attestation: add option for MAA fallback to verify azure's snp-sev id key digest (#1257)

* Convert enforceIDKeyDigest setting to enum

* Use MAA fallback in Azure SNP attestation

* Only create MAA provider if MAA fallback is enabled

---------

Signed-off-by: Daniel Weiße <dw@edgeless.systems>
Co-authored-by: Thomas Tendyck <tt@edgeless.systems>

internal/attestation/idkeydigest/BUILD.bazel

@@ -3,7 +3,10 @@ load("//bazel/go:go_test.bzl", "go_test")
 
 go_library(
     name = "idkeydigest",
-    srcs = ["idkeydigest.go"],
+    srcs = [
+        "enforceidkeydigest_string.go",
+        "idkeydigest.go",
+    ],
     importpath = "github.com/edgelesssys/constellation/v2/internal/attestation/idkeydigest",
     visibility = ["//:__subpackages__"],
     deps = ["//internal/cloud/cloudprovider"],

internal/attestation/idkeydigest/enforceidkeydigest_string.go

@@ -0,0 +1,26 @@
+// Code generated by "stringer -type=EnforceIDKeyDigest"; DO NOT EDIT.
+
+package idkeydigest
+
+import "strconv"
+
+func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[Unknown-0]
+	_ = x[StrictChecking-1]
+	_ = x[MAAFallback-2]
+	_ = x[WarnOnly-3]
+}
+
+const _EnforceIDKeyDigest_name = "UnknownStrictCheckingMAAFallbackWarnOnly"
+
+var _EnforceIDKeyDigest_index = [...]uint8{0, 7, 21, 32, 40}
+
+func (i EnforceIDKeyDigest) String() string {
+	if i >= EnforceIDKeyDigest(len(_EnforceIDKeyDigest_index)-1) {
+		return "EnforceIDKeyDigest(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _EnforceIDKeyDigest_name[_EnforceIDKeyDigest_index[i]:_EnforceIDKeyDigest_index[i+1]]
+}

internal/attestation/idkeydigest/idkeydigest.go

@@ -4,6 +4,8 @@ Copyright (c) Edgeless Systems GmbH
 SPDX-License-Identifier: AGPL-3.0-only
 */
 
+// Package idkeydigest contains policies and type definitions
+// for checking the ID Key Digest value in SEV-SNP attestation.
 package idkeydigest
 
 import (
@@ -11,10 +13,105 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"strings"
 
 	"github.com/edgelesssys/constellation/v2/internal/cloud/cloudprovider"
 )
 
+// Config contains the configuration for ID Key Digest validation.
+type Config struct {
+	IDKeyDigests      IDKeyDigests       `json:"idKeyDigests"`
+	EnforcementPolicy EnforceIDKeyDigest `json:"enforcementPolicy"`
+	MAAURL            string             `json:"maaURL,omitempty"`
+}
+
+//go:generate stringer -type=EnforceIDKeyDigest
+
+// EnforceIDKeyDigest defines the behavior of the validator when the ID key digest is not found in the expected list.
+type EnforceIDKeyDigest uint32
+
+// TODO: Decide on final value naming.
+const (
+	// Unknown is reserved for invalid configurations.
+	Unknown EnforceIDKeyDigest = iota
+	// StrictChecking will return an error if the ID key digest is not found in the expected list.
+	StrictChecking
+	// MAAFallback attempts to verify the attestation using Microsoft Azure Attestation (MAA),
+	// if the ID key digest is not found in the expected list.
+	MAAFallback
+	// WarnOnly logs a warning if the ID key digest is not found in the expected list.
+	// No error is returned.
+	WarnOnly
+)
+
+// UnmarshalJSON implements the json.Unmarshaler interface.
+func (e *EnforceIDKeyDigest) UnmarshalJSON(b []byte) error {
+	return e.unmarshal(func(val any) error {
+		return json.Unmarshal(b, val)
+	})
+}
+
+// MarshalJSON implements the json.Marshaler interface.
+func (e EnforceIDKeyDigest) MarshalJSON() ([]byte, error) {
+	return json.Marshal(e.String())
+}
+
+// UnmarshalYAML implements the yaml.Unmarshaler interface.
+func (e *EnforceIDKeyDigest) UnmarshalYAML(unmarshal func(any) error) error {
+	return e.unmarshal(unmarshal)
+}
+
+// MarshalYAML implements the yaml.Marshaler interface.
+func (e EnforceIDKeyDigest) MarshalYAML() (any, error) {
+	return e.String(), nil
+}
+
+func (e *EnforceIDKeyDigest) unmarshal(unmarshalFunc func(any) error) error {
+	// Check for legacy format: EnforceIDKeyDigest might be a boolean.
+	// If set to true, the value will be set to StrictChecking.
+	// If set to false, the value will be set to WarnOnly.
+	var legacyEnforce bool
+	legacyErr := unmarshalFunc(&legacyEnforce)
+	if legacyErr == nil {
+		if legacyEnforce {
+			*e = StrictChecking
+		} else {
+			*e = WarnOnly
+		}
+		return nil
+	}
+
+	var enforce string
+	if err := unmarshalFunc(&enforce); err != nil {
+		return errors.Join(
+			err,
+			fmt.Errorf("trying legacy format: %w", legacyErr),
+		)
+	}
+
+	*e = EnforcePolicyFromString(enforce)
+	if *e == Unknown {
+		return fmt.Errorf("unknown EnforceIDKeyDigest value: %q", enforce)
+	}
+
+	return nil
+}
+
+// EnforcePolicyFromString returns EnforceIDKeyDigest from string.
+func EnforcePolicyFromString(s string) EnforceIDKeyDigest {
+	s = strings.ToLower(s)
+	switch s {
+	case "strictchecking":
+		return StrictChecking
+	case "maafallback":
+		return MAAFallback
+	case "warnonly":
+		return WarnOnly
+	default:
+		return Unknown
+	}
+}
+
 // IDKeyDigests is a list of trusted digest values for the ID key.
 type IDKeyDigests [][]byte
 

internal/attestation/idkeydigest/idkeydigest_test.go

@@ -117,3 +117,126 @@ func TestUnmarshal(t *testing.T) {
 		})
 	}
 }
+
+func TestEnforceIDKeyDigestMarshal(t *testing.T) {
+	testCases := map[string]struct {
+		input    EnforceIDKeyDigest
+		wantJSON string
+		wantYAML string
+	}{
+		"strict": {
+			input:    StrictChecking,
+			wantJSON: `"StrictChecking"`,
+			wantYAML: "StrictChecking",
+		},
+		"maaFallback": {
+			input:    MAAFallback,
+			wantJSON: `"MAAFallback"`,
+			wantYAML: "MAAFallback",
+		},
+		"warnOnly": {
+			input:    WarnOnly,
+			wantJSON: `"WarnOnly"`,
+			wantYAML: "WarnOnly",
+		},
+	}
+
+	for name, tc := range testCases {
+		t.Run(name, func(t *testing.T) {
+			assert := assert.New(t)
+			require := require.New(t)
+
+			{
+				// YAML
+				yaml, err := yaml.Marshal(tc.input)
+				require.NoError(err)
+				assert.YAMLEq(tc.wantYAML, string(yaml))
+			}
+
+			{
+				// JSON
+				json, err := json.Marshal(tc.input)
+				require.NoError(err)
+				assert.JSONEq(tc.wantJSON, string(json))
+			}
+		})
+	}
+}
+
+func TestEnforceIDKeyDigestUnmarshal(t *testing.T) {
+	testCases := map[string]struct {
+		inputJSON string
+		inputYAML string
+		want      EnforceIDKeyDigest
+		wantErr   bool
+	}{
+		"strict": {
+			inputJSON: `"StrictChecking"`,
+			inputYAML: "StrictChecking",
+			want:      StrictChecking,
+		},
+		"maaFallback": {
+			inputJSON: `"MAAFallback"`,
+			inputYAML: "MAAFallback",
+			want:      MAAFallback,
+		},
+		"warnOnly": {
+			inputJSON: `"WarnOnly"`,
+			inputYAML: "WarnOnly",
+			want:      WarnOnly,
+		},
+		"legacyTrue": {
+			inputJSON: `true`,
+			inputYAML: "true",
+			want:      StrictChecking,
+		},
+		"legacyFalse": {
+			inputJSON: `false`,
+			inputYAML: "false",
+			want:      WarnOnly,
+		},
+		"invalid": {
+			inputJSON: `"invalid"`,
+			inputYAML: "invalid",
+			wantErr:   true,
+		},
+		"invalidType": {
+			inputJSON: `{"object": "invalid"}`,
+			inputYAML: "object: invalid",
+			wantErr:   true,
+		},
+	}
+
+	for name, tc := range testCases {
+		t.Run(name, func(t *testing.T) {
+			assert := assert.New(t)
+			require := require.New(t)
+
+			{
+				// YAML
+				var got EnforceIDKeyDigest
+				err := yaml.Unmarshal([]byte(tc.inputYAML), &got)
+				if tc.wantErr {
+					assert.Error(err)
+					return
+				}
+
+				require.NoError(err)
+				assert.Equal(tc.want, got)
+			}
+
+			{
+				// JSON
+				var got EnforceIDKeyDigest
+				err := json.Unmarshal([]byte(tc.inputJSON), &got)
+				if tc.wantErr {
+					assert.Error(err)
+					return
+				}
+
+				require.NoError(err)
+				assert.Equal(tc.want, got)
+			}
+		})
+	}
+}