Re: [RFC PATCH v1] fw_lockdown: new micro LSM module to prevent loading unsigned firmware

Casey Schaufler
2017-11-10T22:46:00Z

On 11/10/2017 1:02 PM, Mimi Zohar wrote:
> If the kernel is locked down and IMA-appraisal is not enabled, prevent
> loading of unsigned firmware.
>
> Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
> ---
>
> Changelog v1:
> - Lots of minor changes Kconfig, Makefile, fw_lsm.c for such a small patch
>
> security/Kconfig | 1 +
> security/Makefile | 2 ++
> security/fw_lockdown/Kconfig | 6 +++++
> security/fw_lockdown/Makefile | 3 +++
> security/fw_lockdown/fw_lsm.c | 51 +++++++++++++++++++++++++++++++++++++++++++
> 5 files changed, 63 insertions(+)
> create mode 100644 security/fw_lockdown/Kconfig
> create mode 100644 security/fw_lockdown/Makefile
> create mode 100644 security/fw_lockdown/fw_lsm.c
>
> diff --git a/security/Kconfig b/security/Kconfig
> index a4fa8b826039..6e7e5888f823 100644
> --- a/security/Kconfig
> +++ b/security/Kconfig
> @@ -243,6 +243,7 @@ source security/tomoyo/Kconfig
> source security/apparmor/Kconfig
> source security/loadpin/Kconfig
> source security/yama/Kconfig
> +source security/fw_lockdown/Kconfig
>
> source security/integrity/Kconfig
>
> diff --git a/security/Makefile b/security/Makefile
> index 8c4a43e3d4e0..58852dee5e22 100644
> --- a/security/Makefile
> +++ b/security/Makefile
> @@ -9,6 +9,7 @@ subdir-$(CONFIG_SECURITY_TOMOYO) += tomoyo
> subdir-$(CONFIG_SECURITY_APPARMOR) += apparmor
> subdir-$(CONFIG_SECURITY_YAMA) += yama
> subdir-$(CONFIG_SECURITY_LOADPIN) += loadpin
> +subdir-$(CONFIG_SECURITY_FW_LOCKDOWN) += fw_lockdown
>
> # always enable default capabilities
> obj-y += commoncap.o
> @@ -24,6 +25,7 @@ obj-$(CONFIG_SECURITY_TOMOYO) += tomoyo/
> obj-$(CONFIG_SECURITY_APPARMOR) += apparmor/
> obj-$(CONFIG_SECURITY_YAMA) += yama/
> obj-$(CONFIG_SECURITY_LOADPIN) += loadpin/
> +obj-$(CONFIG_SECURITY_FW_LOCKDOWN) += fw_lockdown/
> obj-$(CONFIG_CGROUP_DEVICE) += device_cgroup.o
>
> # Object integrity file lists
> diff --git a/security/fw_lockdown/Kconfig b/security/fw_lockdown/Kconfig
> new file mode 100644
> index 000000000000..d6aef6ce8fee
> --- /dev/null
> +++ b/security/fw_lockdown/Kconfig
> @@ -0,0 +1,6 @@
> +config SECURITY_FW_LOCKDOWN
> + bool "Prevent loading unsigned firmware"
> + depends on LOCK_DOWN_KERNEL
> + default y
> + help
> + Prevent loading unsigned firmware in lockdown mode,
> diff --git a/security/fw_lockdown/Makefile b/security/fw_lockdown/Makefile
> new file mode 100644
> index 000000000000..3a16757fd35d
> --- /dev/null
> +++ b/security/fw_lockdown/Makefile
> @@ -0,0 +1,3 @@
> +obj-$(CONFIG_SECURITY_FW_LOCKDOWN) += fw_lockdown.o
> +
> +fw_lockdown-y := fw_lsm.o
> diff --git a/security/fw_lockdown/fw_lsm.c b/security/fw_lockdown/fw_lsm.c
> new file mode 100644
> index 000000000000..cce03a5c5280
> --- /dev/null
> +++ b/security/fw_lockdown/fw_lsm.c
> @@ -0,0 +1,51 @@
> +/*
> + * fw_lockdown security module
> + *
> + * Copyright (C) 2017 IBM Corporation
> + *
> + * Authors:
> + * Mimi Zohar <zohar@linux.vnet.ibm.com>
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License as published by
> + * the Free Software Foundation; either version 2 of the License, or
> + * (at your option) any later version.
> + */
> +
> +#define pr_fmt(fmt) "fw_lockdown: " fmt
> +
> +#include <linux/module.h>
> +#include <linux/ima.h>
> +#include <linux/lsm_hooks.h>
> +
> +/**
> + * fw_lockdown_read_file - prevent loading of unsigned firmware
> + * @file: pointer to firmware
> + * @read_id: caller identifier
> + *
> + * Prevent loading of unsigned firmware in lockdown mode.
> + */
> +static int fw_lockdown_read_file(struct file *file, enum kernel_read_file_id id)
> +{
> + if (id == READING_FIRMWARE) {
> + if (!is_ima_appraise_enabled() &&
> + !kernel_is_locked_down("Loading of unsigned firmware"))
> + return -EACCES;
> + }
> + return 0;
> +}
> +
> +static struct security_hook_list fw_lockdown_hooks[] = {
> + LSM_HOOK_INIT(kernel_read_file, fw_lockdown_read_file)
> +};
> +
> +static int __init init_fw_lockdown(void)
> +{
> + security_add_hooks(fw_lockdown_hooks, ARRAY_SIZE(fw_lockdown_hooks),
> + "fw_lockdown");

SECURITY_NAME_MAX is 10. Either pick a shorter name or increase
this value. I slightly favor an increase to 16.

> + pr_info("initialized\n");
> + return 0;
> +}
> +
> +late_initcall(init_fw_lockdown);
> +MODULE_LICENSE("GPL");