attest/attestation/layout.go

/*
   Copyright Docker attest authors

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.
*/

package attestation

import (
	"context"
	"encoding/json"
	"fmt"

	containerd "github.com/containerd/containerd/v2/core/images"
	"github.com/distribution/reference"
	"github.com/docker/attest/oci"
	v1 "github.com/google/go-containerregistry/pkg/v1"
	"github.com/google/go-containerregistry/pkg/v1/layout"
	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
)

// implementation of Resolver that closes over attestations from an oci layout.

var _ Resolver = (*LayoutResolver)(nil)

type LayoutResolver struct {
	*Manifest
	*oci.ImageSpec
}

func NewOCILayoutResolver(src *oci.ImageSpec) (*LayoutResolver, error) {
	r := &LayoutResolver{
		ImageSpec: src,
	}
	_, err := r.fetchManifest()
	if err != nil {
		return nil, err
	}
	return r, nil
}

func (r *LayoutResolver) fetchManifest() (*Manifest, error) {
	if r.Manifest == nil {
		m, err := manifestFromOCILayout(r.Identifier, r.ImageSpec.Platform)
		if err != nil {
			return nil, err
		}
		r.Manifest = m
	}

	return r.Manifest, nil
}

func (r *LayoutResolver) Attestations(_ context.Context, predicateType string) ([]*EnvelopeReference, error) {
	var envs []*EnvelopeReference
	dsseMediaType, err := DSSEMediaType(predicateType)
	if err != nil {
		return nil, fmt.Errorf("failed to get DSSE media type for predicate '%s': %w", predicateType, err)
	}
	for _, attestationLayer := range r.Manifest.OriginalLayers {
		mt, err := attestationLayer.Layer.MediaType()
		if err != nil {
			return nil, fmt.Errorf("failed to get layer media type: %w", err)
		}
		mts := string(mt)
		if mts != dsseMediaType {
			continue
		}
		env := new(EnvelopeReference)
		// parse layer blob as json
		layer, err := attestationLayer.Layer.Uncompressed()
		if err != nil {
			return nil, fmt.Errorf("failed to get layer contents: %w", err)
		}
		defer layer.Close()
		err = json.NewDecoder(layer).Decode(env)
		if err != nil {
			return nil, fmt.Errorf("failed to decode envelope: %w", err)
		}
		var uri string
		if len(r.Manifest.OriginalDescriptor.URLs) > 0 {
			uri = r.Manifest.OriginalDescriptor.URLs[0]
		}
		env.ResourceDescriptor = &ResourceDescriptor{
			MediaType: string(mt),
			Digest:    map[string]string{r.Manifest.OriginalDescriptor.Digest.Algorithm: r.Manifest.OriginalDescriptor.Digest.Hex},
			URI:       uri,
		}
		envs = append(envs, env)
	}
	return envs, nil
}

func (r *LayoutResolver) ImageName(_ context.Context) (string, error) {
	return r.SubjectName, nil
}

func (r *LayoutResolver) ImageDescriptor(_ context.Context) (*v1.Descriptor, error) {
	return r.SubjectDescriptor, nil
}

func (r *LayoutResolver) ImagePlatform(_ context.Context) (*v1.Platform, error) {
	return r.ImageSpec.Platform, nil
}

func manifestFromOCILayout(path string, platform *v1.Platform) (*Manifest, error) {
	layoutIndex, err := layout.ImageIndexFromPath(path)
	if err != nil {
		return nil, err
	}

	layoutIndexManifest, err := layoutIndex.IndexManifest()
	if err != nil {
		return nil, fmt.Errorf("failed to get digest: %w", err)
	}

	layoutDescriptor := layoutIndexManifest.Manifests[0]
	layoutDescriptorDigest := layoutDescriptor.Digest
	subjectName := layoutDescriptor.Annotations[ocispec.AnnotationRefName]
	if _, err := reference.ParseNamed(subjectName); err != nil {
		// try the containerd annotation if the org.opencontainers.image.ref.name is not a full name
		subjectName = layoutDescriptor.Annotations[containerd.AnnotationImageName]
		if _, err := reference.ParseNamed(subjectName); err != nil {
			return nil, fmt.Errorf("failed to find subject name in annotations")
		}
	}

	// check if digest refers to an image or an index
	_, err = layoutIndex.Image(layoutDescriptorDigest)
	if err == nil {
		return &Manifest{
			OriginalLayers:     nil,
			OriginalDescriptor: nil,
			SubjectName:        subjectName,
			SubjectDescriptor:  &layoutDescriptor,
		}, nil
	}

	subjectIndex, err := layoutIndex.ImageIndex(layoutDescriptorDigest)
	if err != nil {
		return nil, fmt.Errorf("failed to extract ImageIndex for digest %s: %w", layoutDescriptorDigest.String(), err)
	}
	subjectIndexManifest, err := subjectIndex.IndexManifest()
	if err != nil {
		return nil, fmt.Errorf("failed to extract IndexManifest from ImageIndex: %w", err)
	}
	var subjectDescriptor *v1.Descriptor
	for i := range subjectIndexManifest.Manifests {
		manifest := &subjectIndexManifest.Manifests[i]
		if manifest.Platform != nil {
			if manifest.Platform.Equals(*platform) {
				subjectDescriptor = manifest
				break
			}
		}
	}
	if subjectDescriptor == nil {
		return nil, fmt.Errorf("platform not found in index")
	}
	for i := range subjectIndexManifest.Manifests {
		mf := &subjectIndexManifest.Manifests[i]
		if mf.Annotations[DockerReferenceType] != AttestationManifestType {
			continue
		}

		if mf.Annotations[DockerReferenceDigest] != subjectDescriptor.Digest.String() {
			continue
		}

		attestationImage, err := subjectIndex.Image(mf.Digest)
		if err != nil {
			return nil, fmt.Errorf("failed to extract attestation image with digest %s: %w", mf.Digest.String(), err)
		}
		layers, err := layersFromImage(attestationImage)
		if err != nil {
			return nil, fmt.Errorf("failed to get attestations from image: %w", err)
		}
		attest := &Manifest{
			OriginalLayers:     layers,
			OriginalDescriptor: mf,
			SubjectName:        subjectName,
			SubjectDescriptor:  subjectDescriptor,
		}
		return attest, nil
	}
	return nil, fmt.Errorf("attestation manifest not found")
}