Contents

  1. NetBSD Veriexec subsystem
    1. How it works
    2. Signatures file
      1. Generating fingerprints
      2. veriexecctl
      3. Flags
      4. Examples
    3. Strict levels
    4. Veriexec and layered file systems
    5. Kernel configuration

NetBSD Veriexec subsystem

veriexec(8) is a file integrity subsystem in NetBSD (introduced in 3.0). It is kernel based, hence can provide some protection even in the case of a root compromise.

How it works

First of all (usually at boot time), Veriexec loads 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.

Then, whenever an application tries to open a file, there is a kauth(9) check before actually doing so if the application is permitted to do so. Veriexec hooks in here and checks whether the file has the fingerprint as recorded in the signature file. If not, it acts depending on its mode, e.g., it could deny file access if the fingerprints mismatch.

To secure your system with veriexec, you need a kernel with the option veriexec enabled (as it is in the default kernel), and you need support for the algorithms you want to use (by default, all are activated). Then, you decide which files you want to monitor, generate a signatures file and make it load at boot.

Signatures file

An entry in the Veriexec signatures file looks like this:

/path/to/file algorithm fingerprint flags

With

Generating fingerprints

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

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

veriexecgen

veriexecgen(8) is a tool which automatically creates fingerprints for files or directories. By default, it will create SHA256 fingerprints for /bin, /sbin, /usr/bin, /usr/sbin, /lib, /usr/lib, /libexec and /usr/libexec and save them to /etc/signatures.

You can also specify other algorithms and directories to be searched, e.g., you might want to include fingerprints of the packages you installed (though you have to be careful to recreate fingerprints on the next package update!).

veriexecctl

For controlling the database inside the kernel, you have the tool veriexecctl(8). It can be used for dumping the whole database (dump), flushing it (flush), i.e., deleting all entries in the kernel's table, query an entry, etc.

Flags

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

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            # `ls` is the executed file
$ cp ~/foo /tmp/bar  # `cp` is the executed file
$ rm ~/foo           # `rm` is the executed file

INDIRECT

INDIRECT access means that the file is executed indirectly, and is invoked to interpret a script. This happens usually when scripts have a shebang (\#!) 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

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

Examples

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:

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

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.

It's recommended to first run Veriexec in strict level 0 and verbose level 1 to fine-tune your signatures file and getting used to Veriexec, 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

Or you can also increase while the system is running (though this undermines some of the security Veriexec provides):

sysctl -w kern.veriexec.strict=1

Veriexec and layered file systems

Veriexec can be used on NFS file systems on the client side and on layered file systems such as nullfs. 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, 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, you have to follow the instructions in the previous paragraph and ensure that the files you want protected have fingerprints at all layers. You should also raise securelevel to 2 after all mounts are done to prevent new layers from being mounted, which could compromise Veriexec's protection:

kern.securelevel=2

Kernel configuration

To use Veriexec, aside from creating a signatures file, you need a kernel with Veriexec enabled. It is already enabled in the default (GENERIC) kernel, but if you built your own one, you have to uncomment the following line in the configuration:

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 enabled. Once done, rebuild and reinstall your kernel, see 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
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