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Separate quote verification from EventLog API #85 (#87)

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Tom D 2019-08-28 16:59:46 -07:00 committed by GitHub
parent 2ad969b54a
commit 4ef1479ae1
No known key found for this signature in database
GPG Key ID: 4AEE18F83AFDEB23
3 changed files with 117 additions and 95 deletions
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19
attest/attest.go
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@ -253,6 +253,25 @@ func ParseAIKPublic(version TPMVersion, public []byte) (*AIKPublic, error) {
}
}
// Verify is used to prove authenticity of the PCR measurements. It ensures that
// the quote was signed by the AIK, and that its contents matches the PCR and
// nonce combination.
//
// The nonce is used to prevent replays of Quote and PCRs and is signed by the
// quote. Some TPMs don't support nonces longer than 20 bytes, and if the
// nonce is used to tie additional data to the quote, the additional data should be
// hashed to construct the nonce.
func (a *AIKPublic) Verify(quote Quote, pcrs []PCR, nonce []byte) error {
switch quote.Version {
case TPMVersion12:
return a.validate12Quote(quote, pcrs, nonce)
case TPMVersion20:
return a.validate20Quote(quote, pcrs, nonce)
default:
return fmt.Errorf("quote used unknown tpm version 0x%x", quote.Version)
}
}
// HashAlg identifies a hashing Algorithm.
type HashAlg uint8

154
attest/eventlog.go
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@ -1,16 +1,30 @@
// 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"
"crypto/rsa"
"crypto/sha1"
"encoding/binary"
"fmt"
"io"
"sort"
// Ensure hashes are available.
"crypto/rsa"
"crypto/sha1"
_ "crypto/sha256"
"github.com/google/go-tpm/tpm2"
@ -48,49 +62,26 @@ type Event struct {
// match their data to their digest.
}
// EventLog contains the data required to parse and validate an event log.
type EventLog struct {
// AIKPublic is the activated public key that has been proven to be under the
// control of the TPM.
AIKPublic crypto.PublicKey
// AIKHash is the hash used to generate the quote.
AIKHash crypto.Hash
// Quote is a signature over the values of a PCR.
Quote *Quote
// PCRs are the hash values in a given number of registers.
PCRs []PCR
// Nonce is additional data used to validate the quote signature. It's used
// by the server to prevent clients from re-playing quotes.
Nonce []byte
// MeasurementLog contains the raw event log data, which is matched against
// the PCRs for validation.
MeasurementLog []byte
}
// Validate verifies the signature of the quote agains the public key, that the
// quote matches the PCRs, parses the measurement log, and replays the PCRs.
//
// Events for PCRs not in the quote are dropped.
func (e *EventLog) Validate() (events []Event, err error) {
var pcrs []PCR
switch e.Quote.Version {
case TPMVersion12:
pcrs, err = e.validate12Quote()
case TPMVersion20:
pcrs, err = e.validate20Quote()
default:
return nil, fmt.Errorf("quote used unknown tpm version 0x%x", e.Quote.Version)
}
if err != nil {
return nil, fmt.Errorf("invalid quote: %v", err)
}
rawEvents, err := parseEventLog(e.MeasurementLog)
// ParseEventLog parses an unverified measurement log.
func ParseEventLog(measurementLog []byte) (*EventLog, error) {
rawEvents, err := parseEventLog(measurementLog)
if err != nil {
return nil, fmt.Errorf("parsing measurement log: %v", err)
}
events, err = replayEvents(rawEvents, pcrs)
return &EventLog{rawEvents}, nil
}
// EventLog is a parsed measurement log. This contains unverified data representing
// boot events that must be replayed against PCR values to determine authenticity.
type EventLog struct {
rawEvents []rawEvent
}
// Verify replays the event log against a TPM's PCR values, returning events
// from the event log, or an error if the replayed PCR values did not match the
// provided PCR values.
func (e *EventLog) Verify(pcrs []PCR) ([]Event, error) {
events, err := replayEvents(e.rawEvents, pcrs)
if err != nil {
return nil, fmt.Errorf("pcrs failed to replay: %v", err)
}
@ -114,113 +105,112 @@ type rawPCRComposite struct {
Values tpmutil.U32Bytes
}
func (e *EventLog) validate12Quote() (pcrs []PCR, err error) {
pub, ok := e.AIKPublic.(*rsa.PublicKey)
func (a *AIKPublic) validate12Quote(quote Quote, pcrs []PCR, nonce []byte) error {
pub, ok := a.Public.(*rsa.PublicKey)
if !ok {
return nil, fmt.Errorf("unsupported public key type: %T", e.AIKPublic)
return fmt.Errorf("unsupported public key type: %T", a.Public)
}
quote := sha1.Sum(e.Quote.Quote)
if err := rsa.VerifyPKCS1v15(pub, crypto.SHA1, quote[:], e.Quote.Signature); err != nil {
return nil, fmt.Errorf("invalid quote signature: %v", err)
qHash := sha1.Sum(quote.Quote)
if err := rsa.VerifyPKCS1v15(pub, crypto.SHA1, qHash[:], quote.Signature); err != nil {
return fmt.Errorf("invalid quote signature: %v", err)
}
var att rawAttestationData
if _, err := tpmutil.Unpack(e.Quote.Quote, &att); err != nil {
return nil, fmt.Errorf("parsing quote: %v", err)
if _, err := tpmutil.Unpack(quote.Quote, &att); err != nil {
return fmt.Errorf("parsing quote: %v", err)
}
// TODO(ericchiang): validate Version field.
if att.Nonce != sha1.Sum(e.Nonce) {
return nil, fmt.Errorf("invalid nonce")
if att.Nonce != sha1.Sum(nonce) {
return fmt.Errorf("invalid nonce")
}
if att.Fixed != fixedQuote {
return nil, fmt.Errorf("quote wasn't a QUOT object: %x", att.Fixed)
return fmt.Errorf("quote wasn't a QUOT object: %x", att.Fixed)
}
// See 5.4.1 Creating a PCR composite hash
sort.Slice(e.PCRs, func(i, j int) bool { return e.PCRs[i].Index < e.PCRs[j].Index })
sort.Slice(pcrs, func(i, j int) bool { return pcrs[i].Index < pcrs[j].Index })
var (
pcrMask [3]byte // bitmap indicating which PCRs are active
values []byte // appended values of all PCRs
)
for _, pcr := range e.PCRs {
for _, pcr := range pcrs {
if pcr.Index < 0 || pcr.Index >= 24 {
return nil, fmt.Errorf("invalid PCR index: %d", pcr.Index)
return fmt.Errorf("invalid PCR index: %d", pcr.Index)
}
pcrMask[pcr.Index/8] |= 1 << uint(pcr.Index%8)
values = append(values, pcr.Digest...)
}
composite, err := tpmutil.Pack(rawPCRComposite{3, pcrMask, values})
if err != nil {
return nil, fmt.Errorf("marshaling PCRss: %v", err)
return fmt.Errorf("marshaling PCRs: %v", err)
}
if att.Digest != sha1.Sum(composite) {
return nil, fmt.Errorf("PCRs passed didn't match quote: %v", err)
return fmt.Errorf("PCRs passed didn't match quote: %v", err)
}
return e.PCRs, nil
return nil
}
func (e *EventLog) validate20Quote() (pcrs []PCR, err error) {
sig, err := tpm2.DecodeSignature(bytes.NewBuffer(e.Quote.Signature))
func (a *AIKPublic) validate20Quote(quote Quote, pcrs []PCR, nonce []byte) error {
sig, err := tpm2.DecodeSignature(bytes.NewBuffer(quote.Signature))
if err != nil {
return nil, fmt.Errorf("parse quote signature: %v", err)
return fmt.Errorf("parse quote signature: %v", err)
}
sigHash := e.AIKHash.New()
sigHash.Write(e.Quote.Quote)
sigHash := a.Hash.New()
sigHash.Write(quote.Quote)
switch pub := e.AIKPublic.(type) {
switch pub := a.Public.(type) {
case *rsa.PublicKey:
if sig.RSA == nil {
return nil, fmt.Errorf("rsa public key provided for ec signature")
return fmt.Errorf("rsa public key provided for ec signature")
}
sigBytes := []byte(sig.RSA.Signature)
if err := rsa.VerifyPKCS1v15(pub, e.AIKHash, sigHash.Sum(nil), sigBytes); err != nil {
return nil, fmt.Errorf("invalid quote signature: %v", err)
if err := rsa.VerifyPKCS1v15(pub, a.Hash, sigHash.Sum(nil), sigBytes); err != nil {
return fmt.Errorf("invalid quote signature: %v", err)
}
default:
// TODO(ericchiang): support ecdsa
return nil, fmt.Errorf("unsupported public key type %T", pub)
return fmt.Errorf("unsupported public key type %T", pub)
}
att, err := tpm2.DecodeAttestationData(e.Quote.Quote)
att, err := tpm2.DecodeAttestationData(quote.Quote)
if err != nil {
return nil, fmt.Errorf("parsing quote signature: %v", err)
return fmt.Errorf("parsing quote signature: %v", err)
}
if att.Type != tpm2.TagAttestQuote {
return nil, fmt.Errorf("attestation isn't a quote, tag of type 0x%x", att.Type)
return fmt.Errorf("attestation isn't a quote, tag of type 0x%x", att.Type)
}
if !bytes.Equal([]byte(att.ExtraData), e.Nonce) {
return nil, fmt.Errorf("nonce didn't match: %v", err)
if !bytes.Equal([]byte(att.ExtraData), nonce) {
return fmt.Errorf("nonce didn't match: %v", err)
}
pcrByIndex := map[int][]byte{}
for _, pcr := range e.PCRs {
for _, pcr := range pcrs {
pcrByIndex[pcr.Index] = pcr.Digest
}
n := len(att.AttestedQuoteInfo.PCRDigest)
hash, ok := hashBySize[n]
if !ok {
return nil, fmt.Errorf("quote used unsupported hash algorithm length: %d", n)
return fmt.Errorf("quote used unsupported hash algorithm length: %d", n)
}
var validatedPCRs []PCR
h := hash.New()
for _, index := range att.AttestedQuoteInfo.PCRSelection.PCRs {
digest, ok := pcrByIndex[index]
if !ok {
return nil, fmt.Errorf("quote was over PCR %d which wasn't provided", index)
return fmt.Errorf("quote was over PCR %d which wasn't provided", index)
}
if len(digest) != hash.Size() {
return nil, fmt.Errorf("mismatch pcr and quote hash, pcr hash length=%d, quote hash length=%d", len(digest), hash.Size())
return fmt.Errorf("mismatch pcr and quote hash, pcr hash length=%d, quote hash length=%d", len(digest), hash.Size())
}
h.Write(digest)
validatedPCRs = append(validatedPCRs, PCR{Index: index, Digest: digest})
}
if !bytes.Equal(h.Sum(nil), att.AttestedQuoteInfo.PCRDigest) {
return nil, fmt.Errorf("quote digest didn't match pcrs provided")
return fmt.Errorf("quote digest didn't match pcrs provided")
}
return validatedPCRs, nil
return nil
}
var hashBySize = map[int]crypto.Hash{

33
attest/eventlog_test.go
View File

@ -1,3 +1,17 @@
// 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 (
@ -85,20 +99,19 @@ func testEventLog(t *testing.T, testdata string) {
if err != nil {
t.Fatalf("parsing AIK: %v", err)
}
el := EventLog{
AIKPublic: aik.Public,
AIKHash: aik.Hash,
Quote: &Quote{
if err := aik.Verify(Quote{
Version: dump.Static.TPMVersion,
Quote: dump.Quote.Quote,
Signature: dump.Quote.Signature,
},
Nonce: dump.Quote.Nonce,
PCRs: dump.Log.PCRs,
MeasurementLog: dump.Log.Raw,
}, dump.Log.PCRs, dump.Quote.Nonce); err != nil {
t.Fatalf("verifying quote: %v", err)
}
if _, err := el.Validate(); err != nil {
el, err := ParseEventLog(dump.Log.Raw)
if err != nil {
t.Fatalf("parsing event log: %v", err)
}
if _, err := el.Verify(dump.Log.PCRs); err != nil {
t.Fatalf("validating event log: %v", err)
}
}