Phase 2: fallback_detector module for tier selection

src/Cargo.toml

@@ -61,6 +61,7 @@ windows = { version = "0.62", features = [
     "Win32_System_SystemInformation",
     "Win32_System_Time",
     "Win32_System_SystemServices",
+    "Win32_System_SystemInformation",
     "Win32_System_JobObjects",
 ] }
 windows-core = "0.62"

src/wxc/src/main.rs

@@ -151,8 +151,23 @@ fn print_error_envelope(error: &MxcError) {
 }
 
 fn delete_app_container_profile(name: &str, logger: &mut Logger) -> bool {
-    // Clear BFS policy first
-    let mut bfs = FileSystemBfsManager::new(name.to_string());
+    // Clear BFS policy first. We need an absolute path to `bfscfg.exe`
+    // here for the same security reason as the runner — pass an
+    // authoritative path to `CreateProcessW` rather than a bare name.
+    // If resolution fails (rare; only on hosts where
+    // `GetWindowsDirectoryW` itself returns 0), we log and skip the BFS
+    // clearing step; deleting the AppContainer profile below is still
+    // worth attempting.
+    let bfscfg_path = match wxc_common::fallback_detector::find_bfscfg_exe() {
+        Ok(p) => p,
+        Err(e) => {
+            logger.log_line(&format!(
+                "Skipping BFS policy clear: could not resolve bfscfg.exe ({e})"
+            ));
+            None
+        }
+    };
+    let mut bfs = FileSystemBfsManager::new(name.to_string(), bfscfg_path);
     bfs.remove_configuration(logger);
 
     // Delete the AppContainer profile

src/wxc_common/src/appcontainer_runner.rs

@@ -599,7 +599,20 @@ impl ScriptRunner for AppContainerScriptRunner {
         let principal_id = self.get_principal_id();
         logger.log_line(&format!("AppContainerSID: {principal_id}"));
 
-        let mut bfs_manager = FileSystemBfsManager::new(self.app_container_name.clone());
+        // Resolve `bfscfg.exe` by absolute path so probe and execution
+        // agree on the binary — defeats executable-search-order
+        // hijacking (see `fallback_detector::find_bfscfg_exe`). On
+        // hosts where SystemRoot itself cannot be resolved (a
+        // pathological state on any healthy Windows install) we surface
+        // the resolution error rather than silently demoting to a
+        // weaker isolation tier.
+        let bfscfg_path = match crate::fallback_detector::find_bfscfg_exe() {
+            Ok(p) => p,
+            Err(e) => return ScriptResponse::error(&e.to_string()),
+        };
+
+        let mut bfs_manager =
+            FileSystemBfsManager::new(self.app_container_name.clone(), bfscfg_path);
         if let Err(e) = bfs_manager.configure(&request.policy, logger) {
             return ScriptResponse::error(&e.to_string());
         }