Re: [PATCH] LSM: MntRestrict blocks mounts on symlink targets

[David Quigley <dpquigl@davequigley.com>]

Why is this an LSM and not something further up in the VFS? Why not make 
a sysctl for this and place it further up in the VFS? Has it already 
been rejected from there? If so why not include it in the things covered 
by Yama? From a code perspective I can't find anything wrong code wise 
but it seems odd to make a security module for something which is 
clearly a change to VFS semantics not really a security module.

On 10/03/2013 16:56, Kees Cook wrote:
> 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@chromium.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@google.com>
>> + *      Kees Cook       <keescook@chromium.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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