fix: eliminate TOCTOU race in template file system by reading directly instead of checking existence (CSM-026)

CODESMELL.md

@@ -403,10 +403,14 @@
 
 ---
 
-### CSM-026 — TOCTOU Race Condition in Template File System
-- **File:** `lib/bds/rendering/file_system.ex:28-37`
-- **What:** `Enum.find(&File.regular?/1)` checks existence, then the file is read later (in the `Liquex.FileSystem` impl, Z. 43-49). Between check and read the file can vanish.
-- **Fix:** Just try to read and handle `{:error, :enoent}`. Remove the `Enum.find` existence check and attempt reads directly.
+### ~~CSM-026 — TOCTOU Race Condition in Template File System~~ ✅ FIXED
+- **Fixed:** 2026-05-11
+- **What was done:**
+  - Extracted `candidate_paths/2` — validates the template path and returns all candidate file paths without checking existence.
+  - Added `try_read/2` — attempts `File.read` on each candidate path sequentially, returning `{:ok, contents}` on first success or `{:error, :enoent}` when all fail. No separate existence check.
+  - Simplified `full_path/2` to delegate to `candidate_paths/2` (returns first candidate for backward compatibility with tests).
+  - Rewrote `Liquex.FileSystem` protocol impl to use `try_read/2` directly, eliminating the TOCTOU window between `File.regular?` and `File.read`.
+  - Added 10 tests in `test/bds/csm026_toctou_file_system_test.exs`: atomic read, missing template, multi-root fallthrough, first-root-wins priority, file-deleted-between-calls safety, protocol read, protocol raise on missing, and path validation (empty, absolute, traversal).
 
 ---
 

lib/bds/rendering/file_system.ex

@@ -13,8 +13,8 @@ defmodule BDS.Rendering.FileSystem do
     new([root_path])
   end
 
-  @spec full_path(t(), String.t()) :: String.t()
-  def full_path(%__MODULE__{root_paths: root_paths}, template_path) do
+  @spec candidate_paths(t(), String.t()) :: [String.t()]
+  def candidate_paths(%__MODULE__{root_paths: root_paths}, template_path) do
     normalized_path = to_string(template_path)
 
     cond do
@@ -29,18 +29,30 @@ defmodule BDS.Rendering.FileSystem do
 
       true ->
         filename = ensure_liquid_ext(normalized_path)
-
-        root_paths
-        |> Enum.map(&Path.expand(Path.join(&1, filename)))
-        |> Enum.find(&File.regular?/1)
-        |> case do
-          nil ->
-            Path.expand(Path.join(List.first(root_paths) || ".", filename))
-
-          path ->
-            path
+        Enum.map(root_paths, &Path.expand(Path.join(&1, filename)))
     end
   end
+
+  @spec full_path(t(), String.t()) :: String.t()
+  def full_path(%__MODULE__{} = fs, template_path) do
+    List.first(candidate_paths(fs, template_path))
+  end
+
+  @spec try_read(t(), String.t()) :: {:ok, String.t()} | {:error, :enoent}
+  def try_read(%__MODULE__{} = fs, template_path) do
+    fs
+    |> candidate_paths(template_path)
+    |> try_read_from_paths()
+  end
+
+  defp try_read_from_paths([]), do: {:error, :enoent}
+
+  defp try_read_from_paths([path | rest]) do
+    case File.read(path) do
+      {:ok, contents} -> {:ok, contents}
+      {:error, :enoent} -> try_read_from_paths(rest)
+      {:error, _reason} -> try_read_from_paths(rest)
+    end
   end
 
   defp ensure_liquid_ext(path) do
@@ -50,12 +62,9 @@ end
 
 defimpl Liquex.FileSystem, for: BDS.Rendering.FileSystem do
   def read_template_file(file_system, template_path) do
-    file_system
-    |> BDS.Rendering.FileSystem.full_path(template_path)
-    |> File.read()
-    |> case do
+    case BDS.Rendering.FileSystem.try_read(file_system, template_path) do
       {:ok, contents} -> contents
-      _error -> raise Liquex.Error, message: "No such template '#{template_path}'"
+      {:error, :enoent} -> raise Liquex.Error, message: "No such template '#{template_path}'"
     end
   end
 end

test/bds/csm026_toctou_file_system_test.exs

@@ -0,0 +1,106 @@
+defmodule BDS.TOCTOU.FileSystemTest do
+  use ExUnit.Case, async: true
+
+  alias BDS.Rendering.FileSystem, as: TemplateFileSystem
+
+  describe "try_read/2 eliminates TOCTOU race" do
+    @tag :tmp_dir
+    test "reads file atomically without separate existence check", %{tmp_dir: tmp_dir} do
+      File.write!(Path.join(tmp_dir, "header.liquid"), "HEADER CONTENT")
+      fs = TemplateFileSystem.new(tmp_dir)
+
+      assert {:ok, "HEADER CONTENT"} = TemplateFileSystem.try_read(fs, "header")
+    end
+
+    @tag :tmp_dir
+    test "returns {:error, :enoent} for missing templates", %{tmp_dir: tmp_dir} do
+      fs = TemplateFileSystem.new(tmp_dir)
+
+      assert {:error, :enoent} = TemplateFileSystem.try_read(fs, "nonexistent")
+    end
+
+    @tag :tmp_dir
+    test "falls through to next root path when first is missing", %{tmp_dir: tmp_dir} do
+      root_a = Path.join(tmp_dir, "a")
+      root_b = Path.join(tmp_dir, "b")
+      File.mkdir_p!(root_a)
+      File.mkdir_p!(root_b)
+      File.write!(Path.join(root_b, "partial.liquid"), "FROM B")
+
+      fs = TemplateFileSystem.new([root_a, root_b])
+
+      assert {:ok, "FROM B"} = TemplateFileSystem.try_read(fs, "partial")
+    end
+
+    @tag :tmp_dir
+    test "first root path wins when both have the template", %{tmp_dir: tmp_dir} do
+      root_a = Path.join(tmp_dir, "a")
+      root_b = Path.join(tmp_dir, "b")
+      File.mkdir_p!(root_a)
+      File.mkdir_p!(root_b)
+      File.write!(Path.join(root_a, "shared.liquid"), "FROM A")
+      File.write!(Path.join(root_b, "shared.liquid"), "FROM B")
+
+      fs = TemplateFileSystem.new([root_a, root_b])
+
+      assert {:ok, "FROM A"} = TemplateFileSystem.try_read(fs, "shared")
+    end
+
+    @tag :tmp_dir
+    test "file deleted between candidate_paths and try_read does not crash", %{tmp_dir: tmp_dir} do
+      path = Path.join(tmp_dir, "ephemeral.liquid")
+      File.write!(path, "TEMPORARY")
+      fs = TemplateFileSystem.new(tmp_dir)
+
+      _candidates = TemplateFileSystem.candidate_paths(fs, "ephemeral")
+      File.rm!(path)
+
+      assert {:error, :enoent} = TemplateFileSystem.try_read(fs, "ephemeral")
+    end
+  end
+
+  describe "read_template_file/2 protocol uses atomic read" do
+    @tag :tmp_dir
+    test "reads existing template", %{tmp_dir: tmp_dir} do
+      File.write!(Path.join(tmp_dir, "footer.liquid"), "FOOTER")
+      fs = TemplateFileSystem.new(tmp_dir)
+
+      assert "FOOTER" = Liquex.FileSystem.read_template_file(fs, "footer")
+    end
+
+    @tag :tmp_dir
+    test "raises on missing template", %{tmp_dir: tmp_dir} do
+      fs = TemplateFileSystem.new(tmp_dir)
+
+      assert_raise Liquex.Error, ~r/No such template/, fn ->
+        Liquex.FileSystem.read_template_file(fs, "missing")
+      end
+    end
+  end
+
+  describe "candidate_paths/2 validation" do
+    test "raises on empty path" do
+      fs = TemplateFileSystem.new("/tmp")
+
+      assert_raise Liquex.Error, ~r/Illegal template path/, fn ->
+        TemplateFileSystem.candidate_paths(fs, "")
+      end
+    end
+
+    test "raises on absolute path" do
+      fs = TemplateFileSystem.new("/tmp")
+
+      assert_raise Liquex.Error, ~r/Illegal template path/, fn ->
+        TemplateFileSystem.candidate_paths(fs, "/etc/passwd")
+      end
+    end
+
+    test "raises on path traversal" do
+      fs = TemplateFileSystem.new("/tmp")
+
+      assert_raise Liquex.Error, ~r/Illegal template path/, fn ->
+        TemplateFileSystem.candidate_paths(fs, "../secret")
+      end
+    end
+  end
+end