2 Remote attestation using Keylime #

Keylime is a remote attestation solution that enables you to monitor the health of remote nodes using a TPM as a root of trust for measurement. With Keylime, you can perform several tasks like validation of the PCRs extended during the measured boot, analysis and assertions of the event log, assertion of the value of any PCR in the remote system, and also monitoring the validity of open or executed files.

Keylime also provides a framework for delivering encrypted data to verified nodes. This data is the so-called payload. The framework can also execute custom scripts that are triggered when a machine fails the attested measurements.

Keylime consists of an agent, a verifier, a registrar, and a command-line tool (tenant). Agents are on those systems that need to be attested. The verifier and registrar are on remote systems that perform the registration and attestation of agents. Keep in mind that only the agent role is available on SLE Micro. For details about each component, refer to the following sections.

Before you start installing and registering agents, prepare the verifier and the registrar on remote hosts as described in the following procedure.

The Keylime agent is not present on SLE Micro by default, you need to install it manually. To install the agent, proceed as follows:

Note: SLE Micro does not provide the tenant, registrar or verifier

SLE Micro provides only the Keylime agent capability, therefore, the tasks performed in this chapter cannot be performed from your SLE Micro.

You can register a new agent either by using the CLI tenant or by editing the configuration of the verifier. Using the tenant on the verifier host, run the following:

# keylime_tenant -v 127.0.0.1 \
                -t AGENT \
                -u UUID \
                --cert default \
                -c add
                [--include PATH_TO_ZIP_FILE]

Where:

You can list registered agents by using the reglist command on the verifier host as follows:

# keylime_tenant -v 127.0.0.1 \
                --cert default \
                -c reglist

To remove a registered agent, specify the agent using the -t and -u options and the -c delete command as follows:

# keylime_tenant -v 127.0.0.1 \
              -t AGENT \
              -u UUID \
              -c delete

A secure payload enables you to deliver encrypted data to healthy agents. Typically, these payloads are used to provide keys, passwords, certificates, configurations, or scripts that are further used by the agent.

The secure payload is delivered to the agent in a zip file that must contain a shell script—autorun.sh. The script will be executed only if the agent has been properly registered and verified. To deliver the zip file, use the --include option of the keylime_tenant command.

The script autorun.sh contains steps that will enable the use of passwords, certificates and so on. For example, the script can create a directory structure and copy SSH keys there:

#!/bin/bash
 
 mkdir -p /root/.ssh/
 cp id_rsa* /root/.ssh/
 chmod 600 /root/.ssh/id_rsa*
 cp /root/.ssh/id_rsa.pub /root/.ssh/authorized_keys

In this case, do not forget to include the SSH keys in the zip file.

When using IMA, the kernel calculates a hash of accessed files. The hash is then used to extend the PCR 10 in the TPM and also log a list of accessed files. The verifier can request a signed quote to the agent for PCR 10 to get the logs of all accessed files including the file hashes. Verifiers then compare the accessed files with a local allowlist of approved files. If any of the hashes are not recognized, the system is considered unsafe, and a revocation event is triggered.

For a high-level overview of IMA/EVM, refer to IMA/EVM introduction.

Before Keylime can collect information, IMA/EVM needs to be enabled. To enable the process, boot a kernel of the agent with the parameters: ima_appraise=log and ima_policy=tcb. To use the boot parameters on boot, proceed as follows:

The procedure above uses the default kernel IMA policy, but we recommend creating a new policy to avoid monitoring too many files and therefore creating long logs. For details, refer to the Keylime documentation.

To indicate the expected hashes, use the --allowlistoption of the keylime_tenant command when registering the agent. To view the excluded or ignored files, use the --exclude option of the keylime_tenant command:

# keylime_tenant --allowlist
    -v 127.0.0.1 \
    -u UUID