Save EFI BootX variables when (un)sealing storage key

To do a proper analysis a monitor TUI we need values of BootXXXX and
BootOrder EFI variables. Variable data for them is present in the TPM
log however neither TCG spec nor UEFI spec mandate what part of
EFI_LOAD_OPTION structure is serialized and measured in the TPM log so
the reliable extraction is not possible. For example UEFI on NUC13
device avoids first two fields of this structure.

- save variables to /persist/status/boot_vars/{success,fail} directories
  Export paths by introducing GetBootVariablesDirNames()
- export paths to saved TPM logs by introducing GetTpmLogFileNames()
  and GetTpmLogBackupFileNames()

Signed-off-by: Mikhail Malyshev <[email protected]>

pkg/pillar/evetpm/tpm.go

@@ -15,7 +15,10 @@ import (
 	"io"
 	"math/big"
 	"os"
+	"path/filepath"
+	"regexp"
 	"sort"
+	"strings"
 	"unsafe"
 
 	"github.com/google/go-tpm/legacy/tpm2"
@@ -82,6 +85,10 @@ const (
 	PCRBank256StatusNotSupported
 )
 
+// as defined in https://uefi.org/sites/default/files/resources/UEFI%20Spec%202_6.pdf
+// 3.3 Globally Defined Variables. must be LOWERCASE
+const efiGlobalVariableGUID = "8be4df61-93ca-11d2-aa0d-00e098032b8c"
+
 var (
 	//EcdhKeyFile is the location of the ecdh private key
 	//on devices without a TPM. It is not a constant due to test usage
@@ -120,6 +127,16 @@ var (
 	// it is not a constant due to test usage.
 	measurefsTpmEventLog = types.PersistStatusDir + "/measurefs_tpm_event_log"
 
+	// we do not make backup copies of following directories because we use them
+	// only when we couldn't unseal the key from TPM and remote attestation fails
+	// to get a backup key
+	// directory to store the boot variables's values when the key is sealed
+	bootVariablesSealSuccess = filepath.Join(types.PersistStatusDir, "boot_vars/success")
+	// directory to store the boot variables's values when we failed to unseal the key
+	bootVariablesUnsealFail = filepath.Join(types.PersistStatusDir, "boot_vars/fail")
+	// sysfs directory with boot variables
+	kernelEfiBootVarsPath = "/hostfs/sys/firmware/efi/efivars/"
+
 	// PcrSelection is used as an entropy to generate keys and the selection
 	// of PCRs do not matter as well as the contents but PCR[7] is not changed often
 	// on our devices
@@ -224,6 +241,23 @@ var (
 	}
 )
 
+// GetTpmLogFileNames returns paths to saved TPM logs
+func GetTpmLogFileNames() (string, string) {
+	return measurementLogSealSuccess, measurementLogUnsealFail
+}
+
+// GetTpmLogBackupFileNames returns paths to saved TPM logs for previous boot
+func GetTpmLogBackupFileNames() (string, string) {
+	sealSuccessBackupPath := fmt.Sprintf("%s-backup", measurementLogSealSuccess)
+	unsealFailBackupPath := fmt.Sprintf("%s-backup", measurementLogUnsealFail)
+	return sealSuccessBackupPath, unsealFailBackupPath
+}
+
+// GetBootVariablesDirNames returns paths to saved boot variables directories
+func GetBootVariablesDirNames() (string, string) {
+	return bootVariablesSealSuccess, bootVariablesUnsealFail
+}
+
 // SealedKeyType holds different types of sealed key
 // defined below
 type SealedKeyType uint32
@@ -819,6 +853,12 @@ func SealDiskKey(log *base.LogObject, key []byte, pcrSel tpm2.PCRSelection) erro
 		log.Warnf("copying current TPM measurement log failed: %s", err)
 	}
 
+	// save a copy of the current boot variables, this is also called
+	// if unseal fails to have copy when we fail to unlock the vault.
+	if err := saveBootVariables(bootVariablesSealSuccess); err != nil {
+		log.Warnf("copying current boot variables failed: %s", err)
+	}
+
 	return nil
 }
 
@@ -883,6 +923,13 @@ func UnsealDiskKey(pcrSel tpm2.PCRSelection) ([]byte, error) {
 			evtLogStat = fmt.Sprintf("copying (failed unseal) TPM measurement log failed: %v", errEvtLog)
 		}
 
+		// save a copy of the current boot variables
+		if errSaveVars := saveBootVariables(bootVariablesUnsealFail); errSaveVars != nil {
+			// just report the failure, still give FindMismatchingPCRs a chance so
+			// we can at least have some partial information about why unseal failed.
+			evtLogStat += fmt.Sprintf(" ,copying (failed unseal) boot variables failed: %v", errSaveVars)
+		}
+
 		// try to find out the mismatching PCR index
 		mismatch, errPcrMiss := FindMismatchingPCRs()
 		if errPcrMiss != nil {
@@ -1000,8 +1047,7 @@ func pcrBankSHA256EnabledHelper() bool {
 }
 
 func backupCopiedMeasurementLogs() error {
-	sealSuccessBackupPath := fmt.Sprintf("%s-backup", measurementLogSealSuccess)
-	unsealFailBackupPath := fmt.Sprintf("%s-backup", measurementLogUnsealFail)
+	sealSuccessBackupPath, unsealFailBackupPath := GetTpmLogBackupFileNames()
 
 	if fileutils.FileExists(nil, measurementLogSealSuccess) {
 		if err := os.Rename(measurementLogSealSuccess, sealSuccessBackupPath); err != nil {
@@ -1052,6 +1098,47 @@ func copyMeasurementLog(dstPath string) error {
 	return nil
 }
 
+func saveBootVariables(destPath string) error {
+	if err := os.RemoveAll(destPath); err != nil {
+		return fmt.Errorf("failed to clean directory %s: %w", destPath, err)
+	}
+
+	if err := os.MkdirAll(destPath, 0755); err != nil {
+		return fmt.Errorf("failed to create directory %s: %w", destPath, err)
+	}
+
+	files, err := os.ReadDir(kernelEfiBootVarsPath)
+	if err != nil {
+		return fmt.Errorf("failed to read directory %s: %w", kernelEfiBootVarsPath, err)
+	}
+
+	variableSuffix := fmt.Sprintf("-%s", efiGlobalVariableGUID)
+	bootOrderFileName := fmt.Sprintf(`BootOrder%s`, variableSuffix)
+	regexpStr := fmt.Sprintf(`^Boot[0-9a-fA-F]{4}%s$`, variableSuffix)
+
+	// regexp to match BootXXXX where XXXX is a 4 digit hex number
+	bootVarRegexp := regexp.MustCompile(regexpStr)
+
+	for _, file := range files {
+		variableFileName := file.Name()
+
+		if variableFileName == bootOrderFileName || bootVarRegexp.MatchString(variableFileName) {
+			src := filepath.Join(kernelEfiBootVarsPath, variableFileName)
+
+			//remove suffix for destination file
+			dst := filepath.Join(destPath, variableFileName)
+			dst = strings.TrimSuffix(dst, variableSuffix)
+
+			// copy the file
+			if err := fileutils.CopyFile(src, dst); err != nil {
+				return fmt.Errorf("failed to copy file %s to %s: %w", src, dst, err)
+			}
+		}
+	}
+
+	return nil
+}
+
 func saveDiskKeySealingPCRs() error {
 	trw, err := tpm2.OpenTPM(TpmDevicePath)
 	if err != nil {