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Date: Thu, 3 Oct 2013 13:56:29 -0700
From: Kees Cook <kees@...flux.net>
To: Kees Cook <keescook@...omium.org>
Cc: linux-kernel@...r.kernel.org,
James Morris <james.l.morris@...cle.com>,
Vivek Goyal <vgoyal@...hat.com>,
Tyler Hicks <tyhicks@...onical.com>,
Casey Schaufler <casey@...aufler-ca.com>,
linux-security-module@...r.kernel.org
Subject: Re: [PATCH] LSM: MntRestrict blocks mounts on symlink targets
Pinging on this too. Any feedback?
-Kees
On Sat, Sep 21, 2013 at 03:59:59PM -0700, Kees Cook wrote:
> On systems where certain filesystem contents cannot be entirely trusted,
> it is beneficial to block mounts on symlinks. This makes sure that
> malicious filesystem contents cannot trigger the over-mounting of trusted
> filesystems. (For example, a bind-mounted subdirectory of /var cannot be
> redirected to mount on /etc via a symlink: a daemon cannot elevate privs
> to uid-0.)
>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> security/Kconfig | 6 ++++
> security/Makefile | 2 ++
> security/mntrestrict/Kconfig | 13 ++++++++
> security/mntrestrict/Makefile | 1 +
> security/mntrestrict/mntrestrict.c | 63 ++++++++++++++++++++++++++++++++++++
> 5 files changed, 85 insertions(+)
> create mode 100644 security/mntrestrict/Kconfig
> create mode 100644 security/mntrestrict/Makefile
> create mode 100644 security/mntrestrict/mntrestrict.c
>
> diff --git a/security/Kconfig b/security/Kconfig
> index e9c6ac7..84d41f4 100644
> --- a/security/Kconfig
> +++ b/security/Kconfig
> @@ -121,6 +121,7 @@ source security/selinux/Kconfig
> source security/smack/Kconfig
> source security/tomoyo/Kconfig
> source security/apparmor/Kconfig
> +source security/mntrestrict/Kconfig
> source security/yama/Kconfig
>
> source security/integrity/Kconfig
> @@ -131,6 +132,7 @@ choice
> default DEFAULT_SECURITY_SMACK if SECURITY_SMACK
> default DEFAULT_SECURITY_TOMOYO if SECURITY_TOMOYO
> default DEFAULT_SECURITY_APPARMOR if SECURITY_APPARMOR
> + default DEFAULT_SECURITY_MNTRESTRICT if SECURITY_MNTRESTRICT
> default DEFAULT_SECURITY_YAMA if SECURITY_YAMA
> default DEFAULT_SECURITY_DAC
>
> @@ -150,6 +152,9 @@ choice
> config DEFAULT_SECURITY_APPARMOR
> bool "AppArmor" if SECURITY_APPARMOR=y
>
> + config DEFAULT_SECURITY_MNTRESTRICT
> + bool "MntRestrict" if SECURITY_MNTRESTRICT=y
> +
> config DEFAULT_SECURITY_YAMA
> bool "Yama" if SECURITY_YAMA=y
>
> @@ -164,6 +169,7 @@ config DEFAULT_SECURITY
> default "smack" if DEFAULT_SECURITY_SMACK
> default "tomoyo" if DEFAULT_SECURITY_TOMOYO
> default "apparmor" if DEFAULT_SECURITY_APPARMOR
> + default "mntrestrict" if DEFAULT_SECURITY_MNTRESTRICT
> default "yama" if DEFAULT_SECURITY_YAMA
> default "" if DEFAULT_SECURITY_DAC
>
> diff --git a/security/Makefile b/security/Makefile
> index c26c81e..33f38b1 100644
> --- a/security/Makefile
> +++ b/security/Makefile
> @@ -7,6 +7,7 @@ subdir-$(CONFIG_SECURITY_SELINUX) += selinux
> subdir-$(CONFIG_SECURITY_SMACK) += smack
> subdir-$(CONFIG_SECURITY_TOMOYO) += tomoyo
> subdir-$(CONFIG_SECURITY_APPARMOR) += apparmor
> +subdir-$(CONFIG_SECURITY_MNTRESTRICT) += mntrestrict
> subdir-$(CONFIG_SECURITY_YAMA) += yama
>
> # always enable default capabilities
> @@ -22,6 +23,7 @@ obj-$(CONFIG_SECURITY_SMACK) += smack/built-in.o
> obj-$(CONFIG_AUDIT) += lsm_audit.o
> obj-$(CONFIG_SECURITY_TOMOYO) += tomoyo/built-in.o
> obj-$(CONFIG_SECURITY_APPARMOR) += apparmor/built-in.o
> +obj-$(CONFIG_SECURITY_MNTRESTRICT) += mntrestrict/built-in.o
> obj-$(CONFIG_SECURITY_YAMA) += yama/built-in.o
> obj-$(CONFIG_CGROUP_DEVICE) += device_cgroup.o
>
> diff --git a/security/mntrestrict/Kconfig b/security/mntrestrict/Kconfig
> new file mode 100644
> index 0000000..9a7310d
> --- /dev/null
> +++ b/security/mntrestrict/Kconfig
> @@ -0,0 +1,13 @@
> +config SECURITY_MNTRESTRICT
> + bool "Restrict mounts on symlinks system-wide"
> + depends on SECURITY
> + help
> + Enforces that rule that symlinks cannot be used as mount
> + destinations across the entire system. This is to make sure
> + it is never possible to redirect locations using symlinks
> + when the process doing the mount may not be able to strictly
> + trust the contents of the filesystem where it is happening. For
> + example, making sure a bind mounted subdirectory in /var is not
> + redirected into /etc: stops a daemon without full privileges
> + from elevating to uid-0 by tricking the system init filesystem
> + mounting infrastructure.
> diff --git a/security/mntrestrict/Makefile b/security/mntrestrict/Makefile
> new file mode 100644
> index 0000000..340be1f
> --- /dev/null
> +++ b/security/mntrestrict/Makefile
> @@ -0,0 +1 @@
> +obj-$(CONFIG_SECURITY_MNTRESTRICT) += mntrestrict.o
> diff --git a/security/mntrestrict/mntrestrict.c b/security/mntrestrict/mntrestrict.c
> new file mode 100644
> index 0000000..257ae69
> --- /dev/null
> +++ b/security/mntrestrict/mntrestrict.c
> @@ -0,0 +1,63 @@
> +/*
> + * Mount Restriction Security Module
> + *
> + * Copyright 2011-2013 Google Inc.
> + *
> + * Authors:
> + * Stephan Uphoff <ups@...gle.com>
> + * 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) "MntRestrict LSM: " fmt
> +
> +#include <linux/module.h>
> +#include <linux/security.h>
> +#include <linux/sched.h>
> +#include <linux/fs.h>
> +#include <linux/fs_struct.h>
> +#include <linux/mount.h>
> +#include <linux/path.h>
> +#include <linux/root_dev.h>
> +
> +static int mntrestrict_sb_mount(const char *dev_name, struct path *path,
> + const char *type, unsigned long flags,
> + void *data)
> +{
> + int error = current->total_link_count ? -ELOOP : 0;
> +
> + if (error) {
> + pr_notice("Mount path with symlinks prohibited - pid=%d\n",
> + task_pid_nr(current));
> + }
> +
> + return error;
> +}
> +
> +static struct security_operations mntrestrict_ops = {
> + .name = "mntrestrict",
> + .sb_mount = mntrestrict_sb_mount,
> +};
> +
> +static int __init mntrestrict_init(void)
> +{
> + int error;
> +
> + error = register_security(&mntrestrict_ops);
> +
> + if (error)
> + panic("Could not register MntRestrict security module");
> +
> + pr_info("symlink destinations will be blocked.\n");
> +
> + return error;
> +}
> +security_initcall(mntrestrict_init);
> --
> 1.7.9.5
>
>
> --
> Kees Cook
> Chrome OS Security
> --
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--
Kees Cook @outflux.net
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