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Date:   Mon, 5 Jun 2017 00:07:45 +0200
From:   Mickaël Salaün <mic@...ikod.net>
To:     Matt Brown <matt@...tt.com>, james.l.morris@...cle.com,
        serge@...lyn.com
Cc:     linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        kernel-hardening@...ts.openwall.com,
        Corey Henderson <corman@...mander.com>
Subject: Re: [kernel-hardening] [PATCH v1 1/1] Add Trusted Path Execution as a
 stackable LSM

As was pointed out to me, the first grsecurity's implementation of TPE
date back to earlier days (before Git was used for Linux):
https://github.com/linux-scraping/grsecurity-patches/blob/master/grsec-2.4.5/grsecurity-1.4-LIDS-2.4.5.patch

There seem to be multiple implementations inspired by the Phrack
articles. This may be worth to take a look at this different approaches.

 Mickaël

On 04/06/2017 18:43, Mickaël Salaün wrote:
> Hi,
> 
> If you want to get some information about the history of TPE in
> grsecurity, take a look at
> https://github.com/linux-scraping/linux-grsecurity/ and run git log
> grsecurity/grsec_tpe.c
> 
> Here are some links about TPE (before grsecurity used it):
> * http://phrack.org/issues/52/6.html#article
> * http://phrack.org/issues/53/8.html#article
> * https://lwn.net/Articles/32087/
> *
> https://www.usenix.org/legacy/event/usenix04/tech/freenix/full_papers/rahimi/rahimi_html/
> 
> You may want to adjust the credits.
> 
> A more flexible way to configure TPE options (sysctl) may be considered too.
> 
> Regards,
>  Mickaël
> 
> On 03/06/2017 07:53, Matt Brown wrote:
>> This patch was modified from Brad Spengler's Trusted Path Execution (TPE)
>> feature in Grsecurity and also incorporates logging ideas from
>> cormander's tpe-lkm.
>>
>> Modifications from the Grsecurity implementation of TPE were made to
>> turn it into a stackable LSM using the existing LSM hook bprm_set_creds.
>> Also, denial messages were improved by including the full path of the
>> disallowed program. (This idea was taken from cormander's tpe-lkm)
>>
>> Trusted Path Execution is not a new idea:
>>
>> http://phrack.org/issues/52/6.html#article
>>
>> | A trusted path is one that is inside is a root owned directory that
>> | is not group or world writable.  /bin, /usr/bin, /usr/local/bin, are
>> | (under normal circumstances) considered trusted.  Any non-root
>> | users home directory is not trusted, nor is /tmp.
>>
>> This Trusted Path Execution implementation introduces the following
>> Kconfig options and sysctls. These config behaviors are taken straight
>> from Grsecurity's implementation.
>>
>> CONFIG_SECURITY_TPE (sysctl=kernel.tpe.enabled)
>>
>> This option enables Trusted Path Execution. TPE blocks *untrusted*
>> users from executing files that meet the following conditions:
>>
>> * file is not in a root-owned directory
>> * file is writable by a user other than root
>>
>> NOTE: root is never restricted by TPE
>>
>> CONFIG_SECURITY_TPE_GID (sysctl=kernel.tpe.gid)
>>
>> This option defines a group id that, by default, is the untrusted group.
>> If a user is untrusted then it has the checks described in
>> CONFIG_SECURITY_TPE applied. Otherwise, the user is trusted and the
>> checks are not applied. Since root is never restricted by TPE, you can
>> effectively remove the concept of a trusted or untrusted group by
>> setting this value to 0.
>>
>> CONFIG_SECURITY_TPE_ALL (sysctl=kernel.tpe.restrict_all)
>>
>> This option applies another set of restrictions to all non-root users
>> even if they are trusted. This only allows execution under the
>> following conditions:
>>
>> * file is in a directory owned by the user that is not group or
>>   world-writable
>> * file is in a directory owned by root and writable only by root
>>
>> CONFIG_SECURITY_TPE_INVERT (sysctl=kernel.tpe.gid_invert)
>>
>> This option reverses the trust logic of the gid option and makes
>> kernel.tpe.gid into the trusted group. This means that all other groups
>> become untrusted. This sysctl is helpful when you want TPE restrictions
>> to apply to most of the users on the system.
>>
>> Threat Models:
>>
>> 1. Attacker on system executing exploit on system vulnerability
>>
>> *  If attacker uses a binary as a part of their system exploit, TPE can
>>    frustrate their efforts
>>
>> *  Issues:
>>    *  Can be bypassed by interpreted languages such as python. You can run
>>       malicious code by doing: python -c 'evil code'
>>
>> 2. Attacker on system replaces binary used by a privileged user with a
>>    malicious one
>>
>> *  This situation arises when administrator of a system leaves a binary
>>    as world writable.
>>
>> *  TPE is very effective against this threat model
>>
>> Signed-off-by: Matt Brown <matt@...tt.com>
>> ---
>>  MAINTAINERS               |   5 ++
>>  include/linux/lsm_hooks.h |   5 ++
>>  security/Kconfig          |   1 +
>>  security/Makefile         |   2 +
>>  security/security.c       |   1 +
>>  security/tpe/Kconfig      |  57 +++++++++++++++
>>  security/tpe/Makefile     |   3 +
>>  security/tpe/tpe_lsm.c    | 175 ++++++++++++++++++++++++++++++++++++++++++++++
>>  8 files changed, 249 insertions(+)
>>  create mode 100644 security/tpe/Kconfig
>>  create mode 100644 security/tpe/Makefile
>>  create mode 100644 security/tpe/tpe_lsm.c
>>
>> diff --git a/MAINTAINERS b/MAINTAINERS
>> index 38d3e4e..1952bd6 100644
>> --- a/MAINTAINERS
>> +++ b/MAINTAINERS
>> @@ -11357,6 +11357,11 @@ T:	git git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git yama/tip
>>  S:	Supported
>>  F:	security/yama/
>>  
>> +TPE SECURITY MODULE
>> +M:	Matt Brown <matt@...tt.com>
>> +S:	Supported
>> +F:	security/tpe/
>> +
>>  SENSABLE PHANTOM
>>  M:	Jiri Slaby <jirislaby@...il.com>
>>  S:	Maintained
>> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
>> index e29d4c6..d017f49 100644
>> --- a/include/linux/lsm_hooks.h
>> +++ b/include/linux/lsm_hooks.h
>> @@ -1920,5 +1920,10 @@ void __init loadpin_add_hooks(void);
>>  #else
>>  static inline void loadpin_add_hooks(void) { };
>>  #endif
>> +#ifdef CONFIG_SECURITY_TPE
>> +void __init tpe_add_hooks(void);
>> +#else
>> +static inline void tpe_add_hooks(void) { };
>> +#endif
>>  
>>  #endif /* ! __LINUX_LSM_HOOKS_H */
>> diff --git a/security/Kconfig b/security/Kconfig
>> index 34fb609..30e60cd 100644
>> --- a/security/Kconfig
>> +++ b/security/Kconfig
>> @@ -245,6 +245,7 @@ source security/tomoyo/Kconfig
>>  source security/apparmor/Kconfig
>>  source security/loadpin/Kconfig
>>  source security/yama/Kconfig
>> +source security/tpe/Kconfig
>>  
>>  source security/integrity/Kconfig
>>  
>> diff --git a/security/Makefile b/security/Makefile
>> index f2d71cd..f8b5197 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-$(CONFIG_SECURITY_TPE)		+= tpe
>>  
>>  # always enable default capabilities
>>  obj-y					+= commoncap.o
>> @@ -24,6 +25,7 @@ obj-$(CONFIG_SECURITY_TOMOYO)		+= tomoyo/
>>  obj-$(CONFIG_SECURITY_APPARMOR)		+= apparmor/
>>  obj-$(CONFIG_SECURITY_YAMA)		+= yama/
>>  obj-$(CONFIG_SECURITY_LOADPIN)		+= loadpin/
>> +obj-$(CONFIG_SECURITY_TPE)		+= tpe/
>>  obj-$(CONFIG_CGROUP_DEVICE)		+= device_cgroup.o
>>  
>>  # Object integrity file lists
>> diff --git a/security/security.c b/security/security.c
>> index d0e07f2..ab0dc26 100644
>> --- a/security/security.c
>> +++ b/security/security.c
>> @@ -62,6 +62,7 @@ int __init security_init(void)
>>  	capability_add_hooks();
>>  	yama_add_hooks();
>>  	loadpin_add_hooks();
>> +	tpe_add_hooks();
>>  
>>  	/*
>>  	 * Load all the remaining security modules.
>> diff --git a/security/tpe/Kconfig b/security/tpe/Kconfig
>> new file mode 100644
>> index 0000000..84fe1b7
>> --- /dev/null
>> +++ b/security/tpe/Kconfig
>> @@ -0,0 +1,57 @@
>> +config SECURITY_TPE
>> +	bool "Trusted Path Execution (TPE)"
>> +	default n
>> +	help
>> +	  If you say Y here, you will be able to choose a gid to add to the
>> +	  supplementary groups of users you want to mark as "untrusted."
>> +	  These users will not be able to execute any files that are not in
>> +	  root-owned directories writable only by root.  If the sysctl option
>> +	  is enabled, a sysctl option with name "tpe" is created.
>> +
>> +config SECURITY_TPE_ALL
>> +	bool "Partially restrict all non-root users"
>> +	depends on SECURITY_TPE
>> +	help
>> +	  If you say Y here, all non-root users will be covered under
>> +	  a weaker TPE restriction.  This is separate from, and in addition to,
>> +	  the main TPE options that you have selected elsewhere.  Thus, if a
>> +	  "trusted" GID is chosen, this restriction applies to even that GID.
>> +	  Under this restriction, all non-root users will only be allowed to
>> +	  execute files in directories they own that are not group or
>> +	  world-writable, or in directories owned by root and writable only by
>> +	  root.  If the sysctl option is enabled, a sysctl option with name
>> +	  "tpe_restrict_all" is created.
>> +
>> +config SECURITY_TPE_INVERT
>> +	bool "Invert GID option"
>> +	depends on SECURITY_TPE
>> +	help
>> +	  If you say Y here, the group you specify in the TPE configuration will
>> +	  decide what group TPE restrictions will be *disabled* for.  This
>> +	  option is useful if you want TPE restrictions to be applied to most
>> +	  users on the system.  If the sysctl option is enabled, a sysctl option
>> +	  with name "tpe_invert" is created.  Unlike other sysctl options, this
>> +	  entry will default to on for backward-compatibility.
>> +
>> +config SECURITY_TPE_GID
>> +	int
>> +	default SECURITY_TPE_UNTRUSTED_GID if (SECURITY_TPE && !SECURITY_TPE_INVERT)
>> +	default SECURITY_TPE_TRUSTED_GID if (SECURITY_TPE && SECURITY_TPE_INVERT)
>> +
>> +config SECURITY_TPE_UNTRUSTED_GID
>> +	int "GID for TPE-untrusted users"
>> +	depends on SECURITY_TPE && !SECURITY_TPE_INVERT
>> +	default 1005
>> +	help
>> +	  Setting this GID determines what group TPE restrictions will be
>> +	  *enabled* for.  If the sysctl option is enabled, a sysctl option
>> +	  with name "tpe_gid" is created.
>> +
>> +config SECURITY_TPE_TRUSTED_GID
>> +	int "GID for TPE-trusted users"
>> +	depends on SECURITY_TPE && SECURITY_TPE_INVERT
>> +	default 1005
>> +	help
>> +	  Setting this GID determines what group TPE restrictions will be
>> +	  *disabled* for.  If the sysctl option is enabled, a sysctl option
>> +	  with name "tpe_gid" is created.
>> diff --git a/security/tpe/Makefile b/security/tpe/Makefile
>> new file mode 100644
>> index 0000000..e1bd8ef
>> --- /dev/null
>> +++ b/security/tpe/Makefile
>> @@ -0,0 +1,3 @@
>> +obj-$(CONFIG_SECURITY_TPE) := tpe_lsm.o
>> +
>> +tpe-y := tpe_lsm.o
>> diff --git a/security/tpe/tpe_lsm.c b/security/tpe/tpe_lsm.c
>> new file mode 100644
>> index 0000000..075ca02
>> --- /dev/null
>> +++ b/security/tpe/tpe_lsm.c
>> @@ -0,0 +1,175 @@
>> +/*
>> + * Trusted Path Execution Security Module
>> + *
>> + * Copyright 2017 Matt Brown
>> + *
>> + * Author: Matt Brown <matt@...tt.com>
>> + *
>> + * 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.
>> + */
>> +#include <linux/kernel.h>
>> +#include <linux/uidgid.h>
>> +#include <linux/ratelimit.h>
>> +#include <linux/limits.h>
>> +#include <linux/cred.h>
>> +#include <linux/slab.h>
>> +#include <linux/lsm_hooks.h>
>> +#include <linux/sysctl.h>
>> +#include <linux/binfmts.h>
>> +#include <linux/string_helpers.h>
>> +
>> +#define TPE_GLOBAL_UID(x) from_kuid_munged(&init_user_ns, (x))
>> +#define TPE_GLOBAL_GID(x) from_kgid_munged(&init_user_ns, (x))
>> +#define global_root(x) uid_eq((x), GLOBAL_ROOT_UID)
>> +#define global_nonroot(x) (!uid_eq((x), GLOBAL_ROOT_UID))
>> +#define global_nonroot_gid(x) (!gid_eq((x), GLOBAL_ROOT_GID))
>> +
>> +static int tpe_enabled __read_mostly = IS_ENABLED(CONFIG_SECURITY_TPE);
>> +static kgid_t tpe_gid __read_mostly = KGIDT_INIT(CONFIG_SECURITY_TPE_GID);
>> +static int tpe_all __read_mostly = IS_ENABLED(CONFIG_SECURITY_TPE_ALL);
>> +static int tpe_invert __read_mostly = IS_ENABLED(CONFIG_SECURITY_TPE_INVERT);
>> +
>> +int print_tpe_error(struct file *file, char *reason1, char *reason2)
>> +{
>> +	char *filepath;
>> +
>> +	filepath = kstrdup_quotable_file(file, GFP_KERNEL);
>> +
>> +	if (!filepath)
>> +		return -ENOMEM;
>> +
>> +	pr_warn_ratelimited("TPE: Denied execution of %s Reason: %s%s%s\n",
>> +		(IS_ERR(filepath) ? "failed fetching file path" : filepath),
>> +		reason1, reason2 ? " and " : "", reason2 ?: "");
>> +	kfree(filepath);
>> +	return -EPERM;
>> +}
>> +
>> +/*
>> + * Return 0 if the hook is successful and permission is granted.
>> + * Otherwise return the proper error message
>> + *
>> + */
>> +static int tpe_bprm_set_creds(struct linux_binprm *bprm)
>> +{
>> +	struct file *file = bprm->file;
>> +	struct inode *inode = d_backing_inode(file->f_path.dentry->d_parent);
>> +	struct inode *file_inode = d_backing_inode(file->f_path.dentry);
>> +	const struct cred *cred = current_cred();
>> +	char *reason1 = NULL;
>> +	char *reason2 = NULL;
>> +
>> +	if (!tpe_enabled)
>> +		return 0;
>> +
>> +	/* never restrict root */
>> +	if (global_root(cred->uid))
>> +		return 0;
>> +
>> +	if (!tpe_all)
>> +		goto general_tpe_check;
>> +
>> +	/* TPE_ALL: restrictions enforced even if the gid is trusted */
>> +	if (global_nonroot(inode->i_uid) && !uid_eq(inode->i_uid, cred->uid))
>> +		reason1 = "directory not owned by user";
>> +	else if (inode->i_mode & 0002)
>> +		reason1 = "file in world-writable directory";
>> +	else if ((inode->i_mode & 0020) && global_nonroot_gid(inode->i_gid))
>> +		reason1 = "file in group-writable directory";
>> +	else if (file_inode->i_mode & 0002)
>> +		reason1 = "file is world-writable";
>> +
>> +	if (reason1)
>> +		goto end;
>> +
>> +general_tpe_check:
>> +	/* determine if group is trusted */
>> +	if (tpe_invert && !in_group_p(tpe_gid))
>> +		reason2 = "not in trusted group";
>> +	else if (!tpe_invert && in_group_p(tpe_gid))
>> +		reason2 = "in untrusted group";
>> +	else
>> +		return 0;
>> +
>> +	/* main TPE checks */
>> +	if (global_nonroot(inode->i_uid))
>> +		reason1 = "file in non-root-owned directory";
>> +	else if (inode->i_mode & 0002)
>> +		reason1 = "file in world-writable directory";
>> +	else if ((inode->i_mode & 0020) && global_nonroot_gid(inode->i_gid))
>> +		reason1 = "file in group-writable directory";
>> +	else if (file_inode->i_mode & 0002)
>> +		reason1 = "file is world-writable";
>> +
>> +end:
>> +	if (reason1)
>> +		return print_tpe_error(file, reason1, reason2);
>> +	else
>> +		return 0;
>> +}
>> +
>> +static struct security_hook_list tpe_hooks[] = {
>> +	LSM_HOOK_INIT(bprm_set_creds, tpe_bprm_set_creds),
>> +};
>> +
>> +#ifdef CONFIG_SYSCTL
>> +struct ctl_path tpe_sysctl_path[] = {
>> +	{ .procname = "kernel", },
>> +	{ .procname = "tpe", },
>> +	{ }
>> +};
>> +
>> +static struct ctl_table tpe_sysctl_table[] = {
>> +	{
>> +		.procname	= "enabled",
>> +		.data		= &tpe_enabled,
>> +		.maxlen		= sizeof(int),
>> +		.mode		= 0600,
>> +		.proc_handler	= proc_dointvec,
>> +	},
>> +	{
>> +		.procname	= "gid",
>> +		.data		= &tpe_gid,
>> +		.maxlen		= sizeof(int),
>> +		.mode		= 0600,
>> +		.proc_handler	= proc_dointvec,
>> +	},
>> +	{
>> +		.procname	= "gid_invert",
>> +		.data		= &tpe_invert,
>> +		.maxlen		= sizeof(int),
>> +		.mode		= 0600,
>> +		.proc_handler	= proc_dointvec,
>> +	},
>> +	{
>> +		.procname	= "restrict_all",
>> +		.data		= &tpe_all,
>> +		.maxlen		= sizeof(int),
>> +		.mode		= 0600,
>> +		.proc_handler	= proc_dointvec,
>> +	},
>> +	{ }
>> +};
>> +static void __init tpe_init_sysctl(void)
>> +{
>> +	if (!register_sysctl_paths(tpe_sysctl_path, tpe_sysctl_table))
>> +		panic("TPE: sysctl registration failed.\n");
>> +}
>> +#else
>> +static inline void tpe_init_sysctl(void) { }
>> +#endif /* CONFIG_SYSCTL */
>> +
>> +
>> +void __init tpe_add_hooks(void)
>> +{
>> +	pr_info("TPE: securing systems like it's 1998\n");
>> +	security_add_hooks(tpe_hooks, ARRAY_SIZE(tpe_hooks), "tpe");
>> +	tpe_init_sysctl();
>> +}
>>
> 



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