cli/pkg/cmd/attestation/verify/policy.go

package verify

import (
	"errors"
	"fmt"
	"regexp"

	"github.com/sigstore/sigstore-go/pkg/fulcio/certificate"
	"github.com/sigstore/sigstore-go/pkg/verify"

	"github.com/cli/cli/v2/pkg/cmd/attestation/artifact"
	"github.com/cli/cli/v2/pkg/cmd/attestation/verification"
)

const hostRegex = `^[a-zA-Z0-9-]+\.[a-zA-Z0-9-]+.*$`

func expandToGitHubURL(tenant, ownerOrRepo string) string {
	if tenant == "" {
		return fmt.Sprintf("(?i)^https://github.com/%s/", ownerOrRepo)
	}
	return fmt.Sprintf("(?i)^https://%s.ghe.com/%s/", tenant, ownerOrRepo)
}

func newEnforcementCriteria(opts *Options) (verification.EnforcementCriteria, error) {
	var c verification.EnforcementCriteria

	// Set SANRegex using either the opts.SignerRepo or opts.SignerWorkflow values
	if opts.SignerRepo != "" {
		signedRepoRegex := expandToGitHubURL(opts.Tenant, opts.SignerRepo)
		c.SANRegex = signedRepoRegex
	} else if opts.SignerWorkflow != "" {
		validatedWorkflowRegex, err := validateSignerWorkflow(opts)
		if err != nil {
			return verification.EnforcementCriteria{}, err
		}

		c.SANRegex = validatedWorkflowRegex
	} else {
		// If neither of those values were set, default to the provided SANRegex and SAN values
		c.SANRegex = opts.SANRegex
		c.SAN = opts.SAN
	}

	// if the DenySelfHostedRunner option is set to true, set the
	// RunnerEnvironment extension to the GitHub hosted runner value
	if opts.DenySelfHostedRunner {
		c.Certificate.RunnerEnvironment = verification.GitHubRunner
	} else {
		// if Certificate.RunnerEnvironment value is set to the empty string
		// through the second function argument,
		// no certificate matching will happen on the RunnerEnvironment field
		c.Certificate.RunnerEnvironment = ""
	}

	// If the Repo option is provided, set the SourceRepositoryURI extension
	if opts.Repo != "" {
		// If the Tenant options is also provided, set the SourceRepositoryURI extension
		// using the specific URI format
		if opts.Tenant != "" {
			c.Certificate.SourceRepositoryURI = fmt.Sprintf("https://%s.ghe.com/%s", opts.Tenant, opts.Repo)
		} else {
			c.Certificate.SourceRepositoryURI = fmt.Sprintf("https://github.com/%s", opts.Repo)
		}
	}

	// If the tenant option is provided, set the SourceRepositoryOwnerURI extension
	// using the specific URI format
	if opts.Tenant != "" {
		c.Certificate.SourceRepositoryOwnerURI = fmt.Sprintf("https://%s.ghe.com/%s", opts.Tenant, opts.Owner)
	} else {
		c.Certificate.SourceRepositoryOwnerURI = fmt.Sprintf("https://github.com/%s", opts.Owner)
	}

	// if the tenant is provided and OIDC issuer provided matches the default
	// use the tenant-specific issuer
	if opts.Tenant != "" && opts.OIDCIssuer == verification.GitHubOIDCIssuer {
		c.Certificate.Issuer = fmt.Sprintf(verification.GitHubTenantOIDCIssuer, opts.Tenant)
	} else {
		// otherwise use the custom OIDC issuer provided as an option
		c.Certificate.Issuer = opts.OIDCIssuer
	}

	c.PredicateType = opts.PredicateType

	return c, nil
}

func buildCertificateIdentityOption(c verification.EnforcementCriteria) (verify.PolicyOption, error) {
	sanMatcher, err := verify.NewSANMatcher(c.SAN, c.SANRegex)
	if err != nil {
		return nil, err
	}

	// Accept any issuer, we will verify the issuer as part of the extension verification
	issuerMatcher, err := verify.NewIssuerMatcher("", ".*")
	if err != nil {
		return nil, err
	}

	extensions := certificate.Extensions{
		RunnerEnvironment: c.Certificate.RunnerEnvironment,
	}

	certId, err := verify.NewCertificateIdentity(sanMatcher, issuerMatcher, extensions)
	if err != nil {
		return nil, err
	}

	return verify.WithCertificateIdentity(certId), nil
}

func buildSigstoreVerifyPolicy(c verification.EnforcementCriteria, a artifact.DigestedArtifact) (verify.PolicyBuilder, error) {
	artifactDigestPolicyOption, err := verification.BuildDigestPolicyOption(a)
	if err != nil {
		return verify.PolicyBuilder{}, err
	}

	certIdOption, err := buildCertificateIdentityOption(c)
	if err != nil {
		return verify.PolicyBuilder{}, err
	}

	policy := verify.NewPolicy(artifactDigestPolicyOption, certIdOption)
	return policy, nil
}

func validateSignerWorkflow(opts *Options) (string, error) {
	// we expect a provided workflow argument be in the format [HOST/]/<OWNER>/<REPO>/path/to/workflow.yml
	// if the provided workflow does not contain a host, set the host
	match, err := regexp.MatchString(hostRegex, opts.SignerWorkflow)
	if err != nil {
		return "", err
	}

	if match {
		return fmt.Sprintf("^https://%s", opts.SignerWorkflow), nil
	}

	// if the provided workflow did not match the expect format
	// we move onto creating a signer workflow using the provided host name
	if opts.Hostname == "" {
		return "", errors.New("unknown host")
	}

	return fmt.Sprintf("^https://%s/%s", opts.Hostname, opts.SignerWorkflow), nil
}