt_sb->s_bdev);
+ pr_info("dev(%u,%u): %s\n",
+ MAJOR(mnt_sb->s_bdev->bd_dev),
+ MINOR(mnt_sb->s_bdev->bd_dev),
+ ro ? "read-only" : "writable");
+ } else
+ pr_info("mnt_sb lacks block device, treating as: writable\n");
+
+ if (!ro) {
+ if (!register_sysctl_paths(loadpin_sysctl_path,
+ loadpin_sysctl_table))
+ pr_notice("sysctl registration failed!\n");
+ else
+ pr_info("load pinning can be disabled.\n");
+ } else
+ pr_info("load pinning engaged.\n");
+}
+#else
+static void check_pinning_enforcement(struct super_block *mnt_sb)
+{
+ pr_info("load pinning engaged.\n");
+}
+#endif
+
+static void loadpin_sb_free_security(struct super_block *mnt_sb)
+{
+ /*
+ * When unmounting the filesystem we were using for load
+ * pinning, we acknowledge the superblock release, but make sure
+ * no other modules or firmware can be loaded.
+ */
+ if (!IS_ERR_OR_NULL(pinned_root) && mnt_sb == pinned_root) {
+ pinned_root = ERR_PTR(-EIO);
+ pr_info("umount pinned fs: refusing further loads\n");
+ }
+}
+
+static int loadpin_read_file(struct file *file, enum kernel_read_file_id id)
+{
+ struct super_block *load_root;
+ const char *origin = kernel_read_file_id_str(id);
+
+ /* This handles the older init_module API that has a NULL file. */
+ if (!file) {
+ if (!enabled) {
+ report_load(origin, NULL, "old-api-pinning-ignored");
+ return 0;
+ }
+
+ report_load(origin, NULL, "old-api-denied");
+ return -EPERM;
+ }
+
+ load_root = file->f_path.mnt->mnt_sb;
+
+ /* First loaded module/firmware defines the root for all others. */
+ spin_lock(&pinned_root_spinlock);
+ /*
+ * pinned_root is only NULL at startup. Otherwise, it is either
+ * a valid reference, or an ERR_PTR.
+ */
+ if (!pinned_root) {
+ pinned_root = load_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(pinned_root);
+ report_load(origin, file, "pinned");
+ } else {
+ spin_unlock(&pinned_root_spinlock);
+ }
+
+ if (IS_ERR_OR_NULL(pinned_root) || load_root != pinned_root) {
+ if (unlikely(!enabled)) {
+ report_load(origin, file, "pinning-ignored");
+ return 0;
+ }
+
+ report_load(origin, file, "denied");
+ return -EPERM;
+ }
+
+ return 0;
+}
+
+static struct security_hook_list loadpin_hooks[] = {
+ LSM_HOOK_INIT(sb_free_security, loadpin_sb_free_security),
+ LSM_HOOK_INIT(kernel_read_file, loadpin_read_file),
+};
+
+void __init loadpin_add_hooks(void)
+{
+ pr_info("ready to pin (currently %sabled)", enabled ? "en" : "dis");
+ security_add_hooks(loadpin_hooks, ARRAY_SIZE(loadpin_hooks));
+}
+
+/* Should not be mutable after boot, so not listed in sysfs (perm == 0). */
+module_param(enabled, int, 0);
+MODULE_PARM_DESC(enabled, "Pin module/firmware loading (default: true)");
diff --git a/security/security.c b/security/security.c
index 3644b0344d29..ce02178c892f 100644
--- a/security/security.c
+++ b/security/security.c
@@ -60,6 +60,7 @@ int __init security_init(void)
*/
capability_add_hooks();
yama_add_hooks();
+ loadpin_add_hooks();
/*
* Load all the remaining security modules.
--
2.6.3