fix(R-1187 b): restored links point only downward — no absolute target, no '..', no link on the way

Third automated security review finding, red-proved.

controller/internal/appexport/restore.go

@@ -1203,6 +1203,34 @@ func extractTarGz(tgzPath, destDir string) error {
 	}
 }
 
+// safeDownwardLink is the rule for a restored link: a relative target with no „.." component, and no symbolic link on
+// the way — not among the folders of the link's own path, not among the existing components of its target.
+func safeDownwardLink(rt *os.Root, name, target string) bool {
+	if target == "" || filepath.IsAbs(target) {
+		return false
+	}
+	for _, part := range strings.Split(filepath.ToSlash(target), "/") {
+		if part == ".." {
+			return false
+		}
+	}
+	if parentThroughLink(rt, name) {
+		return false
+	}
+	acc := filepath.Dir(name)
+	for _, part := range strings.Split(filepath.Clean(target), string(os.PathSeparator)) {
+		acc = filepath.Join(acc, part)
+		fi, err := rt.Lstat(acc)
+		if err != nil {
+			break // not there (yet): nothing further down can be a link that exists now
+		}
+		if fi.Mode()&os.ModeSymlink != 0 {
+			return false
+		}
+	}
+	return true
+}
+
 // parentThroughLink is true when any folder on name's path (inside the root) is a symbolic link.
 func parentThroughLink(rt *os.Root, name string) bool {
 	dir := filepath.Dir(name)
@@ -1285,23 +1313,16 @@ func extractTar(tarPath, destDir string) ([]string, error) {
 			out.Close()
 			rt.Chmod(name, os.FileMode(hdr.Mode).Perm())
 		case tar.TypeSymlink:
+			// A restored link may only point DOWNWARD inside the folder, through real folders: no absolute target, no
+			// „..", and no link on the way — neither on the link's own path nor in its target. Such a link can never
+			// resolve outside the app's folder, whatever else the folder holds. Everything else is refused and listed.
+			// (Three rounds of the automated security review: a link written through, a linked parent, a target through
+			// a link. Pinned by TestR1187b_*.)
+			if !safeDownwardLink(rt, name, hdr.Linkname) {
+				refused = append(refused, hdr.Name)
+				continue
+			}
 			dst := hdr.Linkname
-			if filepath.IsAbs(dst) {
-				refused = append(refused, hdr.Name)
-				continue
-			}
-			// The text check below is exact only when no folder on the link's path is itself a link (a „a" → „." makes
-			// „a/b/l" really „b/l"): such an entry is refused (the security review's second case, pinned by
-			// TestR1187b_NoLinkThroughALinkedParent).
-			if parentThroughLink(rt, name) {
-				refused = append(refused, hdr.Name)
-				continue
-			}
-			resolved := filepath.Clean(filepath.Join(filepath.Dir(filepath.Join(root, name)), dst))
-			if resolved != root && !strings.HasPrefix(resolved, root+string(os.PathSeparator)) {
-				refused = append(refused, hdr.Name)
-				continue
-			}
 			if dir := filepath.Dir(name); dir != "." {
 				if err := rt.MkdirAll(dir, 0755); err != nil {
 					return refused, err