Merge pull request #56 from chrisccoulson/add-measure-snap-system-epoch

Add MeasureSnapSystemEpochToTPM

snapmodel_policy.go

@@ -30,6 +30,14 @@ import (
 	"golang.org/x/xerrors"
 )
 
+const zeroSnapSystemEpoch uint32 = 0
+
+func computeSnapSystemEpochDigest(alg tpm2.HashAlgorithmId, epoch uint32) tpm2.Digest {
+	h := alg.NewHash()
+	binary.Write(h, binary.LittleEndian, epoch)
+	return h.Sum(nil)
+}
+
 func computeSnapModelDigest(alg tpm2.HashAlgorithmId, model *asserts.Model) (tpm2.Digest, error) {
 	signKeyId, err := base64.RawURLEncoding.DecodeString(model.SignKeyID())
 	if err != nil {
@@ -73,11 +81,16 @@ type SnapModelProfileParams struct {
 // a PCR policy that is bound to a specific set of device models. It is the responsibility of snap-bootstrap to verify the integrity
 // of the model that it has measured.
 //
-// The profile consists of 2 measurements (where H is the digest algorithm supplied via params.PCRAlgorithm):
-//  H(uint32(0))
+// The profile consists of 2 measurements:
+//  digestEpoch
 //  digestModel
 //
-// digestModel is computed as follows:
+// digestEpoch is currently hardcoded as (where H is the digest algorithm supplied via params.PCRAlgorithm):
+//  digestEpoch = H(uint32(0))
+//
+// A future version of this package may allow another epoch to be supplied.
+//
+// digestModel is computed as follows (where H is the digest algorithm supplied via params.PCRAlgorithm):
 //  digest1 = H(tpm2.HashAlgorithmSHA384 || sign-key-sha3-384 || brand-id)
 //  digest2 = H(digest1 || model)
 //  digestModel = H(digest2 || series || grade)
@@ -98,9 +111,7 @@ func AddSnapModelProfile(profile *PCRProtectionProfile, params *SnapModelProfile
 		return errors.New("no models provided")
 	}
 
-	h := params.PCRAlgorithm.NewHash()
-	binary.Write(h, binary.LittleEndian, uint32(0))
-	profile.ExtendPCR(params.PCRAlgorithm, params.PCRIndex, h.Sum(nil))
+	profile.ExtendPCR(params.PCRAlgorithm, params.PCRIndex, computeSnapSystemEpochDigest(params.PCRAlgorithm, zeroSnapSystemEpoch))
 
 	var subProfiles []*PCRProtectionProfile
 	for _, model := range params.Models {
@@ -119,9 +130,7 @@ func AddSnapModelProfile(profile *PCRProtectionProfile, params *SnapModelProfile
 	return nil
 }
 
-// MeasureSnapModelToTPM measures a digest of the supplied model assertion to the specified PCR for all supported PCR banks.
-// See the documentation for AddSnapModelProfile for details of how the digest of the model is computed.
-func MeasureSnapModelToTPM(tpm *TPMConnection, pcrIndex int, model *asserts.Model) error {
+func measureSnapPropertyToTPM(tpm *TPMConnection, pcrIndex int, computeDigest func(tpm2.HashAlgorithmId) (tpm2.Digest, error)) error {
 	pcrSelection, err := tpm.GetCapabilityPCRs(tpm.HmacSession().IncludeAttrs(tpm2.AttrAudit))
 	if err != nil {
 		return xerrors.Errorf("cannot determine supported PCR banks: %w", err)
@@ -136,13 +145,29 @@ func MeasureSnapModelToTPM(tpm *TPMConnection, pcrIndex int, model *asserts.Mode
 			continue
 		}
 
-		digest, err := computeSnapModelDigest(s.Hash, model)
+		digest, err := computeDigest(s.Hash)
 		if err != nil {
-			return xerrors.Errorf("cannot compute snap mode digest for algorithm %v: %w", s.Hash, err)
+			return xerrors.Errorf("cannot compute digest for algorithm %v: %w", s.Hash, err)
 		}
 
 		digests = append(digests, tpm2.TaggedHash{HashAlg: s.Hash, Digest: digest})
 	}
 
 	return tpm.PCRExtend(tpm.PCRHandleContext(pcrIndex), digests, tpm.HmacSession())
 }
+
+// MeasureSnapSystemEpochToTPM measures a digest of uint32(0) to the specified PCR for all supported PCR banks. See the documentation
+// for AddSnapModelProfile for more details.
+func MeasureSnapSystemEpochToTPM(tpm *TPMConnection, pcrIndex int) error {
+	return measureSnapPropertyToTPM(tpm, pcrIndex, func(alg tpm2.HashAlgorithmId) (tpm2.Digest, error) {
+		return computeSnapSystemEpochDigest(alg, zeroSnapSystemEpoch), nil
+	})
+}
+
+// MeasureSnapModelToTPM measures a digest of the supplied model assertion to the specified PCR for all supported PCR banks.
+// See the documentation for AddSnapModelProfile for details of how the digest of the model is computed.
+func MeasureSnapModelToTPM(tpm *TPMConnection, pcrIndex int, model *asserts.Model) error {
+	return measureSnapPropertyToTPM(tpm, pcrIndex, func(alg tpm2.HashAlgorithmId) (tpm2.Digest, error) {
+		return computeSnapModelDigest(alg, model)
+	})
+}

snapmodel_policy_test.go

@@ -20,6 +20,7 @@
 package secboot_test
 
 import (
+	"encoding/binary"
 	"time"
 
 	"github.com/canonical/go-tpm2"
@@ -558,3 +559,52 @@ func (s *snapModelMeasureSuite) TestMeasureSnapModelToTPMTest7(c *C) {
 		}, "Jv8_JiHiIzJVcO9M55pPdqSDWUvuhfDIBJUS-3VW7F_idjix7Ffn5qMxB21ZQuij"),
 	})
 }
+
+func (s *snapModelMeasureSuite) testMeasureSnapSystemEpochToTPM(c *C, pcrIndex int) {
+	pcrSelection, err := s.tpm.GetCapabilityPCRs()
+	c.Assert(err, IsNil)
+
+	var pcrs []int
+	for i := 0; i < 24; i++ {
+		pcrs = append(pcrs, i)
+	}
+	var readPcrSelection tpm2.PCRSelectionList
+	for _, s := range pcrSelection {
+		readPcrSelection = append(readPcrSelection, tpm2.PCRSelection{Hash: s.Hash, Select: pcrs})
+	}
+
+	_, origPcrValues, err := s.tpm.PCRRead(readPcrSelection)
+	c.Assert(err, IsNil)
+
+	c.Check(MeasureSnapSystemEpochToTPM(s.tpm, pcrIndex), IsNil)
+
+	_, pcrValues, err := s.tpm.PCRRead(readPcrSelection)
+	c.Assert(err, IsNil)
+
+	for _, s := range pcrSelection {
+		h := s.Hash.NewHash()
+		binary.Write(h, binary.LittleEndian, uint32(0))
+		digest := h.Sum(nil)
+
+		h = s.Hash.NewHash()
+		h.Write(origPcrValues[s.Hash][pcrIndex])
+		h.Write(digest)
+
+		c.Check(pcrValues[s.Hash][pcrIndex], DeepEquals, tpm2.Digest(h.Sum(nil)))
+
+		for _, p := range pcrs {
+			if p == pcrIndex {
+				continue
+			}
+			c.Check(pcrValues[s.Hash][p], DeepEquals, origPcrValues[s.Hash][p])
+		}
+	}
+}
+
+func (s *snapModelMeasureSuite) TestMeasureSnapSystemEpochToTPM1(c *C) {
+	s.testMeasureSnapSystemEpochToTPM(c, 12)
+}
+
+func (s *snapModelMeasureSuite) TestMeasureSnapSystemEpochToTPM2(c *C) {
+	s.testMeasureSnapSystemEpochToTPM(c, 14)
+}