Re: [RFC PATCH v2] fw_lockdown: new micro LSM module to prevent loading unsigned firmware

[Luis R. Rodriguez]

On Mon, Nov 13, 2017 at 06:43:34AM -0500, 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@xxxxxxxxxxxxxxxxxx>
> ---
> Changelog v2:
> - Invert kernel_is_locked_down() test (Luis Rodriquez)
> - Increase LSM name maximum size (15 bytes + null) (Casey)
> 
> 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 +++++++++++++++++++++++++++++++++++++++++++
>  security/security.c           |  2 +-
>  6 files changed, 64 insertions(+), 1 deletion(-)
>  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

For now depending on LOCK_DOWN_KERNEL makes sense given we have no alternative
default system policy. If hashing or a default key is used later for linux-firmware,
those alternatives could be added. I sprinkled some psuedo code of what I mean below.

> +	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..9a5472bc733f
> --- /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@xxxxxxxxxxxxxxxxxx>
> + *
> + * 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;
> +	}

How about just if (id != READING_FIRMWARE) return 0 right away so that
the real code of focus is not always indented. This could let the code
grow nicely.

What I meant above is later we may extend this with:

if hash_available()
	if !valid_hash()
		return -EACCES
else if default_fw_key_available()
	if !fw_signed_default_key()
		return -EACCES;

That could be the way we support a default system policy for firmware
signing, and it would not require any modifications to any firmware
API callers.

Notice though that if we later want to extend support for custom requirements
the semantics behind kernel_read_file() would not suffice to LSMify them, as
such I'd think we'd need another call which lets the security requirements
be passed.

Its unclear if IMA may want to ignore that criteria, as it does the checks in
userspace. If it *can* make use of it, it could do the check-in kernel, of
course.

> +	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");
> +	pr_info("initialized\n");
> +	return 0;
> +}
> +
> +late_initcall(init_fw_lockdown);
> +MODULE_LICENSE("GPL");
> diff --git a/security/security.c b/security/security.c
> index 4bf0f571b4ef..61a0c95ec687 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -32,7 +32,7 @@
>  #define MAX_LSM_EVM_XATTR	2
>  
>  /* Maximum number of letters for an LSM name string */
> -#define SECURITY_NAME_MAX	10
> +#define SECURITY_NAME_MAX	15

Should this small hunk be a separate atomic patch?

  Luis
>  
>  struct security_hook_heads security_hook_heads __lsm_ro_after_init;
>  static ATOMIC_NOTIFIER_HEAD(lsm_notifier_chain);
> -- 
> 2.7.4
> 
> 

-- 
Luis Rodriguez, SUSE LINUX GmbH
Maxfeldstrasse 5; D-90409 Nuernberg