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The goals of the kernel integrity subsystem are to detect if files have been accidentally or maliciously altered, both remotely and locally, appraise a file's measurement against a "good" value stored as an extended attribute, and enforce local file integrity. These goals are complementary to Mandatory Access Control(MAC) protections provided by LSM modules, such as SElinux and Smack, which, depending on policy, can attempt to protect file integrity.

Overview

Features

The following modules provide serveral integrity functions:

Collect – measure a file before it is accessed.
Store – add the measurement to a kernel resident list and, if a
hardware Trusted Platform Module (TPM) is present, extend the IMA
PCR
Attest – if present, use the TPM to sign the IMA PCR value, to
allow a remote validation of the measurement list.
Appraise – enforce local validation of a measurement against a
“good” value stored in an extended attribute of the file.
Protect – protect a file's security extended attributes
(including appraisal hash) against off-line attack.

The first three functions were introduced with Integrity Measurement Architecture (IMA) in 2.6.30. The last two features were originally posted as a single EVM/IMA-appraisal patch set for in the 2.6.36 timeframe, but were subsequently split. EVM was upstreamed in Linux 3.2, using a simplier and more secure method for loading the 'evm-key', based on the new Kernel Key Retention Trusted and Encrypted keys. Support for protecting file metadata based on digital signatures was upstreamed in the Linux 3.3. IMA-appraisal was upstreamed in Linux 3.7.

The goals, design, and benefits of these features are further described in the whitepaper "An Overview of the Linux Integrity Subsystem".

Components

IMA measurement, one component of the kernel's integrity subsystem, is part of an overall Integrity Architecture based on the Trusted Computing Group's open standards, including Trusted Platform Module (TPM), Trusted Boot, Trusted Software Stack (TSS), Trusted Network Connect (TNC), and Platform Trust Services (PTS). The diagram shows how these standards relate, and provides links to the respective specifications and open source implementations. IMA and EVM can still run on platforms without a hardware TPM, although without the hardware guarantee of compromise detection.

Integrity Measurement Architecture (IMA)

IMA is an open source trusted computing component. IMA maintains a runtime measurement list and, if anchored in a hardware Trusted Platform Module(TPM), an aggregate integrity value over this list. The benefit of anchoring the aggregate integrity value in the TPM is that the measurement list cannot be compromised by any software attack, without being detectable. Hence, on a trusted boot system, IMA can be used to attest to the system's runtime integrity.

Enabling IMA

IMA was first included in the 2.6.30 kernel. For distros that enable IMA by default in their kernels, collecting IMA measurements simply requires rebooting the kernel with the boot command line parameter 'ima_tcb'. (Fedora/RHEL may also require the boot command line parameter 'ima=on'.)

To determine if your distro enables IMA by default, mount securityfs (mount -t securityfs security /sys/kernel/security), if it isn't already mounted, and then check if '<securityfs>/integrity/ima' exists. If it exists, IMA is indeed enabled. On systems without IMA enabled, recompile the kernel with the config option 'CONFIG_IMA' enabled.</securityfs>

Controlling IMA

IMA is controlled with several kernel command line parameters:

ima_audit= audit control
Format: { "0" | "1" }
0 -- integrity auditing messages. (Default)
1 -- enable additional informational integrity auditing messages.

(eg. Although file measurements are only added to the measurement list once and cached, if the inode is flushed, subsequent access to the inode will result in re-measuring the file and attempting to add the measurement again to the measurement list. Enabling ima_audit will log such attempts.)

ima_template= template used
Format: { "ima" | "ima-ng" | "ima-sig" }
NEW Linux 3.13 default: "ima-ng"

ima_hash= hash used
Format: { "sha1" | "md5" | "sha256" | "sha512" | "wp512" | ... }
'ima' template default: "sha1"
NEW Linux 3.13 default: "sha256"

ima_tcb
If specified, enables the TCB policy, which meets the needs of the Trusted Computing Base. This means IMA will measure all programs exec'd, files mmap'd for exec, and all files opened for read by uid=0.

IMA Measurements

IMA maintains a runtime measurement list, which can be displayed as shown below.

mount securityfs as /sys/kernel/security

$ su -c 'mkdir /sys/kernel/security'
$ su -c 'mount -t securityfs securityfs /sys/kernel/security'

Modify /etc/fstab to mount securityfs on boot.

display the runtime measurement list (Only root is allowed access to securityfs files.)

Example 1: ** NEW ** 'ima-ng' template
$ su -c 'head -5 /sys/kernel/security/ima/ascii_runtime_measurements'

PCR     template-hash                           filedata-hash                           filename-hint
10 91f34b5c671d73504b274a919661cf80dab1e127 ima-ng sha1:1801e1be3e65ef1eaa5c16617bec8f1274eaf6b3 boot_aggregate 
10 8b1683287f61f96e5448f40bdef6df32be86486a ima-ng sha256:efdd249edec97caf9328a4a01baa99b7d660d1afc2e118b69137081c9b689954 /init 
10 ed893b1a0bc54ea5cd57014ca0a0f087ce71e4af ima-ng sha256:1fd312aa6e6417a4d8dcdb2693693c81892b3db1a6a449dec8e64e4736a6a524 /usr/lib64/ld-2.16.so
10 9051e8eb6a07a2b10298f4dc2342671854ca432b ima-ng sha256:3d3553312ab91bb95ae7a1620fedcc69793296bdae4e987abc5f8b121efd84b8 /etc/ld.so.cache

PCR: default CONFIG_IMA_MEASURE_PCR_IDX is 10
template-hash: sha1 hash(filedata-hash length, filedata-hash, pathname length, pathname)
filedata-hash: sha256 hash(filedata)

Example 2: 'ima' template

PCR     template-hash                           filedata-hash                           filename-hint
10 7971593a7ad22a7cce5b234e4bc5d71b04696af4 ima b5a166c10d153b7cc3e5b4f1eab1f71672b7c524 boot_aggregate
10 2c7020ad8cab6b7419e4973171cb704bdbf52f77 ima e09e048c48301268ff38645f4c006137e42951d0 /init
10 ef7a0aff83dd46603ebd13d1d789445365adb3b3 ima 0f8b3432535d5eab912ad3ba744507e35e3617c1 /init
10 247dba6fc82b346803660382d1973c019243e59f ima 747acb096b906392a62734916e0bb39cef540931 ld-2.9.so
10 341de30a46fa55976b26e55e0e19ad22b5712dcb ima 326045fc3d74d8c8b23ac8ec0a4d03fdacd9618a ld.so.cache

PCR: default CONFIG_IMA_MEASURE_PCR_IDX is 10
template-hash: sha1 hash(filedata-hash, filename-hint)
filedata-hash: sha1 hash(filedata)

The first element in the runtime measurement list, shown above, is the boot_aggregate. The boot_aggregate is a SHA1 hash over tpm registers 0-7, assuming a TPM chip exists, and zeroes, if the TPM chip does not exist.

display the bios measurement list entries, used in calculating the boot aggregate

$ su -c 'head /sys/kernel/security/tpm0/ascii_bios_measurements'

0 f797cb88c4b07745a129f35ea01b47c6c309cda9 08 [S-CRTM Version]
0 dca68da0707a9a52b24db82def84f26fa463b44d 01 [POST CODE]
0 dd9efa31c88f467c3d21d3b28de4c53b8d55f3bc 01 [POST CODE]
0 dd261ca7511a7daf9e16cb572318e8e5fbd22963 01 [POST CODE]
0 df22cabc0e09aabf938bcb8ff76853dbcaae670d 01 [POST CODE]
0 a0d023a7f94efcdbc8bb95ab415d839bdfd73e9e 01 [POST CODE]
0 38dd128dc93ff91df1291a1c9008dcf251a0ef39 01 [POST CODE]
0 dd261ca7511a7daf9e16cb572318e8e5fbd22963 01 [POST CODE]
0 df22cabc0e09aabf938bcb8ff76853dbcaae670d 01 [POST CODE]
0 a0d023a7f94efcdbc8bb95ab415d839bdfd73e9e 01 [POST CODE]

Verifying IMA Measurements

The IMA tests programs are part of the Linux Test Project.

Download, compile, and install the test programs in /usr/local/bin.

$ wget -O ltp-ima-standalone.tar http://downloads.sf.net/project/linux-ima/linux-ima/ltp-ima-standalone-v1.tar.gz
$ tar -xvzf ltp-ima-standalone.tar.gz
ima-tests/
ima-tests/test.h
ima-tests/README
ima-tests/Makefile
ima-tests/ltp-tst-replacement.c
ima-tests/config.h
ima-tests/ima_boot_aggregate.c
ima-tests/ima_measure.c
ima-tests/ima_mmap.c
$ cd ima-tests
$ make 
$ su -c 'make install'

ima_boot_aggregate <tpm_bios file=""></tpm_bios>

Using the TPM's binary bios measurement list, re-calculate the boot aggregate.

$ su -c '/usr/local/bin/ima_boot_aggregate /sys/kernel/security/tpm0/binary_bios_measurements'
000 f797cb88c4b07745a129f35ea01b47c6c309cda9
000 dca68da0707a9a52b24db82def84f26fa463b44d
< snip >
005 6895eb784cdaf843eaad522e639f75d24d4c1ff5
PCR-00: 07274edf7147abda49200100fd668ce2c3a374d7
PCR-01: 48dff4fbf3a34d56a08dfc1504a3a9d707678ff7
PCR-02: 53de584dcef03f6a7dac1a240a835893896f218d
PCR-03: 3a3f780f11a4b49969fcaa80cd6e3957c33b2275
PCR-04: acb44e9dd4594d3f121df2848f572e4d891f0574
PCR-05: df72e880e68a2b52e6b6738bb4244b932e0f1c76
PCR-06: 585e579e48997fee8efd20830c6a841eb353c628
PCR-07: 3a3f780f11a4b49969fcaa80cd6e3957c33b2275
boot_aggregate:b5a166c10d153b7cc3e5b4f1eab1f71672b7c524

and compare the value with the ascii_runtime_measurement list value.

$ su -c 'cat /sys/kernel/security/ima/ascii_runtime_measurements | grep boot_aggregate'
10 7971593a7ad22a7cce5b234e4bc5d71b04696af4 ima b5a166c10d153b7cc3e5b4f1eab1f71672b7c524 boot_aggregate

ima_measure <binary_runtime_measurements> [--validate] [--verify] [--verbose]</binary_runtime_measurements>

using the IMA binary measurement list, calculate the PCR aggregate value

$ su -c '/usr/local/bin/ima_measure /sys/kernel/security/ima/binary_runtime_measurements --validate'
PCRAggr (re-calculated): B4 D1 93 D8 FB 31 B4 DD 36 5D DA AD C1 51 AC 84 FA 88 78 1B

and compare it against the PCR value

$ cat /sys/devices/pnp0/00:0a/pcrs | grep PCR-10
PCR-10: B4 D1 93 D8 FB 31 B4 DD 36 5D DA AD C1 51 AC 84 FA 88 78 1B

IMA re-measuring files

Part of the TCG requirement is that all Trusted Computing Base (TCB) files be measured, and re-measured if the file has changed, before reading/executing the file. IMA detects file changes based on i_version. To re-measure a file after it has changed, the filesystem
must be mounted with i_version support.

Attempt to mount a filesystem with i_version support.
```
$ su -c 'mount -o remount,rw,iversion /home'

mount: you must specify the filesystem type
```
Attempt to remount '/home' with i_version support, shown above, failed. Please install a version of the util-linux-ng-2.15-rc1 package or later.
To automatically mount a filesystem with i_version support, update /etc/fstab.
```
UUID=blah  /home                   ext3    defaults,iversion
```
Mount the root filesystem with i_version.
- For systems with /etc/rc.sysinit, update the mount options
  adding 'iversion':
```
# Remount the root filesystem read-write.
update_boot_stage RCmountfs
if remount_needed ; then
  action $"Remounting root filesystem in read-write mode: " mount -n -o remount,rw,iversion /
fi
```
- For systems using dracut, root 'mount' options can be specified on the boot
  command line using 'rootflags'. Add 'rootflags=i_version'. Unlike 'mount',
  which expects 'iversion', notice that on the boot command line 'i_version'
  contains an underscore.

Linux-audit support

As of Linux-audit 2.0, support for integrity auditing messages is available.

Defining an LSM specific policy

The ima_tcb default measurement policy in linux-2.6.30 measures all system sensitive files - executables, mmapped libraries, and files opened for read by root. These measurements, the measurement list and the aggregate integrity value, can be used to attest to a system's
runtime integrity. Based on these measurements, a remote party can detect whether critical system files have been modified or if malicious software has been executed.

Default policy

dont_measure fsmagic=PROC_SUPER_MAGIC
dont_measure fsmagic=SYSFS_MAGIC
dont_measure fsmagic=DEBUGFS_MAGIC
dont_measure fsmagic=TMPFS_MAGIC
dont_measure fsmagic=SECURITYFS_MAGIC
dont_measure fsmagic=SELINUX_MAGIC
measure func=BPRM_CHECK
measure func=FILE_MMAP mask=MAY_EXEC

< add LSM specific rules here >

measure func=PATH_CHECK mask=MAY_READ uid=0

But not all files opened by root for read, are necessarily part of the Trusted Computing Base (TCB), and therefore do not need to be measured. Linux Security Modules (LSM) maintain file metadata, which can be leveraged to limit the number of files measured.

Examples: adding LSM specific rules

SELinux:
dont_measure obj_type=var_log_t
dont_measure obj_type=auditd_log_t

Smack:
measure subj_user=_ func=INODE_PERM mask=MAY_READ

To replace the default policy 'cat' the custom IMA measurement policy and redirect the output to "< securityfs >/ima/policy". Both dracut and systemd have been modified to load the custom IMA policy. If the IMA policy contains LSM labels, then the LSM policy must be loaded prior to the IMA policy. (eg. if systemd loads the SELinux policy, then systemd must also load the IMA policy.)

systemd commit c8161158 adds support for loading a custom IMA measurement policy. Simply place the custom IMA policy in /etc/ima/ima-policy. systemd will automatically load the custom policy.

dracut commit 0c71fb6 add initramfs support for loading the custom IMA measurement policy. Build and install dracut (git://git.kernel.org/pub/scm/boot/dracut/dracut.git), to load the custom IMA measurement policy(default: /etc/sysconfig/ima-policy).

For more information on defining an LSM specific measurement/appraisal/audit policy, refer to the kernel Documentation/ABI/testing/ima_policy.

IMA-appraisal

IMA currently maintains an integrity measurement list used for remote attestation. The IMA-appraisal extension adds local integrity validation and enforcement of the measurement against a "good" value stored as an extended attribute 'security.ima'. The inital method for validating 'security.ima' are hashed based, which provides file data integrity, and digital signature based, which in addition to providing file data integrity, provides authenticity.

Enabling IMA-appraisal

IMA-appraisal was upstreamed in Linux 3.7. For distros that enable IMA-appraisal by default in their kernels, appraising file measurements requires rebooting the kernel first with the boot command line parameters 'ima_appraise_tcb' and ima_appraise='fix' to label the filesystem. Once labeled, reboot with just the 'ima_appraise_tcb' boot command line parameter.

Refer to compiling the kernel for directions on configuring and building a new kernel with IMA-appraisal support enabled.

Understanding the IMA-appraisal policy

The IMA-appraisal policy extends the measurement policy ABI with two new keywords: appraise/dont_appraise. The default appraise policy appraises all files owned by root. Like the default measurement policy, the default appraisal policy does not appraise pseudo filesystem files (eg. debugfs, tmpfs, securityfs, or selinuxfs.)

Additional rules can be added to the default IMA measurement/appraisal policy, which take advantage of the SELinux labels, for a more fine grained policy. Refer to Documentation/ABI/testing/ima_policy.

Labeling the filesystem with 'security.ima' extended attributes

A new boot parameter 'ima_appraise=' has been defined in order to label existing file systems with the 'security.ima' extended attribute.

ima_appraise= appraise integrity measurements\
Format: { "off" | "log" | "fix" } \

off - is a runtime parameter that turns off integrity appraisal verification.
enforce - verifies and enforces runtime file integrity. [default]
fix - for non-digitally signed files, updates the 'security.ima' xattr to reflect the existing file hash.

After building a kernel with IMA-appraisal enabled and verified that the filesystems are mounted with i_version support, to label the filesystem, reboot with the boot command line options 'ima_appraise_tcb' and 'ima_appraise=fix'. Opening a file owned by root, will cause the 'security.ima' extended attributes to be written. For example, to label the entire filesystem, execute:

find / \$ -fstype rootfs -o ext4 -type f \$ -uid 0 -exec head -n 1 '{}' >/dev/null \\;

Labeling 'immutable' files with digital signatures

'Immutable' files, such as ELF executables, can be digitally signed, storing the digital signature in the 'security.ima' xattr. Creating the digital signature requires generating an RSA private/public key pair. The private key is used to sign the file, while the public key is used to verify the signature. For example, to digitally sign all kernel modules, replace <rsa private="" key="">, below, with the pathname to your RSA private key, and execute:</rsa>

find /lib/modules -name "\*.ko" -type f -uid 0 -exec evmctl sign --imasig '{}' <RSA private key> \;

Running with IMA-appraisal

Once the filesystem has been properly labeled, before rebooting, re-install the new labeled kernel. Modify the initramfs to load the RSA public key on the IMA keyring, using evmctl. Reboot with the 'ima_appraise_tcb' and ,possibly, the 'rootflags=i_version' options.

Linux Extended Verification Module (EVM)

EVM detects offline tampering of the security extended attributes (e.g. security.selinux, security.SMACK64, security.ima), which is the basis for LSM permission decisions and, with the IMA-appraisal extension, integrity appraisal decisions. EVM provides a framework, and two methods for detecting offline tampering of the security extended attributes. The initial method maintains an HMAC-sha1 across a set of security extended attributes, storing the HMAC as the extended attribute 'security.evm'. The other method is based on a digital signature of the security extended attributes hash. To verify the integrity of an extended attribute, EVM exports evm_verifyxattr(), which re-calculates either the HMAC or the hash, and compares it with the version stored in 'security.evm'.

Enabling EVM

EVM was upstreamed in Linux 3.2. EVM-digital-signatures is currently in the Linux 3.3 release candidate.

Refer to compiling the kernel, for directions on configuring and building a new kernel with EVM support.

Running EVM

EVM is configured automatically to protect standard “security” extended attributes:

security.ima (IMA's stored “good” hash for the file)
security.selinux (the selinux label/context on the file)
security.SMACK64 (Smack's label on the file)
security.capability (Capability's label on executables)

EVM protects the configured extended attributes with an HMAC across their data, keyed with an EVM key provided at boot time. EVM looks for this key named 'evm-key' on root's key ring. Refer to trusted and EVM encrypted keys, for directions on creating EVM keys. Once loaded, EVM can be activated by writing a '1' to the evm securityfs file: **echo "1" >/sys/kernel/security/evm**

Before EVM is activated, any requested integrity appraisals are unknown, so the EVM startup should be done early in the boot process, preferably entirely within the kernel and initramfs (which are measured by trusted grub) and before any reference to the real root filesystem. To build an initramfs with EVM enabled, build and install dracut (git://git.kernel.org/pub/scm/boot/dracut/dracut.git), which contains the trusted and EVM dracut modules.

Labeling the filesystem with 'security.evm'

A new boot parameter 'evm=fix' has been defined in order to label existing file systems with the 'security.evm' extended attribute.

After building a kernel with EVM, IMA-appraisal, and trusted and encrypted keys enabled, installed the trusted and EVM dracut modules, created the EVM key, and verified that the filesystems are mounted, including root, with i_version support, to label the filesystem, reboot with the command line options 'ima_tcb', 'ima_appraise_tcb', 'ima_appraise=fix', 'evm=fix' and, possibly, 'rootflags=i_version'.

Once EVM is started, as existing file metadata changes or as new files are created, EVM assumes that the LSM has approved such changes, and automatically updates the HMACs accordingly, assuming the existing value is valid. In fix mode, opening a file owned by root, will fix the 'security.ima' extended attribute, causing the 'security.evm' extended attribute to be written as well, regardless if the existing security 'ima' or 'evm' extended attributes are valid. To label the entire filesystem, execute:

find / -fstype ext4 -type f -uid 0 -exec head -n 1 '{}' >/dev/null \;

The following sign_file script can be used to label all 'ELF' files with EVM and IMA digital signatures, and all other other files with just an EVM digital signature.

sign_file:

#!/bin/sh
#label "immutable" files with EVM/IMA digital signatures
#label everything else with just EVM digital signatures

file $1 | grep 'ELF' > /dev/null
if [ $? -eq 0 ]; then
     evmctl sign --imasig $1 /home/zohar/privkey_evm.pem
else
     evmctl sign --imahash $1 /home/zohar/privkey_evm.pem
fi

Instead of opening the file using head, digitally sign the files:

find / $ -fstype rootfs -o -fstype ext3 -o -fstype ext4 $ -type f -exec sign_file.sh {} \;

Once the filesystem has been properly labeled, before rebooting, re-install the new labeled kernel. Modify the initramfs to load the RSA public keys on the EVM and IMA keyring. Reboot with just the 'ima_tcb', 'ima_appraise_tcb' and, possibly, 'rootflags=i_version' options.

Compiling the kernel with EVM/IMA-appraisal enabled

For those unfamiliar with building a linux kernel, here is a short list of existing websites.

Configure and make the linux kernel

IMA, IMA-appraisal, EVM, and trusted/encrypted keys have all been upstreamed. Depending on the distro, some of these options might already have been enabled. For distros with recent kernels, download the distro's kernel source and recompile the kernel with the additional .config options, below. Refer to the distro's documentation for building the kernel from source.

For IMA, enable the following .config options:

CONFIG_INTEGRITY=y
CONFIG_IMA=y
CONFIG_IMA_MEASURE_PCR_IDX=10
CONFIG_IMA_AUDIT=y
CONFIG_IMA_LSM_RULES=y

For IMA-appraisal, enable the following .config options:

CONFIG_INTEGRITY_SIGNATURE=y
CONFIG_INTEGRITY=y
CONFIG_IMA_APPRAISE=y

EVM has a dependency on encrypted keys, which should be encrypted/decrypted using a trusted key, but could be encrypted/decrypted with a user-defined key instead. For EVM, enable the following .config options:

CONFIG_TCG_TPM=y

CONFIG_KEYS=y
CONFIG_TRUSTED_KEYS=y
CONFIG_ENCRYPTED_KEYS=y

CONFIG_INTEGRITY_SIGNATURE=y
CONFIG_INTEGRITY=y
CONFIG_EVM=y

For the new template architecture, which was upstreamed in Linux 3.13, use git to clone the 3.13 kernel.

$ cd ~/src/kernel
$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git

$ cd linux-stable
$ git remote update
$ git checkout --track -b linux-3.13.y origin/linux-3.13.y

In addition to the above .config options, enable the following options:
CONFIG_IMA_NG_TEMPLATE=y
CONFIG_IMA_DEFAULT_TEMPLATE="ima-ng"
CONFIG_IMA_DEFAULT_HASH_SHA256=y

Install the new kernel

Before installing the new kernel, clone dracut (git://git.kernel.org/pub/scm/boot/dracut/dracut.git). By default, masterkey and integrity are not enabled in the dracut git tree. Edit module-setup in both directories, changing the check() return value to 0. Then 'make' and 'install' dracut. Once an updated version of dracut is installed, continue with installing the kernel as normal.

$ su -c "make modules_install install"

Creating trusted and EVM encrypted keys

Trusted and encrypted keys are two new key types (upstreamed in 2.6.38) added to the existing kernel key ring service. Both of these new types are variable length symmetic keys and, in both cases, are created in the kernel. User space sees, stores, and loads only encrypted blobs. Trusted Keys require the availability of a Trusted Platform Module (TPM) chip for greater security, while encrypted keys can be used on any system. All user level blobs, are displayed and loaded in hex ascii for convenience, and are integrity verified.

Depending on the distro, trusted and encrypted keys might not be enabled. Refer to compiling the kernel, for directions on configuring and building a new kernel with trusted and encrypted key support.

The trusted and EVM dracut modules, by default, looks for the trusted and EVM encrypted keys in /etc/keys. To create and save the kernel master and EVM keys,

$ su -c 'mkdir -p /etc/keys'

# To create and save the kernel master key (trusted type):
$ su -c 'modprobe trusted encrypted'
$ su -c 'keyctl add trusted kmk-trusted "new 32" @u'
$ su -c 'keyctl pipe `keyctl search @u trusted kmk-trusted` >/etc/keys/kmk-trusted.blob'

# Create the EVM encrypted key
$ su -c 'keyctl add encrypted evm-key "new trusted:kmk-trusted 32" @u'
$ su -c 'keyctl pipe `keyctl search @u encrypted evm-key` >/etc/keys/evm-trusted.blob'

For those systems which don't have a TPM, but want to experiment with EVM, create a user key of 32 random bytes, and an EVM user encrypted key. Unlike trusted/encrypted keys, user type key data is visible to userspace.

$ su -c 'mkdir -p /etc/keys'

# To create and save the kernel master key (user type):
$ su -c 'modprobe trusted encrypted'
$ su -c 'keyctl add user kmk-user "`dd if=/dev/urandom bs=1 count=32 2>/dev/null`" @u'
$ su -c 'keyctl pipe `keyctl search @u user kmk-user` > /etc/keys/kmk-user.blob'

# Create the EVM encrypted key
$ su -c 'keyctl add encrypted evm-key "new user:kmk-user 32" @u'
$ su -c 'keyctl pipe `keyctl search @u encrypted evm-key` >/etc/keys/evm-user.blob'

Update /etc/sysconfig/masterkey to reflect using a 'user-defined' master key type.

MULTIKERNELMODE="NO"
MASTERKEYTYPE="user"
MASTERKEY="/etc/keys/kmk-${MASTERKEYTYPE}.blob"

Similarly update /etc/sysconfig/evm or on the boot command line specify the EVM key filename (eg. 'evmkey=/etc/keys/evm-user.blob'.)

Digital Signatures: generating an RSA public/private key pair

When using EVM/IMA-appraisal digital signatures, modify Dracut or initramfs-tools to load both the EVM and IMA digital signatures on their respective keyrings.

Getting and building ima-evm-utils

ima-evem-utils are used to sign content with private key or load public keys.

They can be cloned from repository with following command:

git clone git://linux-ima.git.sourceforge.net/gitroot/linux-ima/ima-evm-utils.git
cd ima-evm-utils
./autogen.sh
./configure
make
sudo make install

Generate private key

openssl tools is used to generate RSA keys.

# generate unencrypted private key
openssl genrsa -out privkey_evm.pem 1024

# or generate encrypted (password protected) private key
openssl genrsa -des3 -out privkey_evm.pem 1024

# or convert unencrypted key to encrypted on
openssl rsa -in /etc/keys/privkey_evm.pem -out privkey_evm_enc.pem -des3
or
openssl pkcs8 -topk8 -in /etc/keys/privkey_evm.pem -out privkey_evm_enc.pem

Read public key

openssl rsa -pubout -in privkey_evm.pem -out pubkey_evm.pem

Loading the public keys onto the IMA & EVM keyrings:

ima_id=`keyctl newring _ima @u`
evmctl import /etc/keys/pubkey_ima.pem $ima_id

Loading the EVM RSA public key on the EVM keyring:

evm_id=`keyctl newring _evm @u`
evmctl import /etc/keys/pubkey_evm.pem $evm_id

initramfs-tools & GRUB example

To enable IMA/EVM in initramfs-tools it is necessary to add just 2 files
to /etc/initramfs-tools directory

/etc/initramfs-tools/hooks/ima.sh

#!/bin/sh

echo "Adding IMA binaries"

. /usr/share/initramfs-tools/hook-functions

copy_exec /etc/keys/evm-key
copy_exec /etc/keys/pubkey_evm.pem
copy_exec /etc/ima_policy
copy_exec /bin/keyctl
copy_exec /usr/bin/evmctl /bin/evmctl

/etc/initramfs-tools/scripts/local-top/ima.sh

#!/bin/sh -e

PREREQ=""

# Output pre-requisites
prereqs()
{
        echo "$PREREQ"
}

case "$1" in
    prereqs)
        prereqs
        exit 0
        ;;
esac

grep -q "ima=off" /proc/cmdline && exit 1

mount -n -t securityfs securityfs /sys/kernel/security

IMA_POLICY=/sys/kernel/security/ima/policy
LSM_POLICY=/etc/ima_policy

grep -v "^#" $LSM_POLICY >$IMA_POLICY

# import EVM HMAC key
keyctl show |grep -q kmk || keyctl add user kmk "testing123" @u
keyctl add encrypted evm-key "load `cat /etc/keys/evm-key`" @u
#keyctl revoke kmk

# import Module public key
mod_id=`keyctl newring _module @u`
evmctl import /etc/keys/pubkey_evm.pem $mod_id

# import IMA public key
ima_id=`keyctl newring _ima @u`
evmctl import /etc/keys/pubkey_evm.pem $ima_id

# import EVM public key
evm_id=`keyctl newring _evm @u`
evmctl import /etc/keys/pubkey_evm.pem $evm_id

# enable EVM
echo "1" > /sys/kernel/security/evm

# enable module checking
#echo "1" > /sys/kernel/security/module_check

IMA policy example: /etc/ima-policy

#
# Integrity measure policy
# 
# Do not measure anything, but appraise everything
#
# PROC_SUPER_MAGIC
dont_appraise fsmagic=0x9fa0
# SYSFS_MAGIC
dont_appraise fsmagic=0x62656572
# DEBUGFS_MAGIC
dont_appraise fsmagic=0x64626720
# TMPFS_MAGIC
dont_appraise fsmagic=0x01021994
# RAMFS_MAGIC
dont_appraise fsmagic=0x858458f6
# DEVPTS_SUPER_MAGIC
dont_appraise fsmagic=0x1cd1
# BIFMT
dont_appraise fsmagic=0x42494e4d
# SECURITYFS_MAGIC
dont_appraise fsmagic=0x73636673
# SELINUXFS_MAGIC
dont_appraise fsmagic=0xf97cff8c
appraise

generate new initramfs

update-initramfs -k 3.4.0-rc5-kds+ -u

Edit GRUB bootloader /boot/grub/custom.cfg

menuentry 'IMA' {
    set gfxpayload=$linux_gfx_mode
    insmod gzio
    insmod part_msdos
    insmod ext2
    set root='(hd0,msdos1)'
    # add following string to kernel command line to enable "fix" mode: "ima_appraise=fix evm=fix"
    linux   /boot/vmlinuz-3.4.0-rc5-kds+ root=/dev/sda1 ro nosplash ima_audit=1 ima_tcb=1 ima_appraise_tcb=1
    initrd  /boot/initrd.img-3.4.0-rc5-kds+
}

Frequently asked questions

Why is the first entry in the IMA measurement list (/sys/kernel/security ima/ascii_runtime_measurements) are 0's?

The first entry is the TPM boot aggregate containing PCR values 0 -
7. Enable the TPM in BIOS and take ownership.
How do I take ownership of the TPM?

To take ownership of the TPM, download the tpm-tools, start tcsd (eg. 'service tcsd start'), and execute "tpm_takeownership -u -z". This will set the SRK key to the well-known secret(20 zeroes) and prompt for the TPM owner password.
Why are there 0x00 entries in the measurement list?

The measurement list is invalidated, when a regular file is opened for read and, at the same time, opened for write. In the majority of cases, these files should not have been measured in the first place (eg. log files). In other cases, the application needs to be fixed.
Why aren't files re-measured and added to the IMA measurement list
after being updated?

To detect files changing, the filesystem needs to be mounted with i_version support. For the root filesystem, either update /etc/rc.sysinit or add 'rootflags=iversion' boot command line option. For all other filesystems, modify /etc/fstab.
Why doesn't the measurement list verify?

On some systems, after a suspend/resume, the TPM measurement list does not verify. On those systems, add the boot command line option "tpm.suspend_pcr=< unused PCR >".
Why are there two /init entries in the measurement list?

The first '/init' is from the initramfs. The second /init is from the root filesystem (eg. /sbin/init). The IMA ng/nglong template patches will provide additional metadata to help correlate measurement entries and files.
Why am I unable to boot the new EVM/IMA-appraisal enabled kernel?

After building a new kernel with EVM/IMA-appraisal enabled, the filesystem must be labeled with 'security.evm' and 'security.ima' extended attributes. After creating an EVM
key, boot the new kernel with the 'ima_tcb', 'evm=fix', 'ima_appraise_tcb', 'ima_appraise=fix', and, possibly, 'rootflags=i_version' boot
command line options. Refer to [labeling the filesystem] #Labeling_the_filesystem_with_.27security.evm.27) with 'security.evm'.
How do I enable the measurement policy for local/remote attestation, without enabling IMA-appraisal?

Boot with the 'ima_tcb' command line option.
How do I enable the appraise policy, without the measurement policy?

Boot with the 'ima_appraise_tcb' command line option.

Integrity Measurement Architecture (IMA) Wiki