constellation/internal/attestation/vtpm/attestation.go
2022-12-12 17:45:35 +01:00

284 lines
9.5 KiB
Go

/*
Copyright (c) Edgeless Systems GmbH
SPDX-License-Identifier: AGPL-3.0-only
*/
package vtpm
import (
"bytes"
"crypto"
"crypto/rsa"
"crypto/sha256"
"encoding/json"
"errors"
"fmt"
"io"
"github.com/edgelesssys/constellation/v2/internal/attestation/measurements"
tpmClient "github.com/google/go-tpm-tools/client"
"github.com/google/go-tpm-tools/proto/attest"
tpmProto "github.com/google/go-tpm-tools/proto/tpm"
tpmServer "github.com/google/go-tpm-tools/server"
"github.com/google/go-tpm/tpm2"
)
var (
// AzurePCRSelection are the PCR values verified for Azure Constellations.
// PCR[0] is excluded due to changing rarely, but unpredictably.
// PCR[6] is excluded due to being different for any 2 VMs. See: https://trustedcomputinggroup.org/wp-content/uploads/TCG_PCClient_PFP_r1p05_v23_pub.pdf#%5B%7B%22num%22%3A157%2C%22gen%22%3A0%7D%2C%7B%22name%22%3A%22XYZ%22%7D%2C33%2C400%2C0%5D
// PCR[10] is excluded since its value is derived from a digest of PCR[0-7]. See: https://sourceforge.net/p/linux-ima/wiki/Home/#ima-measurement-list
AzurePCRSelection = tpm2.PCRSelection{
Hash: tpm2.AlgSHA256,
PCRs: []int{1, 2, 3, 4, 5, 7, 8, 9, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23},
}
// GCPPCRSelection are the PCR values verified for GCP Constellations.
// On GCP firmware and other host controlled systems are static. This results in the same PCRs for any 2 VMs using the same image.
GCPPCRSelection = tpmClient.FullPcrSel(tpm2.AlgSHA256)
// AWSPCRSelection are the PCR values verified for AWS based Constellations.
// PCR[1] is excluded. See: https://trustedcomputinggroup.org/wp-content/uploads/TCG_PCClient_PFP_r1p05_v23_pub.pdf#%5B%7B%22num%22:157,%22gen%22:0%7D,%7B%22name%22:%22XYZ%22%7D,33,400,0%5D
// PCR[10] is excluded since its value is derived from a digest of PCR[0-7]. See: https://sourceforge.net/p/linux-ima/wiki/Home/#ima-measurement-list
AWSPCRSelection = tpm2.PCRSelection{
Hash: tpm2.AlgSHA256,
PCRs: []int{0, 2, 3, 4, 5, 6, 7, 8, 9, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23},
}
// QEMUPCRSelection are the PCR values verified for QEMU based Constellations.
// PCR[1] is excluded. See: https://trustedcomputinggroup.org/wp-content/uploads/TCG_PCClient_PFP_r1p05_v23_pub.pdf#%5B%7B%22num%22:157,%22gen%22:0%7D,%7B%22name%22:%22XYZ%22%7D,33,400,0%5D
// PCR[10] is excluded since its value is derived from a digest of PCR[0-7]. See: https://sourceforge.net/p/linux-ima/wiki/Home/#ima-measurement-list
QEMUPCRSelection = tpm2.PCRSelection{
Hash: tpm2.AlgSHA256,
PCRs: []int{0, 2, 3, 4, 5, 6, 7, 8, 9, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23},
}
)
type (
// GetTPMAttestationKey loads a TPM key to perform attestation.
GetTPMAttestationKey func(tpm io.ReadWriter) (*tpmClient.Key, error)
// GetTPMTrustedAttestationPublicKey verifies and returns the attestation public key.
GetTPMTrustedAttestationPublicKey func(akPub []byte, instanceInfo []byte) (crypto.PublicKey, error)
// GetInstanceInfo returns VM metdata.
GetInstanceInfo func(tpm io.ReadWriteCloser) ([]byte, error)
// ValidateCVM validates confidential computing capabilities of the instance issuing the attestation.
ValidateCVM func(attestation AttestationDocument) error
// VerifyUserData verifies signed user data.
VerifyUserData func(pub crypto.PublicKey, hash crypto.Hash, hashed, sig []byte) error
)
// AttestationLogger is a logger used to print warnings and infos during attestation validation.
type AttestationLogger interface {
Infof(format string, args ...any)
Warnf(format string, args ...any)
}
// AttestationDocument contains the TPM attestation with signed user data.
type AttestationDocument struct {
// Attestation contains the TPM event log, PCR values and quotes, and public key of the key used to sign the attestation.
Attestation *attest.Attestation
// InstanceInfo is used to verify the provided public key.
InstanceInfo []byte
// arbitrary data, signed by the TPM.
UserData []byte
UserDataSignature []byte
}
// Issuer handles issuing of TPM based attestation documents.
type Issuer struct {
openTPM TPMOpenFunc
getAttestationKey GetTPMAttestationKey
getInstanceInfo GetInstanceInfo
}
// NewIssuer returns a new Issuer.
func NewIssuer(openTPM TPMOpenFunc, getAttestationKey GetTPMAttestationKey, getInstanceInfo GetInstanceInfo) *Issuer {
return &Issuer{
openTPM: openTPM,
getAttestationKey: getAttestationKey,
getInstanceInfo: getInstanceInfo,
}
}
// Issue generates an attestation document using a TPM.
func (i *Issuer) Issue(userData []byte, nonce []byte) ([]byte, error) {
tpm, err := i.openTPM()
if err != nil {
return nil, fmt.Errorf("opening TPM: %w", err)
}
defer tpm.Close()
// Load the TPM's attestation key
aK, err := i.getAttestationKey(tpm)
if err != nil {
return nil, fmt.Errorf("loading attestation key: %w", err)
}
defer aK.Close()
// Create an attestation using the loaded key
attestation, err := aK.Attest(tpmClient.AttestOpts{Nonce: nonce})
if err != nil {
return nil, fmt.Errorf("creating attestation: %w", err)
}
// Fetch instance info of the VM
instanceInfo, err := i.getInstanceInfo(tpm)
if err != nil {
return nil, fmt.Errorf("fetching instance info: %w", err)
}
// Sign user provided data using the loaded key
userDataSigned, err := aK.SignData(userData)
if err != nil {
return nil, fmt.Errorf("signing user data: %w", err)
}
attDoc := AttestationDocument{
Attestation: attestation,
InstanceInfo: instanceInfo,
UserData: userData,
UserDataSignature: userDataSigned,
}
return json.Marshal(attDoc)
}
// Validator handles validation of TPM based attestation.
type Validator struct {
expected measurements.M
getTrustedKey GetTPMTrustedAttestationPublicKey
validateCVM ValidateCVM
verifyUserData VerifyUserData
log AttestationLogger
}
// NewValidator returns a new Validator.
func NewValidator(expected measurements.M, getTrustedKey GetTPMTrustedAttestationPublicKey,
validateCVM ValidateCVM, verifyUserData VerifyUserData, log AttestationLogger,
) *Validator {
return &Validator{
expected: expected,
getTrustedKey: getTrustedKey,
validateCVM: validateCVM,
verifyUserData: verifyUserData,
log: log,
}
}
// Validate a TPM based attestation.
func (v *Validator) Validate(attDocRaw []byte, nonce []byte) ([]byte, error) {
if v.log != nil {
v.log.Infof("Validating attestation document")
}
var attDoc AttestationDocument
if err := json.Unmarshal(attDocRaw, &attDoc); err != nil {
return nil, fmt.Errorf("unmarshaling TPM attestation document: %w", err)
}
// Verify and retrieve the trusted attestation public key using the provided instance info
aKP, err := v.getTrustedKey(attDoc.Attestation.AkPub, attDoc.InstanceInfo)
if err != nil {
return nil, fmt.Errorf("validating attestation public key: %w", err)
}
// Validate confidential computing capabilities of the VM
if err := v.validateCVM(attDoc); err != nil {
return nil, fmt.Errorf("verifying VM confidential computing capabilities: %w", err)
}
// Verify the TPM attestation
if _, err := tpmServer.VerifyAttestation(
attDoc.Attestation,
tpmServer.VerifyOpts{
Nonce: nonce,
TrustedAKs: []crypto.PublicKey{aKP},
AllowSHA1: false,
},
); err != nil {
return nil, fmt.Errorf("verifying attestation document: %w", err)
}
// Verify PCRs
quoteIdx, err := GetSHA256QuoteIndex(attDoc.Attestation.Quotes)
if err != nil {
return nil, err
}
for idx, pcr := range v.expected {
if !bytes.Equal(pcr.Expected[:], attDoc.Attestation.Quotes[quoteIdx].Pcrs.Pcrs[idx]) {
if !pcr.WarnOnly {
return nil, fmt.Errorf("untrusted PCR value at PCR index %d", idx)
}
if v.log != nil {
v.log.Warnf("Encountered untrusted PCR value at index %d", idx)
}
}
}
// Verify signed user data
digest := sha256.Sum256(attDoc.UserData)
if err = v.verifyUserData(aKP, crypto.SHA256, digest[:], attDoc.UserDataSignature); err != nil {
return nil, fmt.Errorf("verifying signed user data: %w", err)
}
if v.log != nil {
v.log.Infof("Successfully validated attestation document")
}
return attDoc.UserData, nil
}
// GetSHA256QuoteIndex performs safety checks and returns the index for SHA256 PCR quotes.
func GetSHA256QuoteIndex(quotes []*tpmProto.Quote) (int, error) {
if len(quotes) == 0 {
return 0, fmt.Errorf("attestation is missing quotes")
}
for idx, quote := range quotes {
if quote == nil {
return 0, fmt.Errorf("quote is nil")
}
if quote.Pcrs == nil {
return 0, fmt.Errorf("no PCR data in attestation")
}
if quote.Pcrs.Hash == tpmProto.HashAlgo_SHA256 {
return idx, nil
}
}
return 0, fmt.Errorf("attestation did not include SHA256 hashed PCRs")
}
// VerifyPKCS1v15 is a convenience function to call rsa.VerifyPKCS1v15.
func VerifyPKCS1v15(pub crypto.PublicKey, hash crypto.Hash, hashed, sig []byte) error {
key, ok := pub.(*rsa.PublicKey)
if !ok {
return errors.New("key is not an RSA public key")
}
return rsa.VerifyPKCS1v15(key, hash, hashed, sig)
}
// GetSelectedMeasurements returns a map of Measurments for the PCRs in selection.
func GetSelectedMeasurements(open TPMOpenFunc, selection tpm2.PCRSelection) (measurements.M, error) {
tpm, err := open()
if err != nil {
return nil, err
}
defer tpm.Close()
pcrList, err := tpmClient.ReadPCRs(tpm, selection)
if err != nil {
return nil, err
}
m := make(measurements.M)
for i, pcr := range pcrList.Pcrs {
if len(pcr) != 32 {
return nil, fmt.Errorf("invalid measurement: invalid length: %d", len(pcr))
}
m[i] = measurements.Measurement{
Expected: *(*[32]byte)(pcr),
}
}
return m, nil
}