tdx-measure: assemble Boot0000 by hand

It's always nicer to see how a value is constructed/structured rather
than just hard-coding it. Now that we know how that value is derived,
we can also be more confident that it won't change in the future (it's
mostly comprised of constants).

tools/tdx-measure/rtmr/rtmr.go

@@ -63,6 +63,19 @@ func (r *Rtmr) extendSeparator() {
  r.hashAndExtend([]byte{0, 0, 0, 0})
 }
 
+const (
+ mediaProtocolType = 4
+ piwgFirmwareFileSubType = 6
+ piwgFirmwareVolumeSubType = 7
+ endOfPathType = 0x7f
+)
+
+const (
+ loadOptionActive = 1 << 0
+ loadOptionHidden = 1 << 3
+ loadOptionCategoryApp = 1 << 8
+)
+
 var (
  efiGlobalVariable = [16]byte{
  0x61, 0xdf, 0xe4, 0x8b,
@@ -78,8 +91,110 @@ var (
  0xa3, 0xbc,
  0xda, 0xd0, 0x0e, 0x67, 0x65, 0x6f,
  }
+ fvNameGUID = [16]byte{
+ 0xc9, 0xbd, 0xb8, 0x7c,
+ 0xeb, 0xf8,
+ 0x34, 0x4f,
+ 0xaa, 0xea,
+ 0x3e, 0xe4, 0xaf, 0x65, 0x16, 0xa1,
+ }
+ fileGUID = [16]byte{
+ 0x21, 0xaa, 0x2c, 0x46,
+ 0x14, 0x76,
+ 0x03, 0x45,
+ 0x83, 0x6e,
+ 0x8a, 0xb6, 0xf4, 0x66, 0x23, 0x31,
+ }
 )
 
+// buildFilePathList assembles the device path passed by QEMU for boot option Boot0000.
+func buildFilePathList() ([]byte, error) {
+ // The structure of `EFI_DEVICE_PATH_PROTOCOL` is described in Unified
+ // Extensible Firmware Interface (UEFI) Specification, Release 2.10 Errata
+ // A, 10.2 EFI Device Path Protocol.
+ var buffer bytes.Buffer
+ addNode := func(ty uint8, subType uint8, data []byte) error {
+ if err := buffer.WriteByte(ty); err != nil {
+ return err
+ }
+ if err := buffer.WriteByte(subType); err != nil {
+ return err
+ }
+ length := len(data) + 4
+ if err := binary.Write(&buffer, binary.LittleEndian, uint16(length)); err != nil {
+ return err
+ }
+ if _, err := buffer.Write(data); err != nil {
+ return err
+ }
+ return nil
+ }
+
+ // The structure of PIWG Firmware Volume Device Path nodes is described in
+ // UEFI Platform Initialization Specification, Version 1.8 Errata A, II-8.2
+ // Firmware Volume Media Device Path.
+ if err := addNode(mediaProtocolType, piwgFirmwareVolumeSubType, fvNameGUID[:]); err != nil {
+ return nil, err
+ }
+
+ // The structure of PIWG Firmware File Device Path nodes is described in
+ // UEFI Platform Initialization Specification, Version 1.8 Errata A, II-8.3
+ // Firmware File Media Device Path.
+ if err := addNode(mediaProtocolType, piwgFirmwareFileSubType, fileGUID[:]); err != nil {
+ return nil, err
+ }
+
+ // The structure of End Entire Device Path nodes is described in Unified
+ // Extensible Firmware Interface (UEFI) Specification, Release 2.10 Errata
+ // A, 10.3.1 Generic Device Path Structures, Table 10.2: Device Path End
+ // Structure.
+ if err := addNode(endOfPathType, 0xff, []byte{}); err != nil {
+ return nil, err
+ }
+
+ return buffer.Bytes(), nil
+}
+
+// buildEfiLoadOption assembles the EFI_LOAD_OPTION passed by QEMU for boot option Boot0000.
+func buildEfiLoadOption() ([]byte, error) {
+ // The structure of `EFI_LOAD_OPTION` is described in Unified Extensible
+ // Firmware Interface (UEFI) Specification, Release 2.10 Errata A, 3.1.3
+ // Load Options.
+
+ var attributes uint32 = loadOptionActive | loadOptionHidden | loadOptionCategoryApp
+ filePathList, err := buildFilePathList()
+ if err != nil {
+ return nil, err
+ }
+ description := "UiApp"
+ optionalData := []byte{}
+
+ var buffer bytes.Buffer
+ if err := binary.Write(&buffer, binary.LittleEndian, attributes); err != nil {
+ return nil, err
+ }
+ if err := binary.Write(&buffer, binary.LittleEndian, uint16(len(filePathList))); err != nil {
+ return nil, err
+ }
+ unicodeDescription := utf16.Encode([]rune(description))
+ for _, codepoint := range unicodeDescription {
+ if err := binary.Write(&buffer, binary.LittleEndian, codepoint); err != nil {
+ return nil, err
+ }
+ }
+ // null terminator
+ if err := binary.Write(&buffer, binary.LittleEndian, uint16(0)); err != nil {
+ return nil, err
+ }
+ if _, err := buffer.Write(filePathList); err != nil {
+ return nil, err
+ }
+ if _, err := buffer.Write(optionalData); err != nil {
+ return nil, err
+ }
+ return buffer.Bytes(), nil
+}
+
 // CalcRtmr0 calculates RTMR[0] for the given firmware.
 func CalcRtmr0(firmware []byte) ([48]byte, error) {
  var rtmr Rtmr
@@ -116,14 +231,11 @@ func CalcRtmr0(firmware []byte) ([48]byte, error) {
  }
 
  rtmr.extendVariableValue([]byte{0, 0}) // BootOrder
- // This is an EFI path.
- // TODO(freax13): Don't just hard-code the whole thing.
- boot0000 := "090100002c0055006900410070007000000004071400c9bdb87cebf8344faaea3ee4af6516a10406140021aa2c4614760345836e8ab6f46623317fff0400"
- boot, err := hex.DecodeString(boot0000)
+ boot0000, err := buildEfiLoadOption()
  if err != nil {
- panic(err)
+ return [48]byte{}, err
  }
- rtmr.extendVariableValue(boot)
+ rtmr.extendVariableValue(boot0000)
 
  rtmr.extendSeparator()