Add build environment track

* Draft levels Environment L0-L2
* Add build environment model

Signed-off-by: Marcela Melara <marcela.melara@intel.com>

docs/spec/draft/attested-build-env-levels.md

@@ -0,0 +1,235 @@
+---
+title: Build Environment track
+description: This page gives an overview of the SLSA Build Environment track and its levels, describing their security objectives and general requirements.
+---
+
+
+## Track overview
+
+The SLSA Build Environment track describes increasing levels of integrity and
+trustworthiness of the <dfn>provenance</dfn> of a build's execution context.
+In this track, provenance describes how the [hosted] build platform built the
+base [build image], what build environment they deployed, and the hardware
+platform they used.
+
+| Track/Level   | Requirements | Focus |
+| ------------- | ------------ | ----- |
+| [Environment L0]  | (none)       | (n/a)    |
+| [Environment L1]  | Signed build image provenance exists | Tampering during build image distribution |
+| [Environment L2]  | Attested build environment deployment | Tampering via the build platform's control plane |
+| [Environment L3]  | Hardware-authenticated build environment | Tampering via the compute platform's host interface |
+| [Environment L4]  | Encrypted build environment | Tampering and data leaks by the build platform or compute platform during the build |
+
+> [!IMPORTANT]
+> The Environment track currently requires a [hosted] build platform.
+> A future version of this track may generalize requirements to cover local
+> build environments (e.g., developer laptop).
+
+### Build environment model
+
+<p align="center"><img src="images/build-env-model.svg" alt="Model Build Environment"></p>
+
+The Build Environment track expands upon the [build model] by explicitily
+separating the *build image* and *compute platform* from the abstract build environment
+and build platform.
+
+A typical build environment will go through the following lifecycle:
+
+1.  *Build image creation*: A hosted build platform creates different build
+    images through a separate build process. For the SLSA HABE Track, the
+    hosted build platform outputs SLSA provenance describing this process.
+2.  *Build environment deployment*: The hosted build platform calls into the
+    *host interface* to deploy a new build environment from a given build image
+    on the underlying compute platform.
+    For the SLSA HABE Track, the build platform attests to the *measurement*
+    of the environment's *boot process*.
+3.  *Build dispatch*: When the tenant dispatches a new build, the hosted
+    build platform assigns the build to a deployed build environment. For
+    the SLSA HABE Track, the build platform attests to the binding between
+    a build environment and *build ID*.
+4.  *Build execution*: Finally, the *build executor* running within the
+    environment executes the tenant's build definition.
+
+| Primary Term | Description
+| --- | ---
+| Build ID | An immutable identifier assigned uniquely to a specific execution of a tenant's build. In practice, the build ID may be a cryptographic key or other unique and immutable identfier (e.g., a UUID) associated with the build execution.
+| Build image | The template for a build runtime environment, such as a VM or container image. Individual components of a build image are provided by the hosted build platform, and include the bootable storage volume containing the build executor, a dedicated build platform client, and platform-provided pre-installed guest OS and packages.
+| Build executor | The platform-provided program dedicated to executing the tenant’s build definition, i.e., running the build, within the build image. The build executor must be included in the build image's measurement.
+| Build dispatch | The process of assigning a tenant's build to a pre-deployed build environment on a hosted build platform.
+| Compute platform | The compute system and infrastructure, i.e., the host system (hypervisor and/or OS) and hardware, underlying a build platform. In practice, the compute platform and the build platform may be managed by the same or distinct organizations.
+| Boot process | In the context of builds, the process of loading and executing the layers of firmware and software needed to start up a build environmenton the build platform.
+| Measurement | The cryptographic hash of some system state in the build environment, including software binaries, configuration, or initialized run-time data. Software layers that are commonly measured include the bootloader, kernel, and kernel cmdline.
+
+TODO: Disambiguate similar terms (e.g., image, build job, build runner)
+
+## Environment levels
+
+The lowest level only requires SLSA Build L2 (or higher) Provenance to exist
+for the build image, while higher levels provide increasing auditability of
+the build environment's properties and integrity of the generated provenance
+attestations. The highest levels introduce further requirements for hardware-
+assisted hardening aimed at reducing the trusted computing base of a build.
+
+The primary purpose of the Environment track is to enable [auditing] that a
+build was run in the expected compute environment. The build platform and
+producers can check attestations generated by the build platform against the
+expected properties for a given build environment. This enables any party to
+detect several classes of supply chain threats originating in the build
+environment [TODO: Link here].
+
+As in the Build track, the exact implementation of this track is determined
+by the build platform provider, whether they are a commercial CI/CD service,
+or enterprise organization. While this track describes
+general [TODO: minimum requirements], this track does not dictate the
+following implementation-specific details: the type of build environment
+environment, accepted attestation formats, the type of technologies used to
+meet L3/4 requirements, what SLSA [Build] level the *built artifacts* meet,
+how attestations are distributed, how build environments are identified, and
+what happens on failure.
+
+<section id="environment-l0">
+
+### Environment L0: No guarantees
+
+<dl class="as-table">
+<dt>Summary<dd>
+
+No requirements---L0 represents the lack of any sort of build environment provenance.
+
+<dt>Intended for<dd>
+
+Development or test builds of software that are built and run on the same
+machine, such as unit tests.
+
+<dt>Requirements<dd>
+
+n/a
+
+<dt>Benefits<dd>
+
+n/a
+
+</dl>
+</section>
+
+<section id="environment-l1">
+
+### Environment L1: Signed build image provenance exists
+
+<dl class="as-table">
+<dt>Summary<dd>
+
+The build image (i.e., VM or container image) used to instantiate the build
+environment has SLSA provenance showing how the image was built.
+
+<dt>Intended for<dd>
+
+Build platforms and organizations wanting to ensure a baseline level of
+integrity for build environments at the time of build image generation.
+
+<dt>Software Producer Requirements<dd>
+
+-   MUST run builds using a build image that was built by a hosted build
+    platform that meets Environment L1 requirements.
+
+-   SHOULD verify the build image's SLSA Build provenance for the selected
+    build image, and distribute evidence of the verification to consumers
+    (e.g., using a [VSA])
+
+<dt>Build Platform Requirements<dd>
+
+-   MUST automatially generate and distribute SLSA [Build L1] or higher
+    Provenance for its supplied build images (i.e., VM or container images).
+
+-   The initial state of the build environment's root storage volume
+    MUST be integrity measured and signed.
+
+-   MUST execute builds on infrastructure that meets SLSA [Build L2].
+
+<dt>Benefits<dd>
+
+-   Provides evidence that a build image provided by a hosted build platform
+    was built from the advertised source and build process.
+
+</dl>
+
+</section>
+<section id="environment-l2">
+
+### Environment L2: Attested build environment instantiation
+
+<dl class="as-table">
+<dt>Summary<dd>
+
+The deployed build environment is integrity measured and authenticated
+prior to giving access to the tenant, attesting to the initial state of the
+environment.
+
+<dt>Intended for<dd>
+
+Organizations wanting to ensure that a specific build is running
+in the expected build environment.
+
+All of [Environment L1], plus:
+
+<dt>Software Producer Requirements<dd>
+
+-   MUST run builds in a build environment on a hosted build platform that
+    meets Environment L2 requirements.
+
+-   SHOULD verify the build image's Build SLSA provenance for the selected
+    build image, and distribute evidence of the verification to consumers
+    (e.g., using a [VSA])
+
+<dt>Build Platform Requirements<dd>
+
+-   MUST automatially generate and distribute SLSA [Build L2] or higher
+    Provenance for its supplied build images (i.e., VM or container images).
+
+-   The boot process of each build environment MUST be cryptographically measured
+    and signed.
+
+-   MUST distribute the boot and root storage attestations to allow for
+    independent verification.
+
+-   MUST uniquely bind a build image to a build environment instance.
+
+-   MUST uniquely and immutably bind an instance of a build to its build
+    environment.
+
+-   MUST execute builds in an environment that meets SLSA [Build L3].
+
+<dt>Benefits<dd>
+
+-   Provides evidence that a build environment deployed by a hosted build
+    platform was initialized from the expected build image.
+
+</dl>
+
+</section>
+<section id="environment-l3">
+
+### Environment L3: Hardware-authenticated build environment
+TODO
+
+</section>
+<section id="environment-l4">
+
+### Environment L4: Encrypted build environment
+
+TODO
+
+</section>
+
+<!-- Link definitions -->
+
+[Build L1]: levels.md#build-l1
+[Build L2]: levels.md#build-l2
+[Build L3]: levels.md#build-l3
+[Environment L0]: #environment-l0
+[Environment L1]: #environment-l1
+[Environment L2]: #environment-l2
+[Environment L3]: #environment-l3
+[Environment L4]: #environment-l4
+[future versions]: future-directions.md
+[hosted]: requirements.md#isolation-strength