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Date: Tue, 12 Apr 2016 16:44:28 -0500
From: "Serge E. Hallyn" <serge@...lyn.com>
To: Kees Cook <keescook@...omium.org>
Cc: James Morris <jmorris@...ei.org>, Joe Perches <joe@...ches.com>,
Mimi Zohar <zohar@...ux.vnet.ibm.com>,
Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
Andrew Morton <akpm@...ux-foundation.org>,
"Serge E. Hallyn" <serge@...lyn.com>,
Jonathan Corbet <corbet@....net>,
Kalle Valo <kvalo@...eaurora.org>,
Mauro Carvalho Chehab <mchehab@....samsung.com>,
Guenter Roeck <linux@...ck-us.net>,
Jiri Slaby <jslaby@...e.com>, Paul Moore <pmoore@...hat.com>,
Stephen Smalley <sds@...ho.nsa.gov>,
Casey Schaufler <casey@...aufler-ca.com>,
Andreas Gruenbacher <agruenba@...hat.com>,
Rasmus Villemoes <linux@...musvillemoes.dk>,
Ulf Hansson <ulf.hansson@...aro.org>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
linux-security-module@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v4 6/6] LSM: LoadPin for kernel file loading restrictions
Quoting Kees Cook (keescook@...omium.org):
> 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@...omium.org>
Acked-by: Serge Hallyn <serge.hallyn@...onical.com>
> ---
> 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@...omium.org>
> +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@...omium.org>
> 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@...omium.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) "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
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