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tpm.go
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tpm.go
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// Copyright 2019 Google Inc.
//
// 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 attest
import (
"bytes"
"crypto/rsa"
"crypto/sha256"
"encoding/base64"
"encoding/binary"
"fmt"
"io"
"strings"
"github.com/google/certificate-transparency-go/asn1"
"github.com/google/certificate-transparency-go/x509"
"github.com/google/go-tpm/tpm2"
"github.com/google/go-tpm/tpmutil"
)
const (
tpmPtManufacturer = 0x00000100 + 5 // PT_FIXED + offset of 5
tpmPtVendorString = 0x00000100 + 6 // PT_FIXED + offset of 6
tpmPtFwVersion1 = 0x00000100 + 11 // PT_FIXED + offset of 11
// Defined in "Registry of reserved TPM 2.0 handles and localities".
nvramCertIndex = 0x1c00002
// Defined in "Registry of reserved TPM 2.0 handles and localities", and checked on a glinux machine.
commonSrkEquivalentHandle = 0x81000001
commonEkEquivalentHandle = 0x81010001
)
var (
akTemplate = tpm2.Public{
Type: tpm2.AlgRSA,
NameAlg: tpm2.AlgSHA256,
Attributes: tpm2.FlagSignerDefault,
RSAParameters: &tpm2.RSAParams{
Sign: &tpm2.SigScheme{
Alg: tpm2.AlgRSASSA,
Hash: tpm2.AlgSHA256,
},
KeyBits: 2048,
},
}
defaultSRKTemplate = tpm2.Public{
Type: tpm2.AlgRSA,
NameAlg: tpm2.AlgSHA256,
Attributes: tpm2.FlagStorageDefault | tpm2.FlagNoDA,
RSAParameters: &tpm2.RSAParams{
Symmetric: &tpm2.SymScheme{
Alg: tpm2.AlgAES,
KeyBits: 128,
Mode: tpm2.AlgCFB,
},
ModulusRaw: make([]byte, 256),
KeyBits: 2048,
},
}
// Default EK template defined in:
// https://trustedcomputinggroup.org/wp-content/uploads/Credential_Profile_EK_V2.0_R14_published.pdf
defaultEKTemplate = tpm2.Public{
Type: tpm2.AlgRSA,
NameAlg: tpm2.AlgSHA256,
Attributes: tpm2.FlagFixedTPM | tpm2.FlagFixedParent | tpm2.FlagSensitiveDataOrigin |
tpm2.FlagAdminWithPolicy | tpm2.FlagRestricted | tpm2.FlagDecrypt,
AuthPolicy: []byte{
0x83, 0x71, 0x97, 0x67, 0x44, 0x84,
0xB3, 0xF8, 0x1A, 0x90, 0xCC, 0x8D,
0x46, 0xA5, 0xD7, 0x24, 0xFD, 0x52,
0xD7, 0x6E, 0x06, 0x52, 0x0B, 0x64,
0xF2, 0xA1, 0xDA, 0x1B, 0x33, 0x14,
0x69, 0xAA,
},
RSAParameters: &tpm2.RSAParams{
Symmetric: &tpm2.SymScheme{
Alg: tpm2.AlgAES,
KeyBits: 128,
Mode: tpm2.AlgCFB,
},
KeyBits: 2048,
ModulusRaw: make([]byte, 256),
},
}
)
type tpm20Info struct {
vendor string
manufacturer TCGVendorID
fwMajor int
fwMinor int
}
func readTPM2VendorAttributes(tpm io.ReadWriter) (tpm20Info, error) {
var vendorInfo string
// The Vendor String is split up into 4 sections of 4 bytes,
// for a maximum length of 16 octets of ASCII text. We iterate
// through the 4 indexes to get all 16 bytes & construct vendorInfo.
// See: TPM_PT_VENDOR_STRING_1 in TPM 2.0 Structures reference.
for i := 0; i < 4; i++ {
caps, _, err := tpm2.GetCapability(tpm, tpm2.CapabilityTPMProperties, 1, tpmPtVendorString+uint32(i))
if err != nil {
return tpm20Info{}, fmt.Errorf("tpm2.GetCapability(PT_VENDOR_STRING_%d) failed: %v", i+1, err)
}
subset, ok := caps[0].(tpm2.TaggedProperty)
if !ok {
return tpm20Info{}, fmt.Errorf("got capability of type %T, want tpm2.TaggedProperty", caps[0])
}
// Reconstruct the 4 ASCII octets from the uint32 value.
vendorInfo += string(subset.Value&0xFF000000) + string(subset.Value&0xFF0000) + string(subset.Value&0xFF00) + string(subset.Value&0xFF)
}
caps, _, err := tpm2.GetCapability(tpm, tpm2.CapabilityTPMProperties, 1, tpmPtManufacturer)
if err != nil {
return tpm20Info{}, fmt.Errorf("tpm2.GetCapability(PT_MANUFACTURER) failed: %v", err)
}
manu, ok := caps[0].(tpm2.TaggedProperty)
if !ok {
return tpm20Info{}, fmt.Errorf("got capability of type %T, want tpm2.TaggedProperty", caps[0])
}
caps, _, err = tpm2.GetCapability(tpm, tpm2.CapabilityTPMProperties, 1, tpmPtFwVersion1)
if err != nil {
return tpm20Info{}, fmt.Errorf("tpm2.GetCapability(PT_FIRMWARE_VERSION_1) failed: %v", err)
}
fw, ok := caps[0].(tpm2.TaggedProperty)
if !ok {
return tpm20Info{}, fmt.Errorf("got capability of type %T, want tpm2.TaggedProperty", caps[0])
}
return tpm20Info{
vendor: strings.Trim(vendorInfo, "\x00"),
manufacturer: TCGVendorID(manu.Value),
fwMajor: int((fw.Value & 0xffff0000) >> 16),
fwMinor: int(fw.Value & 0x0000ffff),
}, nil
}
// ParseEKCertificate parses a raw DER encoded EK certificate blob.
func ParseEKCertificate(ekCert []byte) (*x509.Certificate, error) {
var wasWrapped bool
// TCG PC Specific Implementation section 7.3.2 specifies
// a prefix when storing a certificate in NVRAM. We look
// for and unwrap the certificate if its present.
if len(ekCert) > 5 && bytes.Equal(ekCert[:3], []byte{0x10, 0x01, 0x00}) {
certLen := int(binary.BigEndian.Uint16(ekCert[3:5]))
if len(ekCert) < certLen+5 {
return nil, fmt.Errorf("parsing nvram header: ekCert size %d smaller than specified cert length %d", len(ekCert), certLen)
}
ekCert = ekCert[5 : 5+certLen]
wasWrapped = true
}
// If the cert parses fine without any changes, we are G2G.
if c, err := x509.ParseCertificate(ekCert); err == nil {
return c, nil
}
// There might be trailing nonsense in the cert, which Go
// does not parse correctly. As ASN1 data is TLV encoded, we should
// be able to just get the certificate, and then send that to Go's
// certificate parser.
var cert struct {
Raw asn1.RawContent
}
if _, err := asn1.UnmarshalWithParams(ekCert, &cert, "lax"); err != nil && x509.IsFatal(err) {
return nil, fmt.Errorf("asn1.Unmarshal() failed: %v, wasWrapped=%v", err, wasWrapped)
}
c, err := x509.ParseCertificate(cert.Raw)
if err != nil && x509.IsFatal(err) {
return nil, fmt.Errorf("x509.ParseCertificate() failed: %v", err)
}
return c, nil
}
const (
manufacturerIntel = "Intel"
intelEKCertServiceURL = "https://ekop.intel.com/ekcertservice/"
)
func intelEKURL(ekPub *rsa.PublicKey) string {
pubHash := sha256.New()
pubHash.Write(ekPub.N.Bytes())
pubHash.Write([]byte{0x1, 0x00, 0x01})
return intelEKCertServiceURL + base64.URLEncoding.EncodeToString(pubHash.Sum(nil))
}
func readEKCertFromNVRAM20(tpm io.ReadWriter) (*x509.Certificate, error) {
ekCert, err := tpm2.NVReadEx(tpm, nvramCertIndex, tpm2.HandleOwner, "", 0)
if err != nil {
return nil, fmt.Errorf("reading EK cert: %v", err)
}
return ParseEKCertificate(ekCert)
}
func quote20(tpm io.ReadWriter, akHandle tpmutil.Handle, hashAlg tpm2.Algorithm, nonce []byte) (*Quote, error) {
sel := tpm2.PCRSelection{Hash: hashAlg}
numPCRs := 24
for pcr := 0; pcr < numPCRs; pcr++ {
sel.PCRs = append(sel.PCRs, pcr)
}
quote, sig, err := tpm2.Quote(tpm, akHandle, "", "", nonce, sel, tpm2.AlgNull)
if err != nil {
return nil, err
}
rawSig, err := tpmutil.Pack(sig.Alg, sig.RSA.HashAlg, sig.RSA.Signature)
return &Quote{
Version: TPMVersion20,
Quote: quote,
Signature: rawSig,
}, err
}
func readAllPCRs20(tpm io.ReadWriter, alg tpm2.Algorithm) (map[uint32][]byte, error) {
numPCRs := 24
out := map[uint32][]byte{}
// The TPM 2.0 spec says that the TPM can partially fulfill the
// request. As such, we repeat the command up to 8 times to get all
// 24 PCRs.
for i := 0; i < numPCRs; i++ {
// Build a selection structure, specifying all PCRs we do
// not have the value for.
sel := tpm2.PCRSelection{Hash: alg}
for pcr := 0; pcr < numPCRs; pcr++ {
if _, present := out[uint32(pcr)]; !present {
sel.PCRs = append(sel.PCRs, pcr)
}
}
// Ask the TPM for those PCR values.
ret, err := tpm2.ReadPCRs(tpm, sel)
if err != nil {
return nil, fmt.Errorf("tpm2.ReadPCRs(%+v) failed with err: %v", sel, err)
}
// Keep track of the PCRs we were actually given.
for pcr, digest := range ret {
out[uint32(pcr)] = digest
}
if len(out) == numPCRs {
break
}
}
if len(out) != numPCRs {
return nil, fmt.Errorf("failed to read all PCRs, only read %d", len(out))
}
return out, nil
}
// tpmBase defines the implementation of a TPM invariant.
type tpmBase interface {
close() error
tpmVersion() TPMVersion
eks() ([]EK, error)
info() (*TPMInfo, error)
loadAK(opaqueBlob []byte) (*AK, error)
newAK(opts *AKConfig) (*AK, error)
pcrs(alg HashAlg) ([]PCR, error)
measurementLog() ([]byte, error)
}
//TPM interfaces with a TPM device on the system.
type TPM struct {
// tpm refers to a concrete implementation of TPM logic, based on the current
// platform and TPM version.
tpm tpmBase
}
// Close shuts down the connection to the TPM.
func (t *TPM) Close() error {
return t.tpm.close()
}
// EKs returns the endorsement keys burned-in to the platform.
func (t *TPM) EKs() ([]EK, error) {
return t.tpm.eks()
}
// Info returns information about the TPM.
func (t *TPM) Info() (*TPMInfo, error) {
return t.tpm.info()
}
// LoadAK loads a previously-created ak into the TPM for use.
// A key loaded via this function needs to be closed with .Close().
// Only blobs generated by calling AK.Serialize() are valid parameters
// to this function.
func (t *TPM) LoadAK(opaqueBlob []byte) (*AK, error) {
return t.tpm.loadAK(opaqueBlob)
}
// MeasurementLog returns the present value of the System Measurement Log.
//
// This is a low-level API. Consumers seeking to attest the state of the
// platform should use tpm.AttestPlatform() instead.
func (t *TPM) MeasurementLog() ([]byte, error) {
el, err := t.tpm.measurementLog()
if err != nil {
return nil, err
}
// A valid event log contains at least one SpecID event header (28 bytes).
// For TPM 1.2, we would expect at least an event header (32 bytes).
if minValidSize := 28; len(el) < minValidSize {
return nil, fmt.Errorf("event log too short: %d < %d", len(el), minValidSize)
}
return el, nil
}
// NewAK creates an attestation key.
func (t *TPM) NewAK(opts *AKConfig) (*AK, error) {
return t.tpm.newAK(opts)
}
// PCRs returns the present value of Platform Configuration Registers with
// the given digest algorithm.
//
// This is a low-level API. Consumers seeking to attest the state of the
// platform should use tpm.AttestPlatform() instead.
func (t *TPM) PCRs(alg HashAlg) ([]PCR, error) {
return t.tpm.pcrs(alg)
}
func (t *TPM) attestPCRs(ak *AK, nonce []byte, alg HashAlg) (*Quote, []PCR, error) {
pcrs, err := t.PCRs(alg)
if err != nil {
return nil, nil, fmt.Errorf("failed to read %v PCRs: %v", alg, err)
}
quote, err := ak.Quote(t, nonce, alg)
if err != nil {
return nil, nil, fmt.Errorf("failed to quote using %v: %v", alg, err)
}
return quote, pcrs, nil
}
func (t *TPM) attestPlatform(ak *AK, nonce []byte, eventLog []byte) (*PlatformParameters, error) {
out := PlatformParameters{
TPMVersion: t.Version(),
Public: ak.AttestationParameters().Public,
EventLog: eventLog,
}
algs := []HashAlg{HashSHA1}
if t.Version() == TPMVersion20 {
algs = []HashAlg{HashSHA1, HashSHA256}
}
var lastErr error
for _, alg := range algs {
quote, pcrs, err := t.attestPCRs(ak, nonce, alg)
if err != nil {
lastErr = err
continue
}
out.Quotes = append(out.Quotes, *quote)
out.PCRs = append(out.PCRs, pcrs...)
}
if len(out.Quotes) == 0 {
return nil, lastErr
}
return &out, nil
}
// PlatformAttestConfig configures how attestations are generated through
// tpm.AttestPlatform().
type PlatformAttestConfig struct {
// If non-nil, the raw event log will be read from EventLog
// instead of being obtained from the running system.
EventLog []byte
}
// AttestPlatform computes the set of information necessary to attest the
// state of the platform. For TPM 2.0 devices, AttestPlatform will attempt
// to read both SHA1 & SHA256 PCR banks and quote both of them, so bugs in
// platform firmware which break replay for one PCR bank can be mitigated
// using the other.
// The provided config, if not nil, can be used to configure aspects of the
// platform attestation.
func (t *TPM) AttestPlatform(ak *AK, nonce []byte, config *PlatformAttestConfig) (*PlatformParameters, error) {
if config == nil {
config = &PlatformAttestConfig{}
}
var el []byte
if config.EventLog != nil {
el = config.EventLog
} else {
var err error
if el, err = t.MeasurementLog(); err != nil {
return nil, fmt.Errorf("failed to read event log: %v", err)
}
}
return t.attestPlatform(ak, nonce, el)
}
// Version returns the version of the TPM.
func (t *TPM) Version() TPMVersion {
return t.tpm.tpmVersion()
}