Re: [PATCH v4 6/6] LSM: LoadPin for kernel file loading restrictions

[Serge E. Hallyn]

Quoting Kees Cook (keescook@xxxxxxxxxxxx):
> 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>

Acked-by: Serge Hallyn <serge.hallyn@xxxxxxxxxxxxx>

> ---
>  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