StealEmoji: check remote size before downloading

To save on bandwith and avoid OOMs with large files.
Ofc, this relies on the remote server
 (a) sending a content-length header and
 (b) being honest about the size.

Common fedi servers seem to provide the header and (b) at least raises
the required privilege of an malicious actor to a server infrastructure
admin of an explicitly allowed host.

A more complete defense which still works when faced with
a malicious server requires changes in upstream Finch;
see https://github.com/sneako/finch/issues/224

docs/docs/configuration/cheatsheet.md

@@ -236,7 +236,9 @@ config :pleroma, :mrf_user_allowlist, %{
 #### :mrf_steal_emoji
 * `hosts`: List of hosts to steal emojis from
 * `rejected_shortcodes`: Regex-list of shortcodes to reject
-* `size_limit`: File size limit (in bytes), checked before an emoji is saved to the disk
+* `size_limit`: File size limit (in bytes), checked before download if possible (and remote server honest),
+   otherwise or again checked before saving emoji to the disk
+* `download_unknown_size`: whether to download an emoji when the remote server doesn’t report its size in advance
 
 #### :mrf_activity_expiration
 

lib/pleroma/web/activity_pub/mrf/steal_emoji_policy.ex

@@ -13,6 +13,10 @@ defmodule Pleroma.Web.ActivityPub.MRF.StealEmojiPolicy do
 
   @pack_name "stolen"
 
+  # Config defaults
+  @size_limit 50_000
+  @download_unknown_size false
+
   defp create_pack() do
     with {:ok, pack} = Pack.create(@pack_name) do
       Pack.save_metadata(
@@ -97,11 +101,28 @@ defp get_extension_if_safe(response) do
     end
   end
 
+  defp is_remote_size_within_limit?(url) do
+    with {:ok, %{status: status, headers: headers} = _response} when status in 200..299 <-
+           Pleroma.HTTP.request(:head, url, nil, [], []) do
+      content_length = :proplists.get_value("content-length", headers, nil)
+      size_limit = Config.get([:mrf_steal_emoji, :size_limit], @size_limit)
+
+      accept_unknown =
+        Config.get([:mrf_steal_emoji, :download_unknown_size], @download_unknown_size)
+
+      content_length <= size_limit or
+        (content_length == nil and accept_unknown)
+    else
+      _ -> false
+    end
+  end
+
   defp maybe_steal_emoji({shortcode, url}) do
     url = Pleroma.Web.MediaProxy.url(url)
 
-    with {:ok, %{status: status} = response} when status in 200..299 <- Pleroma.HTTP.get(url) do
-      size_limit = Config.get([:mrf_steal_emoji, :size_limit], 50_000)
+    with {:remote_size, true} <- {:remote_size, is_remote_size_within_limit?(url)},
+         {:ok, %{status: status} = response} when status in 200..299 <- Pleroma.HTTP.get(url) do
+      size_limit = Config.get([:mrf_steal_emoji, :size_limit], @size_limit)
       extension = get_extension_if_safe(response)
 
       if byte_size(response.body) <= size_limit and extension do

test/pleroma/web/activity_pub/mrf/steal_emoji_policy_test.exs

@@ -32,6 +32,14 @@ defmacro mock_tesla(
            ) do
     quote do
       Tesla.Mock.mock(fn
+        %{method: :head, url: unquote(url)} ->
+          %Tesla.Env{
+            status: unquote(status),
+            body: nil,
+            url: unquote(url),
+            headers: unquote(headers)
+          }
+
         %{method: :get, url: unquote(url)} ->
           %Tesla.Env{
             status: unquote(status),
@@ -46,7 +54,8 @@ defmacro mock_tesla(
   setup do
     clear_config(:mrf_steal_emoji,
       hosts: ["example.org"],
-      size_limit: 284_468
+      size_limit: 284_468,
+      download_unknown_size: true
     )
 
     emoji_path = [:instance, :static_dir] |> Config.get() |> Path.join("emoji/stolen")
@@ -174,5 +183,44 @@ test "reject if host returns error", %{message: message} do
     refute "firedfox" in installed()
   end
 
+  test "reject unknown size", %{message: message} do
+    clear_config([:mrf_steal_emoji, :download_unknown_size], false)
+    mock_tesla()
+
+    refute "firedfox" in installed()
+
+    ExUnit.CaptureLog.capture_log(fn ->
+      assert {:ok, _message} = StealEmojiPolicy.filter(message)
+    end) =~
+      "MRF.StealEmojiPolicy: Failed to fetch https://example.org/emoji/firedfox.png: {:remote_size, false}"
+
+    refute "firedfox" in installed()
+  end
+
+  test "reject too large content-size before download", %{message: message} do
+    clear_config([:mrf_steal_emoji, :download_unknown_size], false)
+    mock_tesla("https://example.org/emoji/firedfox.png", 200, [{"content-length", 2 ** 30}])
+
+    refute "firedfox" in installed()
+
+    ExUnit.CaptureLog.capture_log(fn ->
+      assert {:ok, _message} = StealEmojiPolicy.filter(message)
+    end) =~
+      "MRF.StealEmojiPolicy: Failed to fetch https://example.org/emoji/firedfox.png: {:remote_size, false}"
+
+    refute "firedfox" in installed()
+  end
+
+  test "accepts content-size below limit", %{message: message} do
+    clear_config([:mrf_steal_emoji, :download_unknown_size], false)
+    mock_tesla("https://example.org/emoji/firedfox.png", 200, [{"content-length", 2}])
+
+    refute "firedfox" in installed()
+
+    assert {:ok, _message} = StealEmojiPolicy.filter(message)
+
+    assert "firedfox" in installed()
+  end
+
   defp installed, do: Emoji.get_all() |> Enum.map(fn {k, _} -> k end)
 end