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https://github.com/edgelesssys/constellation.git
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e350ca0f57
* Implement Azure TDX attestation primitives * Add default measurements and claims for Azure TDX * Enable Constellation on Azure TDX --------- Signed-off-by: Daniel Weiße <dw@edgeless.systems>
117 lines
3.5 KiB
Go
117 lines
3.5 KiB
Go
/*
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Copyright (c) Edgeless Systems GmbH
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SPDX-License-Identifier: AGPL-3.0-only
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*/
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/*
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# Azure attestation
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Constellation supports multiple attestation technologies on Azure.
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- SEV - Secure Nested Paging (SEV-SNP)
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TPM attestation verified using an SEV-SNP attestation statement.
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- TDX - Trust Domain Extensions
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TPM attestation verified using a TDX attestation statement.
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- Trusted Launch
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Basic TPM attestation.
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*/
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package azure
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import (
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"bytes"
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"crypto/sha256"
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"encoding/base64"
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"encoding/binary"
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"encoding/json"
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"errors"
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"fmt"
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"io"
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tpmclient "github.com/google/go-tpm-tools/client"
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"github.com/google/go-tpm/legacy/tpm2"
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)
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const (
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// tpmAkIdx is the NV index of the attestation key used by Azure VMs.
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tpmAkIdx = 0x81000003
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)
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// GetAttestationKey reads the attestation key put into the TPM during early boot.
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func GetAttestationKey(tpm io.ReadWriter) (*tpmclient.Key, error) {
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ak, err := tpmclient.LoadCachedKey(tpm, tpmAkIdx, tpmclient.NullSession{})
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if err != nil {
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return nil, fmt.Errorf("reading HCL attestation key from TPM: %w", err)
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}
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return ak, nil
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}
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// HCLAkValidator validates an attestation key issued by the Host Compatibility Layer (HCL).
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// The HCL is written by Azure, and sits between the Hypervisor and CVM OS.
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// The HCL runs in the protected context of the CVM.
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type HCLAkValidator struct{}
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// Validate validates that the attestation key from the TPM is trustworthy. The steps are:
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// 1. runtime data read from the TPM has the same sha256 digest as reported in `report_data` of the SNP report or `TdQuoteBody.ReportData` of the TDX report.
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// 2. modulus reported in runtime data matches modulus from key at idx 0x81000003.
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// 3. exponent reported in runtime data matches exponent from key at idx 0x81000003.
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// The function is currently tested manually on a Azure Ubuntu CVM.
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func (a *HCLAkValidator) Validate(runtimeDataRaw []byte, reportData []byte, rsaParameters *tpm2.RSAParams) error {
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var rtData runtimeData
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if err := json.Unmarshal(runtimeDataRaw, &rtData); err != nil {
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return fmt.Errorf("unmarshalling json: %w", err)
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}
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sum := sha256.Sum256(runtimeDataRaw)
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if len(reportData) < len(sum) {
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return fmt.Errorf("reportData has unexpected size: %d", len(reportData))
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}
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if !bytes.Equal(sum[:], reportData[:len(sum)]) {
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return errors.New("unexpected runtimeData digest in TPM")
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}
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if len(rtData.PublicPart) < 1 {
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return errors.New("did not receive any keys in runtime data")
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}
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rawN, err := base64.RawURLEncoding.DecodeString(rtData.PublicPart[0].N)
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if err != nil {
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return fmt.Errorf("decoding modulus string: %w", err)
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}
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if !bytes.Equal(rawN, rsaParameters.ModulusRaw) {
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return fmt.Errorf("unexpected modulus value in TPM")
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}
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rawE, err := base64.RawURLEncoding.DecodeString(rtData.PublicPart[0].E)
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if err != nil {
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return fmt.Errorf("decoding exponent string: %w", err)
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}
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paddedRawE := make([]byte, 4)
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copy(paddedRawE, rawE)
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exponent := binary.LittleEndian.Uint32(paddedRawE)
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// According to this comment [1] the TPM uses "0" to represent the default exponent "65537".
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// The go tpm library also reports the exponent as 0. Thus we have to handle it specially.
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// [1] https://github.com/tpm2-software/tpm2-tools/pull/1973#issue-596685005
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if !((exponent == 65537 && rsaParameters.ExponentRaw == 0) || exponent == rsaParameters.ExponentRaw) {
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return fmt.Errorf("unexpected N value in TPM")
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}
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return nil
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}
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type runtimeData struct {
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PublicPart []akPub `json:"keys"`
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}
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// akPub are the public parameters of an RSA attestation key.
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type akPub struct {
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E string `json:"e"`
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N string `json:"n"`
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}
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