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Allow platforms to provide run image sboms #211

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Allow platforms to provide run image sboms #211

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Proposes a mechanism for platforms, when running builds, to provide a…
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# Meta
[meta]: #meta
- Name: Run Image SBOM
- Start Date: 2022-03-09
- Author(s): natalieparellano
- Status: Draft <!-- Acceptable values: Draft, Approved, On Hold, Superseded -->
- RFC Pull Request: (leave blank)
- CNB Pull Request: (leave blank)
- CNB Issue: (leave blank)
- Supersedes: https://github.com/buildpacks/rfcs/pull/186

# Summary
[summary]: #summary

This RFC proposes a mechanism for platforms, when running builds, to provide an sbom for the run image that will be included in the final app image.

# Definitions
[definitions]: #definitions

- Run Image: A container image that serves as a base image for application images in the buildpack toolchain.
- SBOM (Software Bill Of Materials) / BOM: A list of components in a piece of software. Software vendors often create products by assembling open source and commercial software components. The SBOM describes the components in a product. In case of buildpacks the SBOM describes the contents of the various layers, buildpacks, base images and the output app container.
- Attestation: authenticated metadata about one or more software artifacts, as per the SLSA Attestation Model (see [here](https://github.com/in-toto/attestation) and [here](https://github.com/sigstore/cosign#in-toto-attestations)).
- Cosign sbom attachment: an sbom object represented as an OCI Image Manifest V1 (see [here](https://github.com/sigstore/cosign/blob/main/specs/SBOM_SPEC.md)).

# Motivation
[motivation]: #motivation

This RFC serves as an addendum to [#RFC 95: Structured SBOMs](https://github.com/buildpacks/rfcs/blob/main/text/0095-sbom.md), which introduced a structured way for buildpacks to provide an sbom for dependencies installed at build time. For a full depiction of dependencies within a container image, container scanning tools would _also_ need an sbom for OS packages that were installed on the run image when it was created.

# What it is
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[what-it-is]: #what-it-is
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- Define the target persona: platform operator, platform implementor, end user.
- If applicable, provide sample error messages, deprecation warnings, or migration guidance.
- The lifecycle could warn if a usable run image sbom is not provided.

Example invocation: `/cnb/lifecycle/exporter -run-image my-run-image -run-image-sbom my-run-image-sbom.json my-app-image`
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# How it Works
[how-it-works]: #how-it-works

Following the invocation above, the exported app image would contain:
* `my-run-image-sbom.json` somewhere in `/layers/sbom`; suggestions:
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* `/layers/sbom/launch/run-image/sbom.<ext>` (this would make `run-image` a reserved buildpack id)
* `/layers/sbom/run-image/sbom.<ext>`
* ?
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The accepted media types would be:
* `application/vnd.cyclonedx+json` with ext `cdx.json`
* `application/spdx+json` with ext `spdx.json`
* `application/vnd.syft+json` with ext `syft.json`
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Related to buildpacks/lifecycle#828, these media types need to be accepted with parameters (e.g. version) so that end users / buildpack authors can maintain stronger contracts about the schema versions of the SBOMs that come out of builds.

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Thinking about how buildpacks "declare" the media types they support, in buildpack.toml - would we want to do the same for the run image? For example, to put a label on the final app image alerting vulnerability scanners which formats are supported. We could put the supported types in a label on the run image. We'd need to take care during rebase that the label is still accurate.

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Added in latest commit - but since we'd be deriving the media type from the file extension, I'm not sure how to derive the parameter. Could we have more flexible file names? Could the platform provide the media type as a separate argument?

* others?

# Drawbacks
[drawbacks]: #drawbacks

Why should we *not* do this? Leaving the location of the run image sbom unspec'd could make it harder for logic-less platforms like Tekton to use the creator. However, a CNB-provided "prepare" operation ([currently under discussion](https://github.com/buildpacks/rfcs/pull/202)) could make this easier. For example, a preparer could look for a run image sbom in a file, attestation, attachment, or layer (picking the first one it finds) and provide the data as a file to the creator. The Tekton task that uses the creator currently already has a ["prepare" step](https://github.com/tektoncd/catalog/blob/4bf8b57aa105f0c7ce05fc122a11b1b0d5822fcd/task/buildpacks/0.3/buildpacks.yaml#L70-L121) which could be modified to invoke a preparer binary.
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# Alternatives
[alternatives]: #alternatives
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Wasn't there an alternative discussed where we would leave it up to the platform to determine the method of attaching an sbom? For instance, if our platform want to use more of an associative artifact (similar to cosign) we could do that. It seems like this RFC only proposes one of what I see possibly being many ways for platforms to want to add this feature. Would it be better if we spec'd out the possible options and a way for further tools to be able to discover how the run image sbom is attached to the app image?

For example, some label that states a) "it's this layer here", or b) "it's located at this URI".

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@natalieparellano natalieparellano Mar 24, 2022 •
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I think there are two problems to solve here - one is how to attach or associate an SBOM with the run image. The other is how to attach or associate a run image SBOM with a CNB-built app image. This RFC is essentially saying we shouldn't worry about the first issue. As for the second issue, this RFC proposes staying consistent with the pattern introduced in https://github.com/buildpacks/rfcs/blob/main/text/0095-sbom.md and put the run image sbom somewhere in /layers/sbom.


- What other designs have been considered? [Adding the sbom as a layer](https://github.com/buildpacks/rfcs/pull/186) to the run image and adding its diffID as a label. Some [concerns](https://github.com/buildpacks/rfcs/pull/186#issuecomment-1043348097) with this proposal included:
* Special tooling is required to create the run image (because we don't know the diffID ahead of time)
* The run image may [fall out of date](https://github.com/buildpacks/rfcs/pull/186#discussion_r773246636) in a way that is not obvious
* We don't want to "pick a winner" just yet for the mechanism for attaching sboms to the run image
- Why is this proposal the best? This frees platforms to choose their own method of attaching sboms to the run image, while providing a standardized way to consume a complete sbom for the app image.
- What is the impact of not doing this? Each platform will have to come up with its own way of attaching run image sboms to the app image.

# Prior Art
[prior-art]: #prior-art

- [#RFC 95: Structured SBOMs](https://github.com/buildpacks/rfcs/blob/main/text/0095-sbom.md)

# Unresolved Questions
[unresolved-questions]: #unresolved-questions

- What should be the path of the run image sbom within the image?
- How should the layer containing the run image sbom be designated? This layer containing the buildpack-provided sbom is referenced in the `io.buildpacks.lifecycle.metadata` label with key `sbom`. #186 proposed `io.buildpacks.base.sbom` for the layer containing the run image sbom, and `io.buildpacks.app.sbom` for the layer containing the buildpack-provided sbom (in addition to `io.buildpacks.lifecycle.metadata`).
- What should be all the accepted media types / extensions?

- What related issues do you consider out of scope for this RFC that could be addressed in the future independently of the solution that comes out of this RFC?
- The existence or behavior of a preparer binary that knows how to download run image sboms.

# Spec. Changes (OPTIONAL)
[spec-changes]: #spec-changes
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Should this section be filled in?

Does this RFC entail any proposed changes to the core specifications or extensions? New fields on the exporter and rebaser. A new label on the app image to designate the layer containing the run image sbom.