Introduce TPM event log infrastructure
This adds support for reading the local TPM event log and for parsing the
resulting blob. Reading the log is implemented as part of our TPM library, but
for reading and processing the event log binary structure we rely on Google's
go-attestation. Since they don't separate their event log processing from the rest
of the package, I imported the relevant files here directly.
Since TPM event logs are really terrible (see included workarounds and
https://github.com/google/go-attestation/blob/master/docs/event-log-disclosure.md)
it's probably a bad idea to use them for anything where we can avoid it.
So this will likely only be used for EFI boot / secure boot attestation and
everything we measure will be part of our TPM library with a much less insane format.
Test Plan:
Manually smoke-tested using a custom fixture on a Ryzen 3000 fTPM.
We cannot really test this until we have a way of generating and loading
secure boot keys since an empty secure boot setup generates no events.
X-Origin-Diff: phab/D622
GitOrigin-RevId: e730a3ea69c4055e411833c80530f630d77788e4
diff --git a/core/pkg/tpm/eventlog/secureboot.go b/core/pkg/tpm/eventlog/secureboot.go
new file mode 100644
index 0000000..5540a0e
--- /dev/null
+++ b/core/pkg/tpm/eventlog/secureboot.go
@@ -0,0 +1,210 @@
+// Copyright 2020 The Monogon Project Authors.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+// 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.
+
+// Taken and pruned from go-attestation under Apache 2.0
+package eventlog
+
+import (
+ "bytes"
+ "errors"
+ "fmt"
+
+ "github.com/google/certificate-transparency-go/x509"
+
+ "git.monogon.dev/source/nexantic.git/core/pkg/tpm/eventlog/internal"
+)
+
+// SecurebootState describes the secure boot status of a machine, as determined
+// by processing its event log.
+type SecurebootState struct {
+ Enabled bool
+
+ // PlatformKeys enumerates keys which can sign a key exchange key.
+ PlatformKeys []x509.Certificate
+ // PlatformKeys enumerates key hashes which can sign a key exchange key.
+ PlatformKeyHashes [][]byte
+
+ // ExchangeKeys enumerates keys which can sign a database of permitted or
+ // forbidden keys.
+ ExchangeKeys []x509.Certificate
+ // ExchangeKeyHashes enumerates key hashes which can sign a database or
+ // permitted or forbidden keys.
+ ExchangeKeyHashes [][]byte
+
+ // PermittedKeys enumerates keys which may sign binaries to run.
+ PermittedKeys []x509.Certificate
+ // PermittedHashes enumerates hashes which permit binaries to run.
+ PermittedHashes [][]byte
+
+ // ForbiddenKeys enumerates keys which must not permit a binary to run.
+ ForbiddenKeys []x509.Certificate
+ // ForbiddenKeys enumerates hashes which must not permit a binary to run.
+ ForbiddenHashes [][]byte
+
+ // PreSeparatorAuthority describes the use of a secure-boot key to authorize
+ // the execution of a binary before the separator.
+ PreSeparatorAuthority []x509.Certificate
+ // PostSeparatorAuthority describes the use of a secure-boot key to authorize
+ // the execution of a binary after the separator.
+ PostSeparatorAuthority []x509.Certificate
+}
+
+// ParseSecurebootState parses a series of events to determine the
+// configuration of secure boot on a device. An error is returned if
+// the state cannot be determined, or if the event log is structured
+// in such a way that it may have been tampered post-execution of
+// platform firmware.
+func ParseSecurebootState(events []Event) (*SecurebootState, error) {
+ // This algorithm verifies the following:
+ // - All events in PCR 7 have event types which are expected in PCR 7.
+ // - All events are parsable according to their event type.
+ // - All events have digests values corresponding to their data/event type.
+ // - No unverifiable events were present.
+ // - All variables are specified before the separator and never duplicated.
+ // - The SecureBoot variable has a value of 0 or 1.
+ // - If SecureBoot was 1 (enabled), authority events were present indicating
+ // keys were used to perform verification.
+ // - If SecureBoot was 1 (enabled), platform + exchange + database keys
+ // were specified.
+ // - No UEFI debugger was attached.
+
+ var (
+ out SecurebootState
+ seenSeparator bool
+ seenAuthority bool
+ seenVars = map[string]bool{}
+ )
+
+ for _, e := range events {
+ if e.Index != 7 {
+ continue
+ }
+
+ et, err := internal.UntrustedParseEventType(uint32(e.Type))
+ if err != nil {
+ return nil, fmt.Errorf("unrecognised event type: %v", err)
+ }
+
+ digestVerify := e.digestEquals(e.Data)
+ switch et {
+ case internal.Separator:
+ if seenSeparator {
+ return nil, fmt.Errorf("duplicate separator at event %d", e.sequence)
+ }
+ seenSeparator = true
+ if !bytes.Equal(e.Data, []byte{0, 0, 0, 0}) {
+ return nil, fmt.Errorf("invalid separator data at event %d: %v", e.sequence, e.Data)
+ }
+ if digestVerify != nil {
+ return nil, fmt.Errorf("invalid separator digest at event %d: %v", e.sequence, digestVerify)
+ }
+
+ case internal.EFIAction:
+ if string(e.Data) == "UEFI Debug Mode" {
+ return nil, errors.New("a UEFI debugger was present during boot")
+ }
+ return nil, fmt.Errorf("event %d: unexpected EFI action event", e.sequence)
+
+ case internal.EFIVariableDriverConfig:
+ v, err := internal.ParseUEFIVariableData(bytes.NewReader(e.Data))
+ if err != nil {
+ return nil, fmt.Errorf("failed parsing EFI variable at event %d: %v", e.sequence, err)
+ }
+ if _, seenBefore := seenVars[v.VarName()]; seenBefore {
+ return nil, fmt.Errorf("duplicate EFI variable %q at event %d", v.VarName(), e.sequence)
+ }
+ seenVars[v.VarName()] = true
+ if seenSeparator {
+ return nil, fmt.Errorf("event %d: variable %q specified after separator", e.sequence, v.VarName())
+ }
+
+ if digestVerify != nil {
+ return nil, fmt.Errorf("invalid digest for variable %q on event %d: %v", v.VarName(), e.sequence, digestVerify)
+ }
+
+ switch v.VarName() {
+ case "SecureBoot":
+ if len(v.VariableData) != 1 {
+ return nil, fmt.Errorf("event %d: SecureBoot data len is %d, expected 1", e.sequence, len(v.VariableData))
+ }
+ out.Enabled = v.VariableData[0] == 1
+ case "PK":
+ if out.PlatformKeys, out.PlatformKeyHashes, err = v.SignatureData(); err != nil {
+ return nil, fmt.Errorf("event %d: failed parsing platform keys: %v", e.sequence, err)
+ }
+ case "KEK":
+ if out.ExchangeKeys, out.ExchangeKeyHashes, err = v.SignatureData(); err != nil {
+ return nil, fmt.Errorf("event %d: failed parsing key exchange keys: %v", e.sequence, err)
+ }
+ case "db":
+ if out.PermittedKeys, out.PermittedHashes, err = v.SignatureData(); err != nil {
+ return nil, fmt.Errorf("event %d: failed parsing signature database: %v", e.sequence, err)
+ }
+ case "dbx":
+ if out.ForbiddenKeys, out.ForbiddenHashes, err = v.SignatureData(); err != nil {
+ return nil, fmt.Errorf("event %d: failed parsing forbidden signature database: %v", e.sequence, err)
+ }
+ }
+
+ case internal.EFIVariableAuthority:
+ a, err := internal.ParseUEFIVariableAuthority(bytes.NewReader(e.Data))
+ if err != nil {
+ // Workaround for: https://github.com/google/go-attestation/issues/157
+ if err == internal.ErrSigMissingGUID {
+ // Versions of shim which do not carry
+ // https://github.com/rhboot/shim/commit/8a27a4809a6a2b40fb6a4049071bf96d6ad71b50
+ // have an erroneous additional byte in the event, which breaks digest
+ // verification. If verification failed, we try removing the last byte.
+ if digestVerify != nil {
+ digestVerify = e.digestEquals(e.Data[:len(e.Data)-1])
+ }
+ } else {
+ return nil, fmt.Errorf("failed parsing EFI variable authority at event %d: %v", e.sequence, err)
+ }
+ }
+ seenAuthority = true
+ if digestVerify != nil {
+ return nil, fmt.Errorf("invalid digest for authority on event %d: %v", e.sequence, digestVerify)
+ }
+ if !seenSeparator {
+ out.PreSeparatorAuthority = append(out.PreSeparatorAuthority, a.Certs...)
+ } else {
+ out.PostSeparatorAuthority = append(out.PostSeparatorAuthority, a.Certs...)
+ }
+
+ default:
+ return nil, fmt.Errorf("unexpected event type: %v", et)
+ }
+ }
+
+ if !out.Enabled {
+ return &out, nil
+ }
+
+ if !seenAuthority {
+ return nil, errors.New("secure boot was enabled but no key was used")
+ }
+ if len(out.PlatformKeys) == 0 && len(out.PlatformKeyHashes) == 0 {
+ return nil, errors.New("secure boot was enabled but no platform keys were known")
+ }
+ if len(out.ExchangeKeys) == 0 && len(out.ExchangeKeyHashes) == 0 {
+ return nil, errors.New("secure boot was enabled but no key exchange keys were known")
+ }
+ if len(out.PermittedKeys) == 0 && len(out.PermittedHashes) == 0 {
+ return nil, errors.New("secure boot was enabled but no keys or hashes were permitted")
+ }
+ return &out, nil
+}