Chapter 20. NetBSD Veriexec subsystem

Table of Contents

20.1. How it works
20.2. Signatures file
20.3. Generating fingerprints
20.4. Strict levels
20.5. Veriexec and layered file systems
20.6. Kernel configuration

Veriexec is NetBSD's file integrity subsystem. It's kernel based, hence can provide some protection even in the case of a root compromise. This chapter applies only to NetBSD 3.0 and onwards.

20.1. How it works

Veriexec works by loading a specification file, also called the signatures file, to the kernel. This file contains information about files Veriexec should monitor, as well as their digital fingerprint (along with the hashing algorithm used to produce this fingerprint), and various flags that will be discussed later.

At the moment, the following hashing algorithms are supported by Veriexec: MD5, SHA1, SHA256, SHA384, SHA512, and RMD160.

20.2. Signatures file

An entry in the Veriexec signatures file looks like this:

	/path/to/file algorithm fingerprint flags

Where the first element, the path, must always be an absolute path. The algorithm is one of the algorithms listed above, and fingerprint is the ASCII fingerprint.

20.3. Generating fingerprints

You can generate ASCII fingerprints for each algorithm using the following tools:

Table 20.1. Veriexec fingerprints tools

Algorithm Tool
MD5 /usr/bin/cksum -a md5
SHA1 /usr/bin/cksum -a sha1
SHA256 /usr/bin/cksum -a sha256
SHA384 /usr/bin/cksum -a sha384
SHA512 /usr/bin/cksum -a sha512
RMD160 /usr/bin/cksum -a rmd160

For example, to generate a MD5 fingerprint for /bin/ls:

	% cksum -a md5 < /bin/ls
	a8b525da46e758778564308ed9b1e493

And to generate a SHA512 fingerprint for /bin/ps:

	% cksum -a sha512 < /bin/ps
	381d4ad64fd47800897446a2026eca42151e03adeae158db5a34d12c529559113d928a9fef9a7c4615d257688d1da4645db004081030d7f080bb7198067eb890

Each entry may be associated with zero or more flags. Currently, these flags indicate how the file the entry is describing should be accessed. Note that this access type is enforced only in strict level 2 (IPS mode) and above.

The access types you can use are DIRECT, INDIRECT, and FILE.

  • DIRECT access means that the file is executed directly, and not invoked as an interpreter for some script, or opened with an editor. Usually, most programs you use will be accessed using this mode:

    	% ls /tmp
    	% cp ~/foo /tmp/bar
    	% rm ~/foo
  • INDIRECT access means that the file is executed indirectly, and is invoked to interpret a script. This happens usually when scripts have a #! magic as their first line. For example, if you have a script with the following as its first line:

    	#!/bin/sh

    And you run it as:

    	% ./script.sh

    Then /bin/sh will be executed indirectly -- it will be invoked to interpret the script.

  • FILE entries refer to everything which is not (or should not) be an executable. This includes shared libraries, configuration files, etc.

    Some examples for Veriexec signature file entries:

    	/bin/ls         MD5 dc2e14dc84bdefff4bf9777958c1b20b DIRECT
    	/usr/bin/perl   MD5 914aa8aa47ebd79ccd7909a09ed61f81 INDIRECT
    	/etc/pf.conf    MD5 950e1dd6fcb3f27df1bf6accf7029f7d FILE

Veriexec allows you to specify more than one way to access a file in an entry. For example, even though /usr/bin/perl is mostly used as an interpreter, it may be desired to be able to execute it directly, too:

	/usr/bin/perl MD5 914aa8aa47ebd79ccd7909a09ed61f81 DIRECT, INDIRECT

Shell scripts using #! magic to be executable also require two access types: We need them to be DIRECT so we can execute them, and we need them to be FILE so that the kernel can feed their contents to the interpreter they define:

	/usr/src/build.sh MD5 e80dbb4c047ecc1d84053174c1e9264a DIRECT, FILE

To make it easier to create signature files, and to make the signature files themselves more readable, Veriexec allows you to use the following aliases:

Table 20.2. Veriexec access type aliases

Alias Expansion
PROGRAM DIRECT
INTERPRETER INDIRECT
SCRIPT DIRECT, FILE
LIBRARY FILE

Sample scripts for generating fingerprints are available in /usr/share/examples/veriexecctl. After you've generated a signatures file, you should save it as /etc/signatures, and enable Veriexec in rc.conf:

	veriexec=YES

20.4. Strict levels

Since different people might want to use Veriexec for different purposes, we also define four strict levels, ranging 0-3, and named learning, IDS, IPS, and lockdown modes.

In strict level 0, learning mode, Veriexec will act passively and simply warn about any anomalies. Combined with verbose level 1, running the system in this mode can help you fine-tune the signatures file. This is also the only strict level in which you can load new entries to the kernel.

Strict level 1, or IDS mode, will deny access to files with a fingerprint mismatch. This mode suits mostly to users who simply want to prevent access to files which might've been maliciously modified by an attacker.

Strict level 2, IPS mode, takes a step towards trying to protect the integrity of monitored files. In addition to preventing access to files with a fingerprint mismatch, it will also deny write access and prevent the removal of monitored files, and enforce the way monitored files are accessed. (as the signatures file specifies).

Lockdown mode (strict level 3) can be used in highly critical situations such as custom made special-purpose machines, or as a last line of defense after an attacker compromised the system and we want to prevent traces from being removed, so we can perform post-mortem analysis. It will prevent the creation of new files, and deny access to files not monitored by Veriexec.

It's recommended to first run Veriexec in strict level 0 and verbose level 1 to fine-tune your signatures file, ensuring that desired applications run correctly, and only then raise the strict level (and lower the verbosity level). You can use /etc/sysctl.conf to auto raise the strict level to the desired level after a reboot:

	kern.veriexec.strict=1

20.5. Veriexec and layered file systems

Veriexec can be used on NFS file systems on the client side and on layered file systems such as the union file system. The files residing on these file systems need only be specified in the /etc/signatures file and that the file systems be mounted prior to the fingerprints being loaded.

If you are going to use layered file systems then you must ensure that you include the fingerprint for files you want protected at every layer. If you fail to do this someone could overwrite a file protected by Veriexec by using a different layer in a layered file system stack. This limitation may be removed in later versions of NetBSD.

It's recommended that if you are not going to use layered file systems nor NFS then these features should be disabled in they kernel configuration. If you need to use layered file systems then you must follow the instructions in the previous paragraph and ensure that the files you want protected have fingerprints at all layers. Also you should raise securelevel to 2 after all mounts are done:

	kern.securelevel=2

To prevent new layers being mounted which could compromise Veriexec's protection.

20.6. Kernel configuration

To use Veriexec, aside from creating a signatures file, you should enable (uncomment) it in your kernel's config file: (e.g. /usr/src/sys/arch/i386/conf/GENERIC):

	pseudo-device veriexec

Then, you need to enable the hashing algorithms you wish to support:

	options VERIFIED_EXEC_FP_MD5
	options VERIFIED_EXEC_FP_SHA1
	options VERIFIED_EXEC_FP_RMD160
	options VERIFIED_EXEC_FP_SHA512
	options VERIFIED_EXEC_FP_SHA384
	options VERIFIED_EXEC_FP_SHA256

Depending on your operating system version and platform, these may already be enable. Once done, rebuild and reinstall your kernel, see Chapter 32, Compiling the kernel for further instructions.

If you do not have the Veriexec device /dev/veriexec, you can create it manually by running the following command:

# cd /dev
# sh MAKEDEV veriexec