security: detect object containment violations at the IR level

It is more efficient to check for object containment violations at the IR
level instead of in the protocol handlers.  OStatus containment is especially
a tricky situation, as the containment rules don't match those of IR and
ActivityPub.

Accordingly, we just always do a final containment check at the IR level
before the object is added to the IR object graph.

lib/pleroma/object/containment.ex

@@ -48,6 +48,9 @@ def contain_origin(id, %{"actor" => _actor} = params) do
     end
   end
 
+  def contain_origin(id, %{"attributedTo" => actor} = params),
+    do: contain_origin(id, Map.put(params, "actor", actor))
+
   def contain_origin_from_id(_id, %{"id" => nil}), do: :error
 
   def contain_origin_from_id(id, %{"id" => other_id} = _params) do
@@ -60,4 +63,9 @@ def contain_origin_from_id(id, %{"id" => other_id} = _params) do
       :error
     end
   end
+
+  def contain_child(%{"object" => %{"id" => id, "attributedTo" => _} = object}),
+    do: contain_origin(id, object)
+
+  def contain_child(_), do: :ok
 end

lib/pleroma/web/activity_pub/activity_pub.ex

@@ -8,6 +8,7 @@ defmodule Pleroma.Web.ActivityPub.ActivityPub do
   alias Pleroma.Conversation
   alias Pleroma.Notification
   alias Pleroma.Object
+  alias Pleroma.Object.Containment
   alias Pleroma.Object.Fetcher
   alias Pleroma.Pagination
   alias Pleroma.Repo
@@ -126,6 +127,7 @@ def insert(map, local \\ true, fake \\ false) when is_map(map) do
          {:ok, map} <- MRF.filter(map),
          {recipients, _, _} = get_recipients(map),
          {:fake, false, map, recipients} <- {:fake, fake, map, recipients},
+         :ok <- Containment.contain_child(map),
          {:ok, map, object} <- insert_full_object(map) do
       {:ok, activity} =
         Repo.insert(%Activity{

test/object/containment_test.exs

@@ -68,4 +68,34 @@ test "users cannot be collided through fake direction spoofing attempts" do
                "[error] Could not decode user at fetch https://n1u.moe/users/rye, {:error, :error}"
     end
   end
+
+  describe "containment of children" do
+    test "contain_child() catches spoofing attempts" do
+      data = %{
+        "id" => "http://example.com/whatever",
+        "type" => "Create",
+        "object" => %{
+          "id" => "http://example.net/~alyssa/activities/1234",
+          "attributedTo" => "http://example.org/~alyssa"
+        },
+        "actor" => "http://example.com/~bob"
+      }
+
+      :error = Containment.contain_child(data)
+    end
+
+    test "contain_child() allows correct origins" do
+      data = %{
+        "id" => "http://example.org/~alyssa/activities/5678",
+        "type" => "Create",
+        "object" => %{
+          "id" => "http://example.org/~alyssa/activities/1234",
+          "attributedTo" => "http://example.org/~alyssa"
+        },
+        "actor" => "http://example.org/~alyssa"
+      }
+
+      :ok = Containment.contain_child(data)
+    end
+  end
 end