post_results.go

// Copyright 2022 OpenSSF Scorecard 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 server

import (
	"bytes"
	"context"
	"crypto/ecdsa"
	"crypto/sha256"
	"crypto/x509"
	_ "embed"
	"encoding/base64"
	"encoding/hex"
	"encoding/json"
	"encoding/pem"
	"errors"
	"fmt"
	"log"
	"net/http"
	"strings"
	"time"

	"github.com/cyberphone/json-canonicalization/go/src/webpki.org/jsoncanonicalizer"
	"github.com/go-openapi/runtime/middleware"
	"github.com/go-openapi/strfmt"
	"github.com/google/go-github/v42/github"
	merkleproof "github.com/transparency-dev/merkle/proof"
	"github.com/transparency-dev/merkle/rfc6962"
	"gocloud.dev/blob"

	"github.com/ossf/scorecard-webapp/app/generated/models"
	"github.com/ossf/scorecard-webapp/app/generated/restapi/operations/results"
	"github.com/ossf/scorecard-webapp/app/server/internal/hashedrekord"
)

const (
	// OID source: https://github.com/sigstore/fulcio/blob/96ef49cc7662912ba37d46f738757e8d8d5b5355/docs/oid-info.md#L33
	// TODO: retrieve these by name.
	fulcioRepoRefKey  = "1.3.6.1.4.1.57264.1.6"
	fulcioRepoPathKey = "1.3.6.1.4.1.57264.1.5"
	fulcioRepoSHAKey  = "1.3.6.1.4.1.57264.1.3"
	resultsBucket     = "gs://ossf-scorecard-results"
	resultsFile       = "results.json"
	noTlogIndex       = 0
)

var (
	errWritingBucket            = errors.New("error writing to GCS bucket")
	errMultipleCerts            = errors.New("multiple certificates found for the entry")
	errEmptyCertRef             = errors.New("cert has empty repository ref")
	errEmptyCertPath            = errors.New("cert has empty repository path")
	errCertMissingURI           = errors.New("certificate has no URIs")
	errCertWorkflowPathEmpty    = errors.New("cert workflow path is empty")
	errMismatchedCertAndRequest = errors.New("repository and branch of cert doesn't match that of request")
	errNotDefaultBranch         = errors.New("branch of cert isn't the repo's default branch")
	errNoTlogEntry              = errors.New("no transparency log entry found")
	errNotRekordEntry           = errors.New("not a rekord entry")
	errMismatchedTlogEntry      = errors.New("tlog entry does not match payload")
)

type certInfo struct {
	repoFullName  string
	repoBranchRef string
	repoSHA       string
	workflowPath  string
	workflowRef   string
}

type tlogEntry struct {
	Body           string `json:"body"`
	IntegratedTime int64  `json:"integratedTime"`
	LogID          string `json:"logID"`
	LogIndex       int64  `json:"logIndex"`
	Verification   *struct {
		InclusionProof *struct {
			Hashes   []string `json:"hashes"`
			RootHash string   `json:"rootHash"`
			TreeSize uint64   `json:"treeSize"`
			LogIndex uint64   `json:"logIndex"`
		} `json:"inclusionProof,omitempty"`
		SignedEntryTimestamp strfmt.Base64 `json:"signedEntryTimestamp,omitempty"`
	} `json:"verification"`
}

//go:embed fulcio_v1.crt.pem
var fulcioRoot []byte

//go:embed fulcio_intermediate.crt.pem
var fulcioIntermediate []byte

//go:embed rekor.pub
var rekorPub []byte

func PostResultsHandler(params results.PostResultParams) middleware.Responder {
	// Sanity check
	host := params.Platform
	orgName := params.Org
	repoName := params.Repo

	// Process
	err := processRequest(host, orgName, repoName, params.Publish)
	if err == nil {
		return results.NewPostResultCreated().WithPayload("successfully verified and published ScorecardResult")
	}
	var vErr verificationError
	if errors.As(err, &vErr) {
		return results.NewPostResultBadRequest().WithPayload(&models.Error{
			Code:    http.StatusBadRequest,
			Message: vErr.Error(),
		})
	}
	log.Println(err)
	return results.NewPostResultDefault(http.StatusInternalServerError).WithPayload(&models.Error{
		Code:    http.StatusInternalServerError,
		Message: "something went wrong and we are looking into it.",
	})
}

func processRequest(host, org, repo string, scorecardResult *models.VerifiedScorecardResult) error {
	ctx := context.Background()
	cert, err := extractAndVerifyCertForPayload(ctx, []byte(scorecardResult.Result), scorecardResult.TlogIndex)
	if err != nil {
		return fmt.Errorf("error extracting cert: %w", err)
	}

	info, err := extractCertInfo(cert)
	if err != nil {
		return fmt.Errorf("error extracting cert info: %w", err)
	}
	if info.repoFullName != fullName(org, repo) ||
		(info.repoBranchRef != scorecardResult.Branch &&
			info.repoBranchRef != fmt.Sprintf("refs/heads/%s", scorecardResult.Branch)) {
		return verificationError{e: errMismatchedCertAndRequest}
	}

	if err := getAndVerifyWorkflowContent(ctx, scorecardResult, info); err != nil {
		return fmt.Errorf("workflow verification failed: %w", err)
	}

	// Save scorecard results (results.json, score.txt) to GCS
	bucketURL := resultsBucket
	objectPath := fmt.Sprintf("%s/%s/%s/%s", host, org, repo, resultsFile)
	if err := writeToBlobStore(ctx, bucketURL, objectPath, []byte(scorecardResult.Result)); err != nil {
		return fmt.Errorf("%w: %v", errWritingBucket, err)
	}

	commitObjectPath := fmt.Sprintf("%s/%s/%s/%s/%s", host, org, repo, info.repoSHA, resultsFile)
	if err := writeToBlobStore(ctx, bucketURL, commitObjectPath, []byte(scorecardResult.Result)); err != nil {
		return fmt.Errorf("%w: %v", errWritingBucket, err)
	}

	return nil
}

func fullName(org, repo string) string {
	return fmt.Sprintf("%s/%s", org, repo)
}

// splitFullPath extracts the org, repo, and path from a full path of the form org/repo/rest/of/path.
func splitFullPath(path string) (org, repo, subPath string, ok bool) {
	parts := strings.SplitN(path, "/", 3)
	if len(parts) < 3 {
		return "", "", "", false
	}
	return parts[0], parts[1], parts[2], true
}

// getAndVerifyWorkflowContent retrieves the workflow content from the repository and verifies it.
// It verifies the branch is a default branch and gets the scorecard workflow from the repository
// from the specific commit and verifies it to ensure that it hasn't been tampered with.
func getAndVerifyWorkflowContent(ctx context.Context,
	scorecardResult *models.VerifiedScorecardResult, info certInfo,
) error {
	org, repo, path, ok := splitFullPath(info.workflowPath)
	if !ok {
		return fmt.Errorf("cert workflow path is malformed")
	}
	workflowRepoFullName := fullName(org, repo)

	// Get the corresponding GitHub repository.
	httpClient := http.DefaultClient
	if scorecardResult.AccessToken != "" {
		httpClient.Transport = githubTransport{
			token: scorecardResult.AccessToken,
		}
	}
	client := github.NewClient(httpClient)
	repoClient, _, err := client.Repositories.Get(ctx, org, repo)
	if err != nil {
		return fmt.Errorf("error getting repository: %w", err)
	}

	// Verify that the branch from the results files is the repo's default branch.
	defaultBranch := repoClient.GetDefaultBranch()
	if scorecardResult.Branch != defaultBranch &&
		scorecardResult.Branch != fmt.Sprintf("refs/heads/%s", defaultBranch) {
		return verificationError{e: errNotDefaultBranch}
	}

	// Use the cert commit SHA if the workflow file is in the repo being analyzed.
	// Otherwise fall back to the workflowRef, which may be a commit SHA, or it may be more vague e.g. refs/heads/main
	opts := &github.RepositoryContentGetOptions{Ref: info.repoSHA}
	if workflowRepoFullName != info.repoFullName {
		opts.Ref = info.workflowRef
	}

	contents, _, _, err := client.Repositories.GetContents(ctx, org, repo, path, opts)
	if err != nil {
		return fmt.Errorf("error downloading workflow contents from repo: %w", err)
	}

	workflowContent, err := contents.GetContent()
	if err != nil {
		return fmt.Errorf("error decoding workflow contents: %w", err)
	}

	verifier := &githubVerifier{
		ctx:    ctx,
		client: client,
	}
	// Verify scorecard workflow.
	return verifyScorecardWorkflow(workflowContent, verifier)
}

func writeToBlobStore(ctx context.Context, bucketURL, filename string, data []byte) error {
	bucket, err := blob.OpenBucket(ctx, bucketURL)
	if err != nil {
		return fmt.Errorf("error from blob.OpenBucket: %w", err)
	}
	defer bucket.Close()

	blobWriter, err := bucket.NewWriter(ctx, filename, nil)
	if err != nil {
		return fmt.Errorf("error from bucket.NewWriter: %w", err)
	}
	if _, err = blobWriter.Write(data); err != nil {
		return fmt.Errorf("error from blobWriter.Write: %w", err)
	}
	if err := blobWriter.Close(); err != nil {
		return fmt.Errorf("error from blobWriter.Close: %w", err)
	}
	return nil
}

func extractAndVerifyCertForPayload(ctx context.Context, payload []byte, tlogIndex int64) (*x509.Certificate, error) {
	var entry *tlogEntry
	var uuid string
	var err error

	// #135 older versions of scorecard action wont send the tlog index, but newer ones will
	if tlogIndex == noTlogIndex {
		// Get most recent Rekor entry uuid.
		uuids, err := getUUIDsByPayload(ctx, payload)
		if err != nil || len(uuids) == 0 {
			return nil, fmt.Errorf("error finding tlog entries corresponding to payload: %w", err)
		}
		uuid = uuids[len(uuids)-1] // ignore past entries.

		// Get tlog entry from the UUID.
		entry, err = getTLogEntryByUUID(ctx, uuid)
		if err != nil {
			return nil, fmt.Errorf("error fetching tlog entry: %w", err)
		}
	} else {
		uuid, entry, err = getTLogEntryByIndex(ctx, tlogIndex)
		if err != nil {
			return nil, fmt.Errorf("error fetching tlog entry: %w", err)
		}
	}

	rekordBody, err := entry.rekord()
	if err != nil {
		return nil, err
	}

	if !rekordBody.Matches(payload) {
		return nil, errMismatchedTlogEntry
	}

	// Verify inclusion proof.
	if err = verifyInclusionProof(uuid, entry); err != nil {
		return nil, fmt.Errorf("unable to verify rekor inclusion proof: %w", err)
	}

	// Extract and verify certificate.
	certs, err := rekordBody.Certs()
	if err != nil || len(certs) == 0 {
		return nil, fmt.Errorf("error extracting certificate from entry: %w", err)
	}
	if len(certs) > 1 {
		return nil, errMultipleCerts
	}

	cert := certs[0]
	if err = verifyCert(certs[0], time.Unix(entry.IntegratedTime, 0)); err != nil {
		return nil, fmt.Errorf("verifying cert: %w", err)
	}
	return cert, nil
}

// getUUIDsByPayload returns the UUIDs of the Rekor entries that contain the given payload.
// It takes the payload as a byte array and converts it to a SHA256 hash.
// It then queries the Rekor server for all entries that contain the hash.
// It returns the UUIDs of the entries that contain the payload.
func getUUIDsByPayload(ctx context.Context, payload []byte) ([]string, error) {
	payloadSHA := sha256.Sum256(payload)
	rekorPayload := struct {
		Hash string `json:"hash"`
	}{
		Hash: fmt.Sprintf("sha256:%s", hex.EncodeToString(payloadSHA[:])),
	}
	jsonPayload, err := json.Marshal(rekorPayload)
	if err != nil {
		return nil, fmt.Errorf("marshaling json payload: %w", err)
	}

	rekorReq, err := http.NewRequestWithContext(ctx,
		http.MethodPost,
		"https://rekor.sigstore.dev/api/v1/index/retrieve",
		bytes.NewBuffer(jsonPayload))
	if err != nil {
		return nil, fmt.Errorf("creating new HTTP request: %w", err)
	}
	rekorReq.Header.Add("Content-Type", "application/json")
	rekorReq.Header.Add("accept", "application/json")
	resp, err := http.DefaultClient.Do(rekorReq)
	if err != nil {
		return nil, fmt.Errorf("looking up Rekor index: %w", err)
	}
	defer resp.Body.Close()

	var rekorResult []string
	if err := json.NewDecoder(resp.Body).Decode(&rekorResult); err != nil {
		return nil, fmt.Errorf("decoding Rekor response: %w", err)
	}

	return rekorResult, nil
}

// tlog entry helper function, url should be a variation of the https://rekor.sigstore.dev/api/v1/log/entries endpoint.
func getTLogEntryFromURL(ctx context.Context, url string) (uuid string, entry *tlogEntry, err error) {
	rekorReq, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
	if err != nil {
		return "", nil, fmt.Errorf("creating new HTTP request: %w", err)
	}
	rekorReq.Header.Add("accept", "application/json")
	resp, err := http.DefaultClient.Do(rekorReq)
	if err != nil {
		return "", nil, fmt.Errorf("looking up Rekor index: %w", err)
	}
	defer resp.Body.Close()

	var rekorResult map[string]tlogEntry
	if err := json.NewDecoder(resp.Body).Decode(&rekorResult); err != nil {
		return "", nil, fmt.Errorf("decoding Rekor response: %w", err)
	}

	for uuid, res := range rekorResult {
		return uuid, &res, nil
	}
	return "", nil, errNoTlogEntry
}

// getTLogEntryByIndex fetches the UUID and tlog entry from Rekor by tlog index.
func getTLogEntryByIndex(ctx context.Context, index int64) (uuid string, entry *tlogEntry, err error) {
	url := fmt.Sprintf("https://rekor.sigstore.dev/api/v1/log/entries?logIndex=%d", index)
	return getTLogEntryFromURL(ctx, url)
}

// getTLogEntryByUUID fetches the tlog entry from Rekor by UUID.
func getTLogEntryByUUID(ctx context.Context, uuid string) (*tlogEntry, error) {
	url := fmt.Sprintf("https://rekor.sigstore.dev/api/v1/log/entries/%s", uuid)
	_, entry, err := getTLogEntryFromURL(ctx, url)
	return entry, err
}

// verifyInclusionProof verifies the inclusion proof of the tlog entry.
// It hex decodes the RootHash from the tlog entry and hex decodes the uuid as the leaf hash.
// It then verifies the merkelproof using the RootHash, LeafHash, and InclusionProof hashes from the
// tlog entry. It also ensures that the timestamp of the tlog entry  was signed by rekor public key.
func verifyInclusionProof(uuid string, e *tlogEntry) error {
	if e == nil || e.Verification == nil || e.Verification.InclusionProof == nil {
		return fmt.Errorf("no inclusion proof provided")
	}
	rootHash, err := hex.DecodeString(e.Verification.InclusionProof.RootHash)
	if err != nil {
		return fmt.Errorf("error decoding hex encoded root hash: %w", err)
	}

	leafHash, err := hex.DecodeString(uuid)
	if err != nil {
		return fmt.Errorf("error decoding hex encoded leaf hash: %w", err)
	}
	if len(leafHash) < 32 {
		return fmt.Errorf("leafHash has unexpected size %d, want 32", len(leafHash))
	}
	if len(leafHash) > 32 {
		leafHash = leafHash[len(leafHash)-32:]
	}

	var hashes [][]byte
	for _, h := range e.Verification.InclusionProof.Hashes {
		hb, err := hex.DecodeString(h)
		if err != nil {
			return fmt.Errorf("error decoding inclusion proof hashes: %w", err)
		}
		hashes = append(hashes, hb)
	}

	if err := merkleproof.VerifyInclusion(rfc6962.DefaultHasher,
		e.Verification.InclusionProof.LogIndex,
		e.Verification.InclusionProof.TreeSize, leafHash, hashes, rootHash); err != nil {
		return fmt.Errorf("%w: %s", err, "verifying inclusion proof")
	}

	// Verify the SignedEntryTimestamp against Rekor's pub key.
	derBytes, _ := pem.Decode(rekorPub)
	if derBytes == nil {
		return errors.New("PEM decoding failed")
	}
	rekorPubKey, err := x509.ParsePKIXPublicKey(derBytes.Bytes)
	if err != nil {
		return fmt.Errorf("parsing Rekor pub key: %w", err)
	}
	rekorECDSA, ok := rekorPubKey.(*ecdsa.PublicKey)
	if !ok {
		return errors.New("public key retrieved from Rekor is not an ECDSA key")
	}

	payload := struct {
		Body           string `json:"body"`
		IntegratedTime int64  `json:"integratedTime"`
		LogID          string `json:"logID"`
		LogIndex       int64  `json:"logIndex"`
	}{
		Body:           e.Body,
		IntegratedTime: e.IntegratedTime,
		LogID:          e.LogID,
		LogIndex:       e.LogIndex,
	}
	jsonPayload, err := json.Marshal(payload)
	if err != nil {
		return fmt.Errorf("json marshalling Rekor payload: %w", err)
	}
	jsonCanonicalized, err := jsoncanonicalizer.Transform(jsonPayload)
	if err != nil {
		return fmt.Errorf("json canonicalizer: %w", err)
	}
	hash := sha256.Sum256(jsonCanonicalized)
	if !ecdsa.VerifyASN1(rekorECDSA, hash[:], e.Verification.SignedEntryTimestamp) {
		return fmt.Errorf("unable to verify")
	}
	return nil
}

// verifyCert verifies the certificate from the tlog entry against the fulcio root cert and
// fulcio intermediate cert.
// It also verifies the certs are not expired by checking the notBefore and notAfter fields based
// on the integratedTime from the tlog entry.
func verifyCert(cert *x509.Certificate, integratedTime time.Time) error {
	// Verify the certificate against Fulcio Root CA
	roots, err := getCertPool(fulcioRoot)
	if err != nil {
		return fmt.Errorf("retrieving Fulcio root: %w", err)
	}
	intermediates, err := getCertPool(fulcioIntermediate)
	if err != nil {
		return fmt.Errorf("retrieving Fulcio root: %w", err)
	}
	if _, err := cert.Verify(x509.VerifyOptions{
		CurrentTime:   cert.NotBefore,
		Roots:         roots,
		Intermediates: intermediates,
		KeyUsages: []x509.ExtKeyUsage{
			x509.ExtKeyUsageCodeSigning,
		},
	}); err != nil {
		return fmt.Errorf("verifying Fulcio issued certificate: %w", err)
	}

	// Verify that cert isn't expired.
	if cert.NotAfter.Before(integratedTime) {
		return fmt.Errorf("certificate expired before signatures were entered in log: %s is before %s",
			cert.NotAfter, integratedTime)
	}
	if cert.NotBefore.After(integratedTime) {
		return fmt.Errorf("certificate was issued after signatures were entered in log: %s is after %s",
			cert.NotAfter, integratedTime)
	}
	return nil
}

// extractCertInfo extracts the repository information from the certificate.
// These certificates are issued by Fulcio and have extensions with the repository information.
// These extensions are extracted and returned as certInfo.
func extractCertInfo(cert *x509.Certificate) (certInfo, error) {
	ret := certInfo{}
	// Get repo reference & path from cert.
	for _, ext := range cert.Extensions {
		if ext.Id.String() == fulcioRepoRefKey {
			if len(ext.Value) == 0 {
				return ret, errEmptyCertRef
			}
			ret.repoBranchRef = string(ext.Value)
		}
		if ext.Id.String() == fulcioRepoPathKey {
			if len(ext.Value) == 0 {
				return ret, errEmptyCertPath
			}
			ret.repoFullName = string(ext.Value)
		}
		if ext.Id.String() == fulcioRepoSHAKey {
			ret.repoSHA = string(ext.Value)
		}
	}

	// Get workflow job ref from the certificate.
	if len(cert.URIs) == 0 {
		return ret, errCertMissingURI
	}
	workflowRef := cert.URIs[0].Path
	if len(workflowRef) == 0 {
		return ret, errCertWorkflowPathEmpty
	}

	// url.URL.Path may have leading slashes
	ret.workflowPath = strings.TrimLeft(workflowRef, "/")
	// Remove repo ref tag from workflow filepath.
	ret.workflowPath, ret.workflowRef, _ = strings.Cut(ret.workflowPath, "@")
	return ret, nil
}

func getCertPool(cert []byte) (*x509.CertPool, error) {
	pool := x509.NewCertPool()

	if ok := pool.AppendCertsFromPEM(cert); !ok {
		return nil, fmt.Errorf("unmarshalling PEM certificate")
	}
	return pool, nil
}

func (t tlogEntry) rekord() (hashedrekord.Body, error) {
	b, err := base64.StdEncoding.DecodeString(t.Body)
	if err != nil {
		return hashedrekord.Body{}, fmt.Errorf("decode rekord body: %w", err)
	}
	var body hashedrekord.Body
	if err := json.Unmarshal(b, &body); err != nil {
		return hashedrekord.Body{}, fmt.Errorf("unmarshal rekord body: %w", err)
	}
	if body.Kind != hashedrekord.Kind {
		return hashedrekord.Body{}, errNotRekordEntry
	}
	return body, nil
}