v0.328.0 (unreleased): security review round 3 — under HDD_PATH only appdata/ is a data root (not backups/)

Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

CHANGELOG.md

@@ -48,6 +48,11 @@ still refuses and the app page still says so.
   options (or a non-root one in its target) is refused. The catalog reader refuses YAML anchors, aliases and merge keys
   (the box's YAML library expands them, the reader would judge a different document). Red-proved
   (`P2/redproof-security-review-round2.txt`).
+- **Security review round 3:** under `${HDD_PATH}` only `appdata/` is a data root — the drive namespace's `backups/`
+  (recovery units with the apps' portable secrets) and its other top-level folders are siblings no app may bind; the
+  household's folders come through `${USERDATA_PATH}`. Every catalog bind already uses `${HDD_PATH}/appdata/…`.
+  Known limit, stated: one app can still name ANOTHER app's `appdata/<name>` (paperless-ngx's folder is `paperless`,
+  so a per-app rule needs a name map) — filed. Red-proved (`P2/redproof-security-review-round3.txt`).
 
 ## v0.327.1 — files.<domain> reaches FileBrowser again: a file route by name, not Docker labels (2026-10-17)
 

controller/internal/privpolicy/privpolicy.go

@@ -512,6 +512,12 @@ func (c *checker) validRoots() []string {
 			continue
 		}
 		if strings.HasPrefix(v, DriveNamespace) || (c.p.SystemDataPath != "" && within(v, filepath.Clean(c.p.SystemDataPath))) {
+			if r == "HDD_PATH" {
+				// round 3: HDD_PATH is the drive's whole namespace — its backups/ (recovery units carry app secrets)
+				// and every other top-level folder are siblings an app has no business binding. Only appdata/ is a
+				// root; the household's folders come through USERDATA_PATH.
+				v = filepath.Join(v, "appdata")
+			}
 			out = append(out, v)
 		}
 	}

controller/internal/privpolicy/privpolicy_test.go

@@ -406,3 +406,14 @@ func TestAdmit_ANotYetCreatedPathBehindASymlink(t *testing.T) {
 		t.Fatal("a resolve error other than not-exist was admitted (fail open)")
 	}
 }
+
+// Round 3: ${HDD_PATH} itself and its siblings of appdata/ (backups/ — the recovery units with app secrets) are no
+// root; ${HDD_PATH}/appdata/... is.
+func TestAdmit_HDDPathRootIsAppdataOnly(t *testing.T) {
+	for _, line := range []string{"      - ${HDD_PATH}:/drive\n", "      - ${HDD_PATH}/backups:/b\n", "      - ${HDD_PATH}/appdata-x:/b\n"} {
+		compose := strings.Replace(base, "      - app_data:/data\n", "      - app_data:/data\n"+line, 1)
+		if err := admit(t, "app", compose, env()); err == nil {
+			t.Fatalf("%q was admitted", line)
+		}
+	}
+}