fix/batches: prevent executor environment leakage to steps

internal/batches/executor/run_steps.go

@@ -316,8 +316,9 @@ func executeSingleStep(
 	}
 	defer cleanup()
 
-	// Resolve step.Env given the current environment.
-	stepEnv, err := step.Env.Resolve(opts.GlobalEnv)
+	// Resolve step.Env given the current environment. Executor control values
+	// must never be selectable by an author-controlled step.
+	stepEnv, err := step.Env.Resolve(withoutReservedExecutorEnv(opts.GlobalEnv))
 	if err != nil {
 		err = errors.Wrap(err, "resolving step environment")
 		opts.UI.StepPreparingFailed(stepIdx+1, err)
@@ -464,6 +465,17 @@ func executeSingleStep(
 	return stdout, stderr, nil
 }
 
+func withoutReservedExecutorEnv(env []string) []string {
+	filtered := make([]string, 0, len(env))
+	for _, variable := range env {
+		name, _, found := strings.Cut(variable, "=")
+		if !found || !strings.HasPrefix(name, "SRC_EXECUTOR_") {
+			filtered = append(filtered, variable)
+		}
+	}
+	return filtered
+}
+
 func setOutputs(stepOutputs batcheslib.Outputs, global map[string]any, stepCtx *template.StepContext) error {
 	for name, output := range stepOutputs {
 		var value bytes.Buffer

internal/batches/executor/run_steps_test.go

@@ -2,6 +2,7 @@ package executor
 
 import (
 	"context"
+	"encoding/json"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -10,9 +11,40 @@ import (
 	"github.com/stretchr/testify/require"
 
 	batcheslib "github.com/sourcegraph/sourcegraph/lib/batches"
+	batchenv "github.com/sourcegraph/sourcegraph/lib/batches/env"
 	"github.com/sourcegraph/sourcegraph/lib/batches/template"
 )
 
+func TestWithoutReservedExecutorEnv(t *testing.T) {
+	env := []string{
+		"ALLOWED=value",
+		"SRC_EXECUTOR_JOB_TOKEN=secret",
+		"SRC_EXECUTOR_FUTURE_SECRET=secret",
+		"VALUE=contains-SRC_EXECUTOR_JOB_TOKEN",
+		"MALFORMED",
+	}
+
+	require.Equal(t, []string{
+		"ALLOWED=value",
+		"VALUE=contains-SRC_EXECUTOR_JOB_TOKEN",
+		"MALFORMED",
+	}, withoutReservedExecutorEnv(env))
+
+	var stepEnv batchenv.Environment
+	require.NoError(t, json.Unmarshal([]byte(`[
+		"ALLOWED",
+		"SRC_EXECUTOR_JOB_TOKEN",
+		"SRC_EXECUTOR_FUTURE_SECRET"
+	]`), &stepEnv))
+	resolved, err := stepEnv.Resolve(withoutReservedExecutorEnv(env[:4]))
+	require.NoError(t, err)
+	require.Equal(t, map[string]string{
+		"ALLOWED":                    "value",
+		"SRC_EXECUTOR_JOB_TOKEN":     "",
+		"SRC_EXECUTOR_FUTURE_SECRET": "",
+	}, resolved)
+}
+
 func TestParseContainerTempPath(t *testing.T) {
 	for _, valid := range []string{"/tmp/tmp.abc-123_456", "/tmp/tmp.abc-123_456\n"} {
 		t.Run("valid_"+valid, func(t *testing.T) {