allow CSP customization on a per-controller basis

lib/philomena_web/plugs/content_security_policy_plug.ex

@@ -1,5 +1,11 @@
 defmodule PhilomenaWeb.ContentSecurityPolicyPlug do
-  alias Plug.Conn
+  import Plug.Conn
+
+  @allowed_sources [
+    :script_src,
+    :frame_src,
+    :style_src
+  ]
 
   def init(opts) do
     opts
@@ -9,18 +15,57 @@ defmodule PhilomenaWeb.ContentSecurityPolicyPlug do
     cdn_uri = cdn_uri()
     camo_uri = camo_uri()
 
-    csp_value =
-      "default-src 'self' #{cdn_uri}; object-src 'none'; " <>
-        "frame-ancestors 'none'; frame-src 'none'; form-action 'self'; " <>
-        "manifest-src 'self'; img-src 'self' data: #{cdn_uri} #{camo_uri}; " <>
-        "block-all-mixed-content"
+    register_before_send(conn, fn conn ->
+      config = get_config(conn)
 
-    Conn.put_resp_header(conn, "content-security-policy", csp_value)
+      script_src = Keyword.get(config, :script_src, [])
+      style_src = Keyword.get(config, :style_src, [])
+      frame_src = Keyword.get(config, :frame_src, nil)
+
+      csp_config = [
+        {:default_src, ["'self'", cdn_uri]},
+        {:script_src, ["'self'", cdn_uri | script_src]},
+        {:style_src, ["'self'", cdn_uri | style_src]},
+        {:object_src, ["'none'"]},
+        {:frame_ancestors, ["'none'"]},
+        {:frame_src, frame_src || ["'none'"]},
+        {:form_action, ["'self'"]},
+        {:manifest_src, ["'self'"]},
+        {:img_src, ["'self'", "data:", cdn_uri, camo_uri]},
+        {:block_all_mixed_content, []}
+      ]
+
+      csp_value =
+        csp_config
+        |> Enum.map(&cspify_element/1)
+        |> Enum.join("; ")
+
+      put_resp_header(conn, "content-security-policy", csp_value)
+    end)
   end
 
+  def permit_source(conn, key, value) when key in @allowed_sources do
+    conn
+    |> get_config()
+    |> Keyword.update(key, [], &[value | &1])
+    |> set_config(conn)
+  end
+
+  defp get_config(conn), do: conn.private[:csp] || []
+  defp set_config(value, conn), do: put_private(conn, :csp, value)
+
   defp cdn_uri, do: Application.get_env(:philomena, :cdn_host) |> to_uri()
   defp camo_uri, do: Application.get_env(:philomena, :camo_host) |> to_uri()
 
   defp to_uri(host) when host in [nil, ""], do: ""
   defp to_uri(host), do: URI.to_string(%URI{scheme: "https", host: host})
+
+  defp cspify_element({key, value}) do
+    key =
+      key
+      |> Atom.to_string()
+      |> String.replace("_", "-")
+
+    Enum.join([key | value], " ")
+  end
 end