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enhancement-16: Remote Allow-List Retrieval

Release Signoff Checklist

  • Enhancement issue in release milestone, which links to pull request in [keylime/enhancements]
  • Core members have approved the issue with the label implementable
  • Design details are appropriately documented
  • Test plan is in place
  • User-facing documentation has been created in [keylime/keylime-docs]

Summary

This enhancement proposes a way to allow Keylime to automatically import IMA Allow-Lists from external sources. These allow-lists will follow a prescribed JSON format that allows the keylime_tenant to download, cryptographically verify and then upload these lists to the verifier. This will be done in versioned manner to allow upgrades and extensions in the future.

Motivation

Keylime currently places the burden of creating and managing allowlists upon the user. Currently the way to generate these allow-lists is to air-gap the machine after it’s been installed and configured and then run a script which crawls the filesystem and records SHA256 hashes into a file. This file is then loaded into keylime with the keylime_tenant command at which point the machine can be put back into circulation and keylime will continually verify it’s state.

This extra process is a burden when provisioning new resources for a cluster. It also means adding keylime to an existing cluster requires reducing the capacity of that cluster as batches of machines are pulled out for allow-list generation. Another downside is that the allow-lists generated are unique per-machine and managing large per-machine allow-lists is another administrative task on cluster operations.

We want to make this process simpler and more automated for more use cases. This will not only help Keylime users scale up their usage, but also increase adoption and make Keylime more attractive for downstream projects.

Goals

  • Create a simple, versioned JSON format spec for allow-lists
  • Make it easy and secure for keylime users to point keylime_tenant at remote allow-lists
  • Allow users to cryptographically verify the signature or checksum (or both) of the allow-lists
  • Document everything and provide at least one real world integration example (Fedora CoreOS)

Non-Goals

This will be the starting point for many future integrations with keylime, but for now we will not:

  • Integrate with other secure supply chain initiatives like in-toto, Tekton/chains or Co-Swids
  • Include non-files in the allow-lists (like SELinux Labels, boot arguments, etc)

Proposal

These are the proposed changes:

  • Document the first version of the allow-list JSON spec

  • Update the allowlist option to handle the new documented JSON spec

  • Add the following arguments to keylime_tenant

    • allowlist-url An HTTP URL that points to the remote JSON data file. This URL must be secure (HTTPS), and non-redirecting (no 30X HTTP codes) to reduce the chance of any MITM attacks. If this option is specified then you can't also have the --allowlist option present.

      If this option is present, then you must use one of allowlist-sig, allowlist-sig-url or allowlist-checksum to verify the contents of the downloaded file.

    • allowlist-sig The cryptographic signature of the remote JSON data file. Cannot be specified if allowlist isn't also present.

    • allowlist-sig-url An HTTP URL that points to the GPG signature of the JSON data file. Again, This URL must be secure (HTTPS), and non-redirecting (no 30X HTTP codes) to reduce the chance of any MITM attacks. This option cannot be specified if allowlist or allowlist-url isn't also present.

    • allowlist-sig-key The key used to verify the allowlist-sig or allowlist-sig-url and is required if one of those options is also present.

    • allowlist-checksum The SHA256 checksum of the allow-list. This can be used to verify that the allow-list hasn't been tampered. You must either verify the checksum or signature (or both) of the allow-list.

  • Expand the way allow-lists are passed between the tenant and the verifier to use JSON instead of the simple text file that exists now. This makes the contract more explicit and allows for expansion in the future.

  • Add routines to validate the JSON that will be utilized by both the tenant and the verifier. The tenant first to provide a better and faster experience for the end user and the verifier to make sure it can't be bypassed.

User Stories

Story 1

As a Fedora CoreOS user, I already know every package that could be present on my system based on my ostree version. I want to just tell Keylime where to find the list of file hashes for my specific version and let it do the rest

Story 2

I have a secure build process that can produce allow-lists for the software my team is building. I can store these allow-lists in an external repository (say an OCI registry). As part of my build/release process I have the cryptographic signatures of these allow-lists. I would like to tell keylime about these lists when deploying and provide the signature to keylime to verify it pulled the expected lists.

Risks and Mitigations

  • We will be updating the internal API between the tenant and the verifier to use the new JSON spec. Care will be taken to make sure this is done in a backwards compatible way so existing instances of keylime can continue to use their existing allowlists. If backwards compatibility can't be maintained good documentation and a utility in scripts/ will be created to help users migrate to the new format.

  • We will need to thoroughly review our handling of the external HTTP connections to make sure they are as secure as possible and don't do things like accept invalid TLS certificates.

  • We will need to thoroughly review our handling of the cryptographic signatures and checksums

Design Details

Propsed JSON spec:

{
    "meta": {
        "version":  "",    // Hash format version to allow for future changes if needed (required)
        "generator": "",   // What generated the hash payload
        "timestamp": ""    // When the hash payload was generated
    },
    "release": "",         // Release Identifier. Specifics here may be different for different compound components
    "hashes": {            // List of hashes to validate (required)
        "/filesystem/path":  [ "HASH1", "HASH2" ],
        "/filesystem/path2": [ "HASH3", "HASH4" ],
    }
}

Test Plan

TBD

Upgrade / Downgrade Strategy

TBD

Drawbacks

If we are concerned about not using existing data formats (like in-toto) we should not implement this design.

Alternatives

Instead of baking the URL retrieval and Hash verification into keylime_tenant itself, we could create an additional utility that would perform those actions and then translate the JSON allow-lists into the current format. Then keylime_tenant could be called on the new resulting allowlist.

I believe this is a less desireable approach because it means we have yet another script that users would have to run in order to get the desired integration. While one more step isn't drastic, it does allow more room for error and creates more friction.

Infrastructure Needed (optional)

For the keylime side there is no additional infrastructure needed. For the end-to-end documented example using Fedore CoreOS we will need to coordinate with that team to make sure the URLs are in place for allow-list retrieval and verification.