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Add support for validating VLEK certificates #67

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merged 1 commit into from
Sep 11, 2023
Merged

Add support for validating VLEK certificates #67

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129 changes: 112 additions & 17 deletions abi/abi.go
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Original file line number Diff line number Diff line change
Expand Up @@ -102,10 +102,13 @@ const (
VcekGUID = "63da758d-e664-4564-adc5-f4b93be8accd"
// VlekGUID is the Versioned Loaded Endorsement Key GUID
VlekGUID = "a8074bc2-a25a-483e-aae6-39c045a0b8a1"
// AskGUID is the AMD signing Key GUID
// AskGUID is the AMD signing Key GUID. Used for the ASVK as well.
AskGUID = "4ab7b379-bbac-4fe4-a02f-05aef327c782"
// ArkGUID is the AMD Root Key GUID
ArkGUID = "c0b406a4-a803-4952-9743-3fb6014cd0ae"
// AsvkGUID may not be defined, but we'd like it to be, so that
// a single machine can use both VCEK and VLEK report signing.
AsvkGUID = "00000000-0000-0000-0000-000000000000"

// ExpectedReportVersion is set by the SNP API specification
// https://www.amd.com/system/files/TechDocs/56860.pdf
Expand Down Expand Up @@ -184,9 +187,8 @@ func ParseSnpPolicy(guestPolicy uint64) (SnpPolicy, error) {
if guestPolicy&uint64(1<<policyReserved1bit) == 0 {
return result, fmt.Errorf("policy[%d] is reserved, must be 1, got 0", policyReserved1bit)
}
validMask := uint64((1 << 21) - 1)
if guestPolicy&^validMask != 0 {
return result, fmt.Errorf("policy[63:21] are reserved mbz, got 0x%x", guestPolicy)
if err := mbz64(guestPolicy, "policy", 63, 21); err != nil {
return result, err
}
result.ABIMinor = uint8(guestPolicy & 0xff)
result.ABIMajor = uint8((guestPolicy >> 8) & 0xff)
Expand Down Expand Up @@ -293,6 +295,14 @@ func mbz(data []uint8, lo, hi int) error {
return nil
}

// Checks a must-be-zero range of a uint64 between bits hi down to lo inclusive.
func mbz64(data uint64, base string, hi, lo int) error {
if (data>>lo)&((1<<(hi-lo+1))-1) != 0 {
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Can you add some comments here? It's not quite clear what being checked here.

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Done.

return fmt.Errorf("mbz range %s[0x%x:0x%x] not all zero: %x", base, lo, hi, data)
}
return nil
}

// ReportToSignatureDER returns the signature component of an attestation report in DER format for
// use in x509 verification.
func ReportToSignatureDER(report []byte) ([]byte, error) {
Expand Down Expand Up @@ -335,6 +345,86 @@ func SignatureAlgo(report []byte) uint32 {
return binary.LittleEndian.Uint32(signatureAlgoSlice(report))
}

// ReportSigner represents which kind of key is expected to have signed the attestation report
type ReportSigner uint8

const (
// VcekReportSigner is the SIGNING_KEY value for if the VCEK signed the attestation report.
VcekReportSigner ReportSigner = iota
// VlekReportSigner is the SIGNING_KEY value for if the VLEK signed the attestation report.
VlekReportSigner
endorseReserved2
endorseReserved3
endorseReserved4
endorseReserved5
endorseReserved6
// NoneReportSigner is the SIGNING_KEY value for if the attestation report is not signed.
NoneReportSigner
)

// SignerInfo represents information about the signing circumstances for the attestation report.
type SignerInfo struct {
// SigningKey represents kind of key by which a report was signed.
SigningKey ReportSigner
// MaskChipKey is true if the host chose to enable CHIP_ID masking, to cause the report's CHIP_ID
// to be all zeros.
MaskChipKey bool
// AuthorKeyEn is true if the VM is launched with an IDBLOCK that includes an author key.
AuthorKeyEn bool
}

// String returns a ReportSigner string rendering.
func (k ReportSigner) String() string {
switch k {
case VcekReportSigner:
return "VCEK"
case VlekReportSigner:
return "VLEK"
case NoneReportSigner:
return "None"
default:
return fmt.Sprintf("UNKNOWN(%d)", byte(k))
}
}

// ParseSignerInfo interprets report[0x48:0x4c] into its component pieces and errors
// on non-zero mbz fields.
func ParseSignerInfo(signerInfo uint32) (result SignerInfo, err error) {
info64 := uint64(signerInfo)
if err = mbz64(info64, "data[0x48:0x4C]", 31, 5); err != nil {
return result, err
}
result.SigningKey = ReportSigner((signerInfo >> 2) & 7)
if result.SigningKey > VlekReportSigner && result.SigningKey < NoneReportSigner {
return result, fmt.Errorf("signing_key values 2-6 are reserved. Got %v", result.SigningKey)
}
result.MaskChipKey = (signerInfo & 2) != 0
result.AuthorKeyEn = (signerInfo & 1) != 0
return result, nil
}

// ComposeSignerInfo returns the uint32 value expected to populate the attestation report byte range
// 0x48:0x4C.
func ComposeSignerInfo(signerInfo SignerInfo) uint32 {
var result uint32
if signerInfo.AuthorKeyEn {
result |= 1
}
if signerInfo.MaskChipKey {
result |= 2
}
result |= uint32(signerInfo.SigningKey) << 2
return result
}

// ReportSignerInfo returns the signer info component of a SEV-SNP raw report.
func ReportSignerInfo(data []byte) (uint32, error) {
if len(data) < 0x4C {
return 0, fmt.Errorf("report too small: %d", len(data))
}
return binary.LittleEndian.Uint32(data[0x48:0x4C]), nil
}

// ReportToProto creates a pb.Report from the little-endian AMD SEV-SNP attestation report byte
// array in SEV SNP ABI format for ATTESTATION_REPORT.
func ReportToProto(data []uint8) (*pb.Report, error) {
Expand All @@ -357,11 +447,11 @@ func ReportToProto(data []uint8) (*pb.Report, error) {
r.CurrentTcb = binary.LittleEndian.Uint64(data[0x38:0x40])
r.PlatformInfo = binary.LittleEndian.Uint64(data[0x40:0x48])

reservedAuthor := binary.LittleEndian.Uint32(data[0x48:0x4C])
if reservedAuthor&0xffffffe0 != 0 {
return nil, fmt.Errorf("mbz bits at offset 0x48 not zero: 0x%08x", reservedAuthor&0xffffffe0)
signerInfo, err := ParseSignerInfo(binary.LittleEndian.Uint32(data[0x48:0x4C]))
if err != nil {
return nil, err
}
r.AuthorKeyEn = reservedAuthor
r.SignerInfo = ComposeSignerInfo(signerInfo)
if err := mbz(data, 0x4C, 0x50); err != nil {
return nil, err
}
Expand Down Expand Up @@ -480,11 +570,10 @@ func ReportToAbiBytes(r *pb.Report) ([]byte, error) {
binary.LittleEndian.PutUint64(data[0x38:0x40], r.CurrentTcb)
binary.LittleEndian.PutUint64(data[0x40:0x48], r.PlatformInfo)

var reservedAuthor uint32
if r.AuthorKeyEn == 1 {
reservedAuthor |= 0x01
if _, err := ParseSignerInfo(r.SignerInfo); err != nil {
return nil, err
}
binary.LittleEndian.PutUint32(data[0x48:0x4C], reservedAuthor)
binary.LittleEndian.PutUint32(data[0x48:0x4C], r.SignerInfo)
copy(data[0x50:0x90], r.ReportData[:])
copy(data[0x90:0xC0], r.Measurement[:])
copy(data[0xC0:0xE0], r.HostData[:])
Expand Down Expand Up @@ -680,12 +769,17 @@ func (c *CertTable) GetByGUIDString(guid string) ([]byte, error) {
// so missing certificates aren't an error. If certificates are missing, you can
// choose to fetch them yourself by calling verify.GetAttestationFromReport.
func (c *CertTable) Proto() *pb.CertificateChain {
var vcek, ask, ark []byte
var err error
vcek, err = c.GetByGUIDString(VcekGUID)
if err != nil {
logger.Warningf("Warning: VCEK certificate not found in data pages: %v", err)
var vcek, vlek, ask, ark []byte
var err, cerr, lerr error
// Whereas a host is permitted to populate its certificate chain blob with both a VCEK and VLEK
// certificate, doing so is unusual since the choice of VCEK vs VLEK is an infrastructural choice.
// To keep the implementation clean, we don't pun vcek and vlek in the same field.
vcek, cerr = c.GetByGUIDString(VcekGUID)
vlek, lerr = c.GetByGUIDString(VlekGUID)
if cerr != nil && lerr != nil {
logger.Warning("Warning: Neither VCEK nor VLEK certificate found in data pages")
}

ask, err = c.GetByGUIDString(AskGUID)
if err != nil {
logger.Warningf("ASK certificate not found in data pages: %v", err)
Expand All @@ -697,6 +791,7 @@ func (c *CertTable) Proto() *pb.CertificateChain {
firmware, _ := c.GetByGUIDString(gce.FirmwareCertGUID)
return &pb.CertificateChain{
VcekCert: vcek,
VlekCert: vlek,
AskCert: ask,
ArkCert: ark,
FirmwareCert: firmware,
Expand Down
42 changes: 40 additions & 2 deletions abi/abi_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -40,6 +40,44 @@ var emptyReport = `
signature: '\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
`

func TestMbz64(t *testing.T) {
tests := []struct {
data uint64
lo int
hi int
wantErr string
}{
{
data: uint64(0),
lo: 0,
hi: 63,
},
{
data: ^uint64(0) &^ (uint64(1<<31) | uint64(1<<32) | uint64(1<<33)),
lo: 31,
hi: 33,
},
{
data: ^uint64(0) &^ (uint64(1<<0x1f) | uint64(1<<0x20)),
lo: 0x1f,
hi: 0x21,
wantErr: "mbz range test[0x1f:0x21] not all zero",
},
{
data: ^uint64(0) &^ (uint64(1<<0x20) | uint64(1<<0x21)),
lo: 0x1f,
hi: 0x21,
wantErr: "mbz range test[0x1f:0x21] not all zero",
},
}
for _, tc := range tests {
err := mbz64(tc.data, "test", tc.hi, tc.lo)
if (tc.wantErr == "" && err != nil) || (tc.wantErr != "" && (err == nil || !strings.Contains(err.Error(), tc.wantErr))) {
t.Errorf("mbz64(0x%x, %d, %d) = %v, want %q", tc.data, tc.hi, tc.lo, err, tc.wantErr)
}
}
}

func TestReportMbz(t *testing.T) {
tests := []struct {
name string
Expand All @@ -50,7 +88,7 @@ func TestReportMbz(t *testing.T) {
{
name: "AuthorKeyEn reserved",
changeIndex: 0x49,
wantErr: "mbz bits at offset 0x48 not zero: 0x0000cc00",
wantErr: "mbz range data[0x48:0x4C][0x5:0x1f] not all zero: cc00",
},
{
name: "pre-report data",
Expand Down Expand Up @@ -92,7 +130,7 @@ func TestReportMbz(t *testing.T) {
name: "Guest policy bit 21",
changeIndex: policyOffset + 2, // Bits 16-23
changeValue: 0x22, // Set bits 17, 21
wantErr: "policy[63:21] are reserved mbz, got 0x220000",
wantErr: "malformed guest policy: mbz range policy[0x15:0x3f] not all zero: 220000",
},
}
reportProto := &spb.Report{}
Expand Down
8 changes: 7 additions & 1 deletion client/client_test.go
View file
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Original file line number Diff line number Diff line change
Expand Up @@ -177,7 +177,13 @@ func TestOpenGetRawExtendedReportClose(t *testing.T) {
}
if UseDefaultSevGuest() {
tcdev := device.(*test.Device)
if err := tcdev.Signer.Vcek.CheckSignature(x509.ECDSAWithSHA384, got, der); err != nil {
infoRaw, _ := abi.ReportSignerInfo(raw)
info, _ := abi.ParseSignerInfo(infoRaw)
reportSigner := tcdev.Signer.Vcek
if info.SigningKey == abi.VlekReportSigner {
reportSigner = tcdev.Signer.Vlek
}
if err := reportSigner.CheckSignature(x509.ECDSAWithSHA384, got, der); err != nil {
t.Errorf("signature with test keys did not verify: %v", err)
}
}
Expand Down
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