Hardened/Grsecurity2 Quickstart

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This document features the grsecurity 2.x security patches, supported kernel configuration options and tools provided by the grsecurity project to lift your system's security to higher standards.

About Grsecurity

The Grsecurity project

The grsecurity project, hosted on http://grsecurity.net, provides various patches to the Linux kernel which enhance your system's overall security. The various features brought by grsecurity are discussed in the next chapter; a comprehensive list is maintained on the grsecurity features page itself.

As grsecurity's features are mostly kernel-based, the majority of this document explains the various kernel features and their respective sysctl operands (if applicable).

Gentoo Hardened integration

The Gentoo Hardened Project maintains security-enhancement features for Gentoo, including but not limited to grsecurity.

Kernel configuration

Throughout this document we will talk about kernel configuration using the kernel variables like CONFIG_GRKERNSEC_PAX_NO_ACL_FLAGS. These are the variables that the kernel build process uses to determine if a certain feature needs to be compiled.

When you configure your kernel through make menuconfig or similar, you receive a user interface through which you can select the various kernel options. If you select the Help button at a certain kernel feature you will see at the top that it lists such a kernel variable.

You can therefore still configure your kernel as you like - with a bit of thinking. And if you can't find a certain option, there's always the possibility to edit /usr/src/linux/.config by hand :)

Of course, to be able to select the various grsecurity kernel options, you must enable grsecurity in your kernel:

KERNEL Activating grsecurity


Fighting the exploitation of software bugs

PaX introduces a couple of security mechanisms that make it harder for attackers to exploit software bugs that involve memory corruption (so don't treat PaX as if it protects against all possible software bugs). The PaX introduction document talks about three possible exploit techniques:

  1. introduce/execute arbitrary code
  2. execute existing code out of original program order
  3. execute existing code in original program order with arbitrary data

One prevention method disallows executable code to be stored in writable memory. When we look at a process, it requires five memory regions:

  1. a data section which contains the statically allocated and global data
  2. a BSS region (Block Started by Symbol) which contains information about the zero-initialized data of the process
  3. a code region , also called the text segment , which contains the executable instructions
  4. a heap which contains the dynamically allocated memory
  5. a stack which contains the local variables

The first PaX prevention method, called NOEXEC, is meant to give control over the runtime code generation. It marks memory pages that do not contain executable code as non-executable. This means that the heap and the stack, which only contain variable data and shouldn't contain executable code, are marked as non-executable. Exploits that place code in these areas with the intention of running it will fail.

NOEXEC does more than this actually, interested readers should focus their attention to the PaX NOEXEC documentation.

The second PaX prevention method, called ASLR (Address Space Layout Randomization), randomize the addresses given to memory requests. Where previously memory was assigned contiguously (which means exploits know where the tasks' memory regions are situated) ASLR randomizes this allocation, rendering techniques that rely on this information useless.

More information about ASLR can be found online.

Enabling PaX

The recommended kernel setting for PaX is:

KERNEL Recommended PaX kernel configuration
# PaX Control
# Non-executable pages
# Address Space Layout Randomization
# Memory Protections

If you are running a non-x86 system you will observe that there is no CONFIG_PAX_NOEXEC. You should select CONFIG_PAX_PAGEEXEC instead as it is the only non-exec implementation around.

Controlling PaX

Not all Linux applications are happy with the PaX security restrictions. These tools include xorg-x11, java, mplayer, xmms and others. If you plan on using them you can elevate the protections for these applications using paxctl.

root #emerge --ask sys-apps/paxctl

pax-utils is a small toolbox which contains useful applications to administrate a PaX aware server.

root #emerge pax-utils

Interesting tools include scanelf and pspax:

  • With scanelf you can scan over library and binary directories and list the various permissions and ELF types that pertain to running an ideal pax/grsec setup
  • With pspax you can display PaX flags/capabilities/xattr from the kernel's perspective

Verifying the PaX settings

Peter Busser has written a regression test suite called paxtest. This tool will check various cases of possible attack vectors and inform you of the result. When you run it, it will leave a logfile called paxtest.log in the current working directory.

root #emerge --ask app-admin/paxtest
root #paxtest blackhat
Executable anonymous mapping             : Killed
Executable bss                           : Killed
Executable data                          : Killed
Executable heap                          : Killed
Executable stack                         : Killed
Executable anonymous mapping (mprotect)  : Killed
Executable bss (mprotect)                : Killed
Executable data (mprotect)               : Killed
Executable heap (mprotect)               : Killed
Executable stack (mprotect)              : Killed
Executable shared library bss (mprotect) : Killed
Executable shared library data (mprotect): Killed
Writable text segments                   : Killed
Anonymous mapping randomisation test     : 16 bits (guessed)
Heap randomisation test (ET_EXEC)        : 13 bits (guessed)
Heap randomisation test (PIE)            : 25 bits (guessed)
Main executable randomisation (ET_EXEC)  : 16 bits (guessed)
Main executable randomisation (PIE)      : 17 bits (guessed)
Shared library randomisation test        : 16 bits (guessed)
Stack randomisation test (SEGMEXEC)      : 23 bits (guessed)
Stack randomisation test (PAGEEXEC)      : No randomisation
Return to function (strcpy)              : Vulnerable
Return to function (memcpy)              : Vulnerable
Return to function (strcpy, RANDEXEC)    : Killed
Return to function (memcpy, RANDEXEC)    : Killed

In the above example run you notice that:

  • strcpy and memcpy are listed as Vulnerable. This is expected and normal - it is simply showing the need for a technology such as ProPolice/SSP
  • there is no randomization for PAGEEXEC. This is normal since our recommended x86 kernel configuration didn't activate the PAGEEXEC setting. However, on arches that support a true NX (non-executable) bit (most of them do, including x86_64), PAGEEXEC is the only method available for NOEXEC and has no performance hit.


Role based access control

There are two basic types of access control mechanisms used to prevent unauthorized access to files (or information in general): DAC (Discretionary Access Control) and MAC (Mandatory Access Control). By default, Linux uses a DAC mechanism: the creator of the file can define who has access to the file. A MAC system however forces everyone to follow rules set by the administrator.

The MAC implementation grsecurity supports is called Role Based Access Control. RBAC associates roles with each user. Each role defines what operations can be performed on certain objects. Given a well-written collection of roles and operations your users will be restricted to perform only those tasks that you tell them they can do. The default "deny-all" ensures you that a user cannot perform an action you haven't thought of.

Configuring the kernel

The recommended kernel setting for RBAC is:

KERNEL Recommended RBAC kernel configuration
# Role Based Access Control Options

Working with gradm

gradm is a tool which allows you to administer and maintain a policy for your system. With it, you can enable or disable the RBAC system, reload the RBAC roles, change your role, set a password for admin mode, etc.

When you install gradm a default policy will be installed in /etc/grsec/policy:

root #emerge --ask sys-apps/gradm

By default, the RBAC policies are not activated. It is the sysadmin's job to determine when the system should have an RBAC policy enforced and not Gentoo's. Before activating the RBAC system you should set an admin password.

root #gradm -P
Setting up grsecurity RBAC password
Password: ## (Enter a well-chosen password)
Re-enter Password: ## (Enter the same password for confirmation)
Password written in /etc/grsec/pw
root #gradm -E

To disable the RBAC system, run gradm -D. If you are not allowed to, you first need to switch to the admin role:

root #gradm -a admin
Password: ## (Enter your admin role password)
root #gradm -D

If you want to leave the admin role, run gradm -u admin:

root #gradm -u admin

Generating a policy

The RBAC system comes with a great feature called "learning mode". The learning mode can generate an anticipatory least privilege policy for your system. This allows for time and money savings by being able to rapidly deploy multiple secure servers.

To use the learning mode, activate it using gradm:

root #gradm -F -L /etc/grsec/learning.log

Now use your system, do the things you would normally do. Try to avoid rsyncing, running locate of any other heavy file I/O operation as this can really slow down the processing time.

When you believe you have used your system sufficiently to obtain a good policy, let gradm process them and propose roles under /etc/grsec/learning.roles:

root #gradm -F -L /etc/grsec/learning.log -O /etc/grsec/learning.roles
You will need to disable the RBAC learning mode before doing this. You can use gradm -D for this.

Audit the /etc/grsec/learning.roles and save it as /etc/grsec/policy (mode 0600) when you are finished.

root #mv /etc/grsec/learning.roles /etc/grsec/policy
root #chmod 0600 /etc/grsec/policy

You will now be able to enable the RBAC system with your new learned policy.

Tweaking the policy

An interesting feature of grsecurity 2.x is Set Operation Support for the configuration file. Currently it supports unions, intersections and differences of sets (of objects in this case).

CODE Example sets
define objset1 {
/root/blah rw
/root/blah2 r
/root/blah3 x
define somename2 {
/root/test1 rw
/root/blah2 rw
/root/test3 h

Here is an example of its use, and the resulting objects that will be added to your subject:

CODE Example set use of the & (ampersand)
subject /somebinary o
$objset1 & $somename2

The above would expand to:

CODE Resulting subject settings
subject /somebinary o
/root/blah2 r

This is the result of the & operator which takes both sets and returns the files that exist in both sets and the permission for those files that exist in both sets.

CODE | Example
subject /somebinary o
$objset1 | $somename2

This example would expand to:

CODE Resulting subject settings
subject /somebinary o
/root/blah rw
/root/blah2 rw
/root/blah3 x
/root/test1 rw
/root/test3 h

This is the result of the | operator which takes both sets and returns the files that exist in either set. If a file exists in both sets, it is returned as well and the mode contains the flags that exist in either set.

CODE - Example
subject /somebinary o
$objset1 - $somename2

This example would expand to:

CODE Resulting subject settings
subject /somebinary o
/root/blah rw
/root/blah2 h
/root/blah3 x

This is the result of the - operator which takes both sets and returns the files that exist in the set on the left but not in the match of the file in set on the right. If a file exists on the left and a match is found on the right (either the filenames are the same, or a parent directory exists in the right set), the file is returned and the mode of the second set is removed from the first set, and that file is returned.

In some obscure pseudo-language you could see this as:

CODE Pseudo-language explanation
if ( ($objset1 contained /tmp/blah rw) and
     ($objset2 contained /tmp/blah r) )
  $objset1 - $objset2 would contain /tmp/blah w
if ( ($objset1 contained /tmp/blah rw) and
     ($objset2 contained / rwx) )
  $objset1 - $objset2 would contain /tmp/blah h

As for order of precedence (from highest to lowest): "-, & |".

If you do not want to bother remembering precedence, parenthesis support is also included, so you can do things like:

CODE Parenthesis example
(($set1 - $set2) | $set3) & $set4

Filesystem protection

Fighting chroot and filesystem abuse

Grsecurity2 includes many patches that prohibits users from gaining unnecessary knowledge about the system. This includes restrictions on /proc usage, chrooting, linking, etc.

Kernel configuration

We recommend the following grsecurity kernel configuration for filesystem protection:

KERNEL Activating filesystem protection
# Filesystem Protections

Triggering the security mechanism

When you're using a kernel compiled with the above (or similar) settings, you will get the option to enable/disable many of the options through the /proc filesystem or via sysctl.

The example below shows an excerpt of a typical /etc/sysctl.conf:

FILE /etc/sysctl.conf
kernel.grsecurity.chroot_deny_sysctl = 1
kernel.grsecurity.chroot_caps = 1
kernel.grsecurity.chroot_execlog = 0
kernel.grsecurity.chroot_restrict_nice = 1
kernel.grsecurity.chroot_deny_mknod = 1
kernel.grsecurity.chroot_deny_chmod = 1
kernel.grsecurity.chroot_enforce_chdir = 1
kernel.grsecurity.chroot_deny_pivot = 1
kernel.grsecurity.chroot_deny_chroot = 1
kernel.grsecurity.chroot_deny_fchdir = 1
kernel.grsecurity.chroot_deny_mount = 1
kernel.grsecurity.chroot_deny_unix = 1
kernel.grsecurity.chroot_deny_shmat = 1

You can enable or disable settings at will using the sysctl command:

Toggling the exec_logging feature ON

root #sysctl -w kernel.grsecurity.exec_logging = 1

Toggling the exec_logging feature OFF

root #sysctl -w kernel.grsecurity.exec_logging = 0

There is a very important sysctl setting pertaining to grsecurity, namely kernel.grsecurity.grsec_lock. When set, you are not able to change any setting anymore.

root #sysctl -w kernel.grsecurity.grsec_lock = 1

Kernel auditing

Extend the system's logging facilities

grsecurity adds extra functionality to the kernel pertaining the logging. With grsecurity's Kernel Auditing the kernel informs you when applications are started, devices (un)mounted, etc.

The various kernel audit settings

The following kernel configuration section can be used to enable grsecurity's kernel audit settings:

KERNEL Activating kernel auditing
# Kernel Auditing

Process restrictions

Executable protection

With grsecurity you can restrict executables. Since most exploits work through one or more running processes this protection can save your system's health.

Network protection

Linux' TCP/IP stack is vulnerable to prediction-based attacks. grsecurity includes randomization patches to counter these attacks. Apart from these you can also enable socket restrictions, disallowing certain groups network access alltogether.

Kernel settings

The following kernel settings enable various executable and network protections:

KERNEL Kernel setting
# Executable Protections
# Network Protections

The hardened toolchain

Although it is outside the scope of this document we mention the use of the hardened toolchain which completes the grsec/PaX model from userspace.

See also

External resources

This article is based on a document formerly found on our main website gentoo.org.
The following people have contributed to the original document: Ned Ludd, Sven Vermeulen
They are listed here as the Wiki history does not provide for any attribution. If you edit the Wiki article, please do not add yourself here, your contributions are recorded on the history page.