AB#2444 Verify Azure trusted launch attestation keys (#203)

Signed-off-by: Daniel Weiße <dw@edgeless.systems>

internal/attestation/azure/trustedlaunch/issuer.go

@@ -7,31 +7,117 @@ SPDX-License-Identifier: AGPL-3.0-only
 package trustedlaunch
 
 import (
+	"context"
+	"crypto/x509"
+	"encoding/json"
+	"errors"
+	"fmt"
 	"io"
+	"net/http"
+	"time"
 
 	"github.com/edgelesssys/constellation/v2/internal/attestation/vtpm"
 	"github.com/edgelesssys/constellation/v2/internal/oid"
 	tpmclient "github.com/google/go-tpm-tools/client"
+	"github.com/google/go-tpm/tpm2"
+)
+
+const (
+	tpmAkIdx     = 0x81000003
+	tpmAkCertIdx = 0x1C101D0
 )
 
 // Issuer for Azure trusted launch TPM attestation.
 type Issuer struct {
 	oid.AzureTrustedLaunch
 	*vtpm.Issuer
+	hClient httpClient
 }
 
 // NewIssuer initializes a new Azure Issuer.
 func NewIssuer() *Issuer {
-	return &Issuer{
-		Issuer: vtpm.NewIssuer(
-			vtpm.OpenVTPM,
-			tpmclient.AttestationKeyRSA,
-			getAttestation,
-		),
+	i := &Issuer{
+		hClient: &http.Client{},
 	}
+	i.Issuer = vtpm.NewIssuer(
+		vtpm.OpenVTPM,
+		getAttestationKey,
+		i.getAttestationCert,
+	)
+	return i
 }
 
-// getAttestation returns nil.
-func getAttestation(tpm io.ReadWriteCloser) ([]byte, error) {
-	return nil, nil
+// akSigner holds the attestation key certificate and the corresponding CA certificate.
+type akSigner struct {
+	AkCert []byte
+	CA     []byte
+}
+
+// getAttestationCert returns the DER encoded certificate of the TPM's attestation key and it's CA.
+func (i *Issuer) getAttestationCert(tpm io.ReadWriteCloser) ([]byte, error) {
+	certDER, err := tpm2.NVReadEx(tpm, tpmAkCertIdx, tpm2.HandleOwner, "", 0)
+	if err != nil {
+		return nil, fmt.Errorf("reading attestation key certificate from TPM: %w", err)
+	}
+
+	cert, err := x509.ParseCertificate(certDER)
+	if err != nil {
+		return nil, fmt.Errorf("parsing attestation key certificate: %w", err)
+	}
+
+	// try to load the CA certificate from the issuing certificate URL
+	// any error is ignored and the next URL is tried
+	// if no CA certificate can be loaded, an error is returned
+	var caCert *x509.Certificate
+	for _, caCertURL := range cert.IssuingCertificateURL {
+		ctx, cancel := context.WithTimeout(context.Background(), time.Second*20)
+		defer cancel()
+		req, err := http.NewRequestWithContext(ctx, http.MethodGet, caCertURL, nil)
+		if err != nil {
+			continue
+		}
+		resp, err := i.hClient.Do(req)
+		if err != nil {
+			continue
+		}
+		defer resp.Body.Close()
+		if resp.StatusCode != http.StatusOK {
+			continue
+		}
+
+		caCertDER, err := io.ReadAll(resp.Body)
+		if err != nil {
+			continue
+		}
+
+		caCert, err = x509.ParseCertificate(caCertDER)
+		if err != nil {
+			continue
+		}
+		break
+	}
+	if caCert == nil {
+		return nil, errors.New("failed to load CA certificate")
+	}
+
+	signerInfo := akSigner{
+		AkCert: certDER,
+		CA:     caCert.Raw,
+	}
+
+	return json.Marshal(signerInfo)
+}
+
+// getAttestationKey reads the Azure trusted launch attesation key.
+func getAttestationKey(tpm io.ReadWriter) (*tpmclient.Key, error) {
+	ak, err := tpmclient.LoadCachedKey(tpm, tpmAkIdx)
+	if err != nil {
+		return nil, fmt.Errorf("reading attestation key from TPM: %w", err)
+	}
+
+	return ak, nil
+}
+
+type httpClient interface {
+	Do(req *http.Request) (*http.Response, error)
 }