sdk: Fix decision log masking of input object

The sdk package allows you to pass arbitrary Go objects as the input
document. This is fine for evaluation (as long as the object can be
parsed to an AST), but is problematic for decision log masking, as the
masking logic expects the input on the event to be either a
map[string]interface{} or a []interface{}, and for inner types of the
object to also be similarly generic. Currently, the decision log masking
silently fails if the input object is not of the correct type.

This patch addresses this by rebuilding the input Go type for the
decision log from the parsed AST of the original input object. This
generates a Go type that does not break the masking logic, provided the
input is of a type that can be masked. The conversion from the AST only
happens if the decision logs plugin is actually registered with the
manager to avoid wasting cycles if decision logs are not enabled.

A test is added to cover the masking case for the SDK.

Signed-off-by: Erik Paulson <epaulson10@gmail.com>

sdk/opa.go

@@ -347,6 +347,17 @@ func (opa *OPA) executeTransaction(ctx context.Context, record *server.Info, wor
 	record.Metrics.Timer(metrics.SDKDecisionEval).Stop()
 
 	if logger := logs.Lookup(s.manager); logger != nil {
+		// Decision log masking requires the event object to be a map[string]interface{},
+		// or a []interface{}, and all internal objects referenced in the mask to be
+		// similarly generic. Convert the input AST back into a JSON-representation to
+		// ensure decision logging will work if the input Go type does not fit these requirements.
+		if record.InputAST != nil {
+			asJSON, err := ast.JSON(record.InputAST)
+			if err != nil {
+				return nil, err
+			}
+			*record.Input = asJSON
+		}
 		if err := logger.Log(ctx, record); err != nil {
 			return result, fmt.Errorf("decision log: %w", err)
 		}

sdk/opa_test.go

@@ -1341,6 +1341,113 @@ main = time.now_ns()
 
 }
 
+func TestDecisionLoggingWithMasking(t *testing.T) {
+
+	ctx := context.Background()
+
+	server := sdktest.MustNewServer(
+		sdktest.MockBundle("/bundles/bundle.tar.gz", map[string]string{
+			"main.rego": `
+package system
+
+main = true
+
+str = "foo"
+
+loopback = input
+`,
+			"log.rego": `
+package system.log
+
+mask["/input/secret"]
+mask["/input/top/secret"]
+mask["/input/dossier/1/highly"]
+`,
+		}),
+	)
+
+	defer server.Stop()
+
+	config := fmt.Sprintf(`{
+		"services": {
+			"test": {
+				"url": %q
+			}
+		},
+		"bundles": {
+			"test": {
+				"resource": "/bundles/bundle.tar.gz"
+			}
+		},
+		"decision_logs": {
+			"console": true
+		}
+	}`, server.URL())
+
+	testLogger := loggingtest.New()
+	opa, err := sdk.New(ctx, sdk.Options{
+		Config:        strings.NewReader(config),
+		ConsoleLogger: testLogger,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	defer opa.Stop(ctx)
+
+	if _, err := opa.Decision(ctx, sdk.DecisionOptions{
+		Input: map[string]interface{}{
+			"secret": "foo",
+			"top": map[string]string{
+				"secret": "bar",
+			},
+			"dossier": []map[string]interface{}{
+				{
+					"very": "private",
+				},
+				{
+					"highly": "classified",
+				},
+			},
+		},
+	}); err != nil {
+		t.Fatal(err)
+	}
+
+	entries := testLogger.Entries()
+
+	if len(entries) != 1 {
+		t.Fatalf("expected 1 entry but got %d", len(entries))
+	}
+
+	expectedErased := []interface{}{
+		"/input/dossier/1/highly",
+		"/input/secret",
+		"/input/top/secret",
+	}
+	erased := entries[0].Fields["erased"].([]interface{})
+	stringLess := func(a, b string) bool {
+		return a < b
+	}
+	if !cmp.Equal(expectedErased, erased, cmpopts.SortSlices(stringLess)) {
+		t.Errorf("Did not get expected result for erased field in decision log:\n%s", cmp.Diff(expectedErased, erased, cmpopts.SortSlices(stringLess)))
+	}
+	errMsg := `Expected masked field "%s" to be removed, but it was present.`
+	input := entries[0].Fields["input"].(map[string]interface{})
+	if _, ok := input["secret"]; ok {
+		t.Errorf(errMsg, "/input/secret")
+	}
+
+	if _, ok := input["top"].(map[string]interface{})["secret"]; ok {
+		t.Errorf(errMsg, "/input/top/secret")
+	}
+
+	if _, ok := input["dossier"].([]interface{})[1].(map[string]interface{})["highly"]; ok {
+		t.Errorf(errMsg, "/input/dossier/1/highly")
+	}
+
+}
+
 func TestDecisionLoggingWithNDBCache(t *testing.T) {
 
 	ctx := context.Background()