attest: Implement discovery of supported PCR banks, rather than always blithely assuming we have exactly SHA1 and SHA256. (#404)

To do this, add a function to attest.TPM called PCRBanks() which enumerates the available PCR banks on a TPM. This requires plumbing through tpmBase and its implementations; the TPM1.2 implementations statically return []HashAlg{HashSHA1}, as one might expect.

To accomplish all of this, the implementation of HashAlg needed to be rethought. Now, instead of a reimplementation of tpm2.Algorithm, it's a lightweight wrapper around it. Dependent methods -- like Hash() and String() -- no longer have case HashSHA1/case HashSHA256 blocks; instead, they simply delegate to go-tpm2 for their implementations. As a result, we should never need to do something like this again.

Also add convenience constants HashSHA384 and HashSHA512.

attest/attest.go

@@ -399,41 +399,30 @@ func (a *AKPublic) VerifyAll(quotes []Quote, pcrs []PCR, nonce []byte) error {
 // HashAlg identifies a hashing Algorithm.
 type HashAlg uint8
 
-// Valid hash algorithms.
+// Known valid hash algorithms.
 var (
 	HashSHA1   = HashAlg(tpm2.AlgSHA1)
 	HashSHA256 = HashAlg(tpm2.AlgSHA256)
+	HashSHA384 = HashAlg(tpm2.AlgSHA384)
+	HashSHA512 = HashAlg(tpm2.AlgSHA512)
 )
 
 func (a HashAlg) cryptoHash() crypto.Hash {
-	switch a {
-	case HashSHA1:
-		return crypto.SHA1
-	case HashSHA256:
-		return crypto.SHA256
+	g := a.goTPMAlg()
+	h, err := g.Hash()
+	if err != nil {
+		panic(fmt.Sprintf("HashAlg %v (corresponding to TPM2.Algorithm %v) has no corresponding crypto.Hash", a, g))
 	}
-	return 0
+	return h
 }
 
 func (a HashAlg) goTPMAlg() tpm2.Algorithm {
-	switch a {
-	case HashSHA1:
-		return tpm2.AlgSHA1
-	case HashSHA256:
-		return tpm2.AlgSHA256
-	}
-	return 0
+	return tpm2.Algorithm(a)
 }
 
 // String returns a human-friendly representation of the hash algorithm.
 func (a HashAlg) String() string {
-	switch a {
-	case HashSHA1:
-		return "SHA1"
-	case HashSHA256:
-		return "SHA256"
-	}
-	return fmt.Sprintf("HashAlg<%d>", int(a))
+	return a.goTPMAlg().String()
 }
 
 // PlatformParameters encapsulates the set of information necessary to attest

attest/eventlog.go

@@ -533,15 +533,7 @@ func ParseEventLog(measurementLog []byte) (*EventLog, error) {
 			return nil, fmt.Errorf("failed to parse spec ID event: %v", err)
 		}
 		for _, alg := range specID.algs {
-			switch tpm2.Algorithm(alg.ID) {
-			case tpm2.AlgSHA1:
-				el.Algs = append(el.Algs, HashSHA1)
-			case tpm2.AlgSHA256:
-				el.Algs = append(el.Algs, HashSHA256)
-			}
-		}
-		if len(el.Algs) == 0 {
-			return nil, fmt.Errorf("measurement log didn't use sha1 or sha256 digests")
+			el.Algs = append(el.Algs, HashAlg(alg.ID))
 		}
 		// Switch to parsing crypto agile events. Don't include this in the
 		// replayed events since it intentionally doesn't extend the PCRs.

attest/tpm.go

@@ -29,6 +29,7 @@ import (
 
 	"github.com/google/go-tpm/legacy/tpm2"
 	"github.com/google/go-tpm/tpmutil"
+	"go.uber.org/multierr"
 )
 
 const (
@@ -312,6 +313,27 @@ func quote20(tpm io.ReadWriter, akHandle tpmutil.Handle, hashAlg tpm2.Algorithm,
 	}, err
 }
 
+func pcrbanks(tpm io.ReadWriter) ([]HashAlg, error) {
+	vals, _, err := tpm2.GetCapability(tpm, tpm2.CapabilityPCRs, 1024, 0)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get TPM available PCR banks: %w", err)
+	}
+
+	var hAlgs []HashAlg
+	var errs error
+	for i, v := range vals {
+		pcrb, ok := v.(tpm2.PCRSelection)
+		if !ok {
+			errs = multierr.Append(errs, fmt.Errorf("failed to convert value %d to tpm2.PCRSelection: %v", i, v))
+			continue
+		}
+
+		hAlgs = append(hAlgs, HashAlg(pcrb.Hash))
+	}
+
+	return hAlgs, errs
+}
+
 func readAllPCRs20(tpm io.ReadWriter, alg tpm2.Algorithm) (map[uint32][]byte, error) {
 	numPCRs := 24
 	out := map[uint32][]byte{}
@@ -357,6 +379,7 @@ type tpmBase interface {
 	eks() ([]EK, error)
 	ekCertificates() ([]EK, error)
 	info() (*TPMInfo, error)
+	pcrbanks() ([]HashAlg, error)
 
 	loadAK(opaqueBlob []byte) (*AK, error)
 	loadAKWithParent(opaqueBlob []byte, parent ParentKeyConfig) (*AK, error)
@@ -483,6 +506,14 @@ func (t *TPM) PCRs(alg HashAlg) ([]PCR, error) {
 	return t.tpm.pcrs(alg)
 }
 
+// PCRBanks returns the list of supported PCR banks on the TPM.
+//
+// This is a low-level API. Consumers seeking to attest the state of the
+// platform should use tpm.AttestPlatform() instead.
+func (t *TPM) PCRBanks() ([]HashAlg, error) {
+	return t.tpm.pcrbanks()
+}
+
 func (t *TPM) attestPCRs(ak *AK, nonce []byte, alg HashAlg) (*Quote, []PCR, error) {
 	pcrs, err := t.PCRs(alg)
 	if err != nil {
@@ -514,9 +545,9 @@ func (t *TPM) attestPlatform(ak *AK, nonce []byte, eventLog []byte) (*PlatformPa
 		EventLog:   eventLog,
 	}
 
-	algs := []HashAlg{HashSHA1}
-	if t.Version() == TPMVersion20 {
-		algs = []HashAlg{HashSHA1, HashSHA256}
+	algs, err := t.PCRBanks()
+	if err != nil {
+		return nil, fmt.Errorf("failed to get PCR banks: %w", err)
 	}
 
 	var lastErr error

attest/tpm12_linux.go

@@ -163,6 +163,10 @@ func allPCRs12(ctx *tspi.Context) (map[uint32][]byte, error) {
 	return PCRs, nil
 }
 
+func (t *trousersTPM) pcrbanks() ([]HashAlg, error) {
+	return []HashAlg{HashSHA1}, nil
+}
+
 func (t *trousersTPM) pcrs(alg HashAlg) ([]PCR, error) {
 	if alg != HashSHA1 {
 		return nil, fmt.Errorf("non-SHA1 algorithm %v is not supported on TPM 1.2", alg)

attest/tpm_windows.go

@@ -376,6 +376,23 @@ func allPCRs12(tpm io.ReadWriter) (map[uint32][]byte, error) {
 	return out, nil
 }
 
+func (t *windowsTPM) pcrbanks() ([]HashAlg, error) {
+	switch t.version {
+	case TPMVersion12:
+		return []HashAlg{HashSHA1}, nil
+
+	case TPMVersion20:
+		tpm, err := t.pcp.TPMCommandInterface()
+		if err != nil {
+			return nil, fmt.Errorf("TPMCommandInterface() failed: %v", err)
+		}
+		return pcrbanks(tpm)
+
+	default:
+		return nil, fmt.Errorf("unsupported TPM version: %x", t.version)
+	}
+}
+
 func (t *windowsTPM) pcrs(alg HashAlg) ([]PCR, error) {
 	var PCRs map[uint32][]byte
 

attest/wrapped_tpm20.go

@@ -462,6 +462,10 @@ func (t *wrappedTPM20) loadKeyWithParent(opaqueBlob []byte, parent ParentKeyConf
 	return &Key{key: newWrappedKey20(hnd, sKey.Blob, sKey.Public, sKey.CreateData, sKey.CreateAttestation, sKey.CreateSignature), pub: pub, tpm: t}, nil
 }
 
+func (t *wrappedTPM20) pcrbanks() ([]HashAlg, error) {
+	return pcrbanks(t.rwc)
+}
+
 func (t *wrappedTPM20) pcrs(alg HashAlg) ([]PCR, error) {
 	PCRs, err := readAllPCRs20(t.rwc, alg.goTPMAlg())
 	if err != nil {

go.mod

@@ -9,6 +9,7 @@ require (
 	github.com/google/go-tpm v0.9.3
 	github.com/google/go-tpm-tools v0.4.4
 	github.com/google/go-tspi v0.3.0
+	go.uber.org/multierr v1.11.0
 	golang.org/x/sys v0.30.0
 )