go-attestation/attest/tpm.go

// 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"
	"encoding/asn1"
	"encoding/binary"
	"fmt"
	"io"
	"math/big"
	"strings"

	"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

	// 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 (
	aikTemplate = tpm2.Public{
		Type:       tpm2.AlgRSA,
		NameAlg:    tpm2.AlgSHA256,
		Attributes: tpm2.FlagSignerDefault | tpm2.FlagNoDA,
		RSAParameters: &tpm2.RSAParams{
			Sign: &tpm2.SigScheme{
				Alg:  tpm2.AlgRSASSA,
				Hash: tpm2.AlgSHA256,
			},
			KeyBits: 2048,
			Modulus: big.NewInt(0),
		},
	}
	defaultSRKTemplate = tpm2.Public{
		Type:       tpm2.AlgRSA,
		NameAlg:    tpm2.AlgSHA256,
		Attributes: tpm2.FlagStorageDefault,
		RSAParameters: &tpm2.RSAParams{
			Symmetric: &tpm2.SymScheme{
				Alg:     tpm2.AlgAES,
				KeyBits: 128,
				Mode:    tpm2.AlgCFB,
			},
			KeyBits: 2048,
			Modulus: big.NewInt(0),
		},
	}
	// 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,
			Exponent:   0,
			ModulusRaw: make([]byte, 256),
		},
	}
)

func readTPM2VendorAttributes(tpm io.ReadWriter) (TCGVendorID, string, 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 TCGVendorID(0), "", fmt.Errorf("tpm2.GetCapability(PT_VENDOR_STRING_%d) failed: %v", i+1, err)
		}
		subset, ok := caps[0].(tpm2.TaggedProperty)
		if !ok {
			return TCGVendorID(0), "", 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 TCGVendorID(0), "", fmt.Errorf("tpm2.GetCapability(PT_MANUFACTURER) failed: %v", err)
	}
	manu, ok := caps[0].(tpm2.TaggedProperty)
	if !ok {
		return TCGVendorID(0), "", fmt.Errorf("got capability of type %T, want tpm2.TaggedProperty", caps[0])
	}

	return TCGVendorID(manu.Value), strings.Trim(vendorInfo, "\x00"), nil
}

func parseCert(ekCert []byte) (*x509.Certificate, error) {
	// 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 := binary.BigEndian.Uint16(ekCert[3:5])
		if len(ekCert) < int(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]
	}

	// 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.Unmarshal(ekCert, &cert); err != nil {
		return nil, err
	}
	c, err := x509.ParseCertificate(cert.Raw)
	if err != nil && x509.IsFatal(err) {
		return nil, err
	}
	return c, 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 parseCert(ekCert)
}

func quote20(tpm io.ReadWriter, aikHandle 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, aikHandle, "", "", 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
}

func allPCRs20(tpm io.ReadWriter) (map[uint32][]byte, crypto.Hash, error) {
	out256, err256 := readAllPCRs20(tpm, tpm2.AlgSHA256)
	if err256 != nil {
		// TPM may not implement active banks with SHA256 - try SHA1.
		out1, err1 := readAllPCRs20(tpm, tpm2.AlgSHA1)
		return out1, crypto.SHA1, err1
	}
	return out256, crypto.SHA256, nil
}