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Make validation of tpm format stricter

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This commit is contained in:
Herman Slatman 2023-03-14 13:59:16 +01:00
parent 213b31bc2c
commit 589a62df74
No known key found for this signature in database
GPG key ID: F4D8A44EA0A75A4F
3 changed files with 539 additions and 84 deletions
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120
acme/challenge.go
View file

@ -11,6 +11,7 @@ import (
"crypto/subtle"
"crypto/tls"
"crypto/x509"
"crypto/x509/pkix"
"encoding/asn1"
"encoding/base64"
"encoding/hex"
@ -27,7 +28,9 @@ import (
"github.com/fxamacker/cbor/v2"
"github.com/google/go-attestation/attest"
x509ext "github.com/google/go-attestation/x509"
"github.com/google/go-tpm/tpm2"
"golang.org/x/exp/slices"
"go.step.sm/crypto/jose"
"go.step.sm/crypto/keyutil"
@ -446,6 +449,8 @@ func deviceAttest01Validate(ctx context.Context, ch *Challenge, db DB, jwk *jose
case "tpm":
data, err := doTPMAttestationFormat(ctx, ch, db, jwk, &att)
if err != nil {
// TODO(hs): we should provide more details in the error reported to the client;
// "Attestation statement cannot be verified" is VERY generic. Also holds true for the other formats.
var acmeError *Error
if errors.As(err, &acmeError) {
if acmeError.Status == 500 {
@ -456,7 +461,19 @@ func deviceAttest01Validate(ctx context.Context, ch *Challenge, db DB, jwk *jose
return WrapErrorISE(err, "error validating attestation")
}
// TODO(hs): more properties to verify? Apple method has nonce, check for permanent identifier.
// TODO(hs): currently this will allow a request for which no PermanentIdentifiers have been
// extracted from the AK certificate. This is currently the case for AK certs from the CLI, as we
// haven't implemented a way for AK certs requested by the CLI to always contain the requested
// PermanentIdentifier. Omitting the check below doesn't allow just any request, as the Order can
// still fail if the challenge value isn't equal to the CSR subject.
if len(data.PermanentIdentifiers) > 0 && !slices.Contains(data.PermanentIdentifiers, ch.Value) { // TODO(hs): add support for HardwareModuleName
subproblem := NewSubproblemWithIdentifier(
ErrorMalformedType,
Identifier{Type: "permanent-identifier", Value: ch.Value},
"challenge identifier %q doesn't match any of the attested hardware identifiers %q", ch.Value, data.PermanentIdentifiers,
)
return storeError(ctx, db, ch, true, NewError(ErrorRejectedIdentifierType, "permanent identifier does not match").AddSubproblems(subproblem))
}
// Update attestation key fingerprint to compare against the CSR
az.Fingerprint = data.Fingerprint
@ -484,22 +501,15 @@ func deviceAttest01Validate(ctx context.Context, ch *Challenge, db DB, jwk *jose
return nil
}
// Borrowed from: https://github.com/golang/crypto/blob/master/acme/acme.go#L748
// TODO(hs): the hash algorithm in use should match the request data
func keyAuthDigest(jwk *jose.JSONWebKey, token string) ([]byte, error) {
th, err := jwk.Thumbprint(crypto.SHA256) // TODO(hs): verify this is the correct thumbprint
digest := sha256.Sum256([]byte(fmt.Sprintf("%s.%s", token, th)))
return digest[:], err
}
var (
oidSubjectAlternativeName = asn1.ObjectIdentifier{2, 5, 29, 17}
)
type tpmAttestationData struct {
Certificate *x509.Certificate
VerifiedChains [][]*x509.Certificate
Fingerprint string
Certificate *x509.Certificate
VerifiedChains [][]*x509.Certificate
PermanentIdentifiers []string
Fingerprint string
}
func doTPMAttestationFormat(ctx context.Context, ch *Challenge, db DB, jwk *jose.JSONWebKey, att *attestationObject) (*tpmAttestationData, error) {
@ -514,7 +524,7 @@ func doTPMAttestationFormat(ctx context.Context, ch *Challenge, db DB, jwk *jose
return nil, NewError(ErrorBadAttestationStatementType, "ver not present")
}
if ver != "2.0" {
return nil, NewError(ErrorBadAttestationStatementType, "%q is not supported", ver)
return nil, NewError(ErrorBadAttestationStatementType, "version %q is not supported", ver)
}
x5c, ok := att.AttStatement["x5c"].([]interface{})
@ -525,11 +535,11 @@ func doTPMAttestationFormat(ctx context.Context, ch *Challenge, db DB, jwk *jose
return nil, NewError(ErrorBadAttestationStatementType, "x5c is empty")
}
der, ok := x5c[0].([]byte)
akCertBytes, ok := x5c[0].([]byte)
if !ok {
return nil, NewError(ErrorBadAttestationStatementType, "x5c is malformed")
}
leaf, err := x509.ParseCertificate(der)
akCert, err := x509.ParseCertificate(akCertBytes)
if err != nil {
return nil, WrapError(ErrorBadAttestationStatementType, err, "x5c is malformed")
}
@ -537,24 +547,24 @@ func doTPMAttestationFormat(ctx context.Context, ch *Challenge, db DB, jwk *jose
intermediates := x509.NewCertPool()
if len(x5c[1:]) > 0 {
for _, v := range x5c[1:] {
der, ok = v.([]byte)
if !ok {
intCertBytes, vok := v.([]byte)
if !vok {
return nil, NewError(ErrorBadAttestationStatementType, "x5c is malformed")
}
cert, err := x509.ParseCertificate(der)
intCert, err := x509.ParseCertificate(intCertBytes)
if err != nil {
return nil, WrapError(ErrorBadAttestationStatementType, err, "x5c is malformed")
}
intermediates.AddCert(cert)
intermediates.AddCert(intCert)
}
}
// TODO(hs): this can be removed when permanent-identifier/hardware-module-name are handled correctly in
// the stdlib in https://cs.opensource.google/go/go/+/refs/tags/go1.19:src/crypto/x509/parser.go;drc=b5b2cf519fe332891c165077f3723ee74932a647;l=362,
// but I doubt that will happen.
if len(leaf.UnhandledCriticalExtensions) > 0 {
unhandledCriticalExtensions := leaf.UnhandledCriticalExtensions[:0]
for _, extOID := range leaf.UnhandledCriticalExtensions {
if len(akCert.UnhandledCriticalExtensions) > 0 {
unhandledCriticalExtensions := akCert.UnhandledCriticalExtensions[:0]
for _, extOID := range akCert.UnhandledCriticalExtensions {
switch {
case extOID.Equal(oidSubjectAlternativeName):
// allow Subject Alternative Names, including PermanentIdentifier, HardwareModuleName, TPM attributes, etc
@ -563,7 +573,7 @@ func doTPMAttestationFormat(ctx context.Context, ch *Challenge, db DB, jwk *jose
unhandledCriticalExtensions = append(unhandledCriticalExtensions, extOID)
}
}
leaf.UnhandledCriticalExtensions = unhandledCriticalExtensions
akCert.UnhandledCriticalExtensions = unhandledCriticalExtensions
}
roots, ok := prov.GetAttestationRoots()
@ -571,7 +581,7 @@ func doTPMAttestationFormat(ctx context.Context, ch *Challenge, db DB, jwk *jose
return nil, NewErrorISE("failed getting tpm attestation root CAs")
}
verifiedChains, err := leaf.Verify(x509.VerifyOptions{
verifiedChains, err := akCert.Verify(x509.VerifyOptions{
Roots: roots,
Intermediates: intermediates,
CurrentTime: time.Now().Truncate(time.Second),
@ -583,15 +593,58 @@ func doTPMAttestationFormat(ctx context.Context, ch *Challenge, db DB, jwk *jose
// TODO(hs): implement revocation check; Verify() doesn't perform CRL check nor OCSP lookup.
// extract and validate Subject Alternative Name extension to contain at least one PermanentIdentifier
var sanExtension pkix.Extension
for _, ext := range akCert.Extensions {
if ext.Id.Equal(oidSubjectAlternativeName) {
sanExtension = ext
}
}
if sanExtension.Value == nil {
return nil, NewError(ErrorBadAttestationStatementType, "AK certificate is missing Subject Alternative Name extension")
}
san, err := x509ext.ParseSubjectAltName(sanExtension) // TODO(hs): move to a package under our control?
if err != nil {
return nil, WrapError(ErrorBadAttestationStatementType, err, "failed parsing Subject Alternative Name extension")
}
var permanentIdentifiers = make([]string, len(san.PermanentIdentifiers))
for i, p := range san.PermanentIdentifiers {
permanentIdentifiers[i] = p.IdentifierValue
}
// TODO(hs): reenable this check when we want to enforce a PermanentIdentifier to be present in
// the AK certificate.
// if len(permanentIdentifiers) == 0 {
// return nil, NewError(ErrorBadAttestationStatementType, "AK certificate doesn't contain a PermanentIdentifier")
// }
// extract and validate pubArea, sig, certInfo and alg properties from the request body
pubArea, ok := att.AttStatement["pubArea"].([]byte)
if !ok {
return nil, NewError(ErrorBadAttestationStatementType, "invalid pubArea in attestation statement")
}
if len(pubArea) == 0 {
return nil, NewError(ErrorBadAttestationStatementType, "pubArea is empty")
}
sig, ok := att.AttStatement["sig"].([]byte)
if !ok {
return nil, NewError(ErrorBadAttestationStatementType, "invalid sig in attestation statement")
}
if len(sig) == 0 {
return nil, NewError(ErrorBadAttestationStatementType, "sig is empty")
}
certInfo, ok := att.AttStatement["certInfo"].([]byte)
if !ok {
return nil, NewError(ErrorBadAttestationStatementType, "invalid certInfo in attestation statement")
}
if len(certInfo) == 0 {
return nil, NewError(ErrorBadAttestationStatementType, "certInfo is empty")
}
alg, ok := att.AttStatement["alg"].(int64)
if !ok {
@ -610,11 +663,6 @@ func doTPMAttestationFormat(ctx context.Context, ch *Challenge, db DB, jwk *jose
return nil, NewError(ErrorBadAttestationStatementType, "invalid alg %d in attestation statement", alg)
}
certInfo, ok := att.AttStatement["certInfo"].([]byte)
if !ok {
return nil, NewError(ErrorBadAttestationStatementType, "invalid certInfo in attestation statement")
}
// recreate the generated key certification parameter values and verify
// the attested key using the public key of the AK.
certificationParameters := &attest.CertificationParameters{
@ -623,7 +671,7 @@ func doTPMAttestationFormat(ctx context.Context, ch *Challenge, db DB, jwk *jose
CreateSignature: sig, // signature over the attested properties
}
verifyOpts := attest.VerifyOpts{
Public: leaf.PublicKey, // public key of the AK that attested the key
Public: akCert.PublicKey, // public key of the AK that attested the key
Hash: hash,
}
if err = certificationParameters.Verify(verifyOpts); err != nil {
@ -636,15 +684,16 @@ func doTPMAttestationFormat(ctx context.Context, ch *Challenge, db DB, jwk *jose
return nil, WrapError(ErrorBadAttestationStatementType, err, "failed decoding attestation data")
}
expectedDigest, err := keyAuthDigest(jwk, ch.Token)
keyAuth, err := KeyAuthorization(ch.Token, jwk)
if err != nil {
return nil, WrapError(ErrorBadAttestationStatementType, err, "failed creating key auth digest")
}
hashedKeyAuth := sha256.Sum256([]byte(keyAuth))
// verify the WebAuthn object contains the expect key authorization digest, which is carried
// within the encoded `certInfo` property of the attestation statement.
if subtle.ConstantTimeCompare(expectedDigest, []byte(tpmCertInfo.ExtraData)) == 0 {
return nil, NewError(ErrorBadAttestationStatementType, "key authorization doesn not match")
if subtle.ConstantTimeCompare(hashedKeyAuth[:], []byte(tpmCertInfo.ExtraData)) == 0 {
return nil, NewError(ErrorBadAttestationStatementType, "key authorization does not match")
}
// decode the (attested) public key and determine its fingerprint
@ -659,8 +708,9 @@ func doTPMAttestationFormat(ctx context.Context, ch *Challenge, db DB, jwk *jose
}
data := &tpmAttestationData{
Certificate: leaf,
VerifiedChains: verifiedChains,
Certificate: akCert,
VerifiedChains: verifiedChains,
PermanentIdentifiers: permanentIdentifiers,
}
if data.Fingerprint, err = keyutil.Fingerprint(publicKey); err != nil {

8
go.mod
View file

@ -14,7 +14,6 @@ require (
github.com/ThalesIgnite/crypto11 v1.2.5 // indirect
github.com/aws/aws-sdk-go v1.44.210 // indirect
github.com/dgraph-io/ristretto v0.1.0 // indirect
github.com/fatih/color v1.9.0 // indirect
github.com/fxamacker/cbor/v2 v2.4.0
github.com/go-chi/chi v4.1.2+incompatible
github.com/go-kit/kit v0.10.0 // indirect
@ -56,7 +55,10 @@ require (
gopkg.in/square/go-jose.v2 v2.6.0
)
require github.com/google/go-attestation v0.4.3
require (
github.com/google/go-attestation v0.4.4-0.20220404204839-8820d49b18d9
golang.org/x/exp v0.0.0-20230310171629-522b1b587ee0
)
require (
cloud.google.com/go/compute v1.18.0 // indirect
@ -147,3 +149,5 @@ require (
// use github.com/smallstep/pkcs7 fork with patches applied
replace go.mozilla.org/pkcs7 => github.com/smallstep/pkcs7 v0.0.0-20230302202335-4c094085c948
replace github.com/google/go-attestation v0.4.4-0.20220404204839-8820d49b18d9 => github.com/smallstep/go-attestation v0.4.4-0.20230113130042-0ad94dd6a52e

495
go.sum
View file

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