Bug 1582512 - Add in exception cases where we disable the javascript load restrictions r=ckerschb

Differential Revision: https://phabricator.services.mozilla.com/D53245

--HG--
extra : moz-landing-system : lando

dom/security/nsContentSecurityUtils.cpp

@@ -706,6 +706,39 @@ bool nsContentSecurityUtils::ValidateScriptFilename(const char* aFilename,
     return true;
   }
 
+  // We only perform a check of this preference on the Main Thread
+  // (because a String-based preference check is only safe on Main Thread.)
+  // The consequence of this is that if a user is using userChromeJS _and_
+  // the scripts they use start a worker - we will enter this function,
+  // skip over this pref check that would normally cause us to allow the
+  // load - and we will block it.
+  // While not ideal, we do not officially support userChromeJS, and hopefully
+  // the usage of workers is even lower than userChromeJS usage.
+  if (NS_IsMainThread()) {
+    // This preference is a file used for autoconfiguration of Firefox
+    // by administrators. It has also been (ab)used by the userChromeJS
+    // project to run legacy-style 'extensions', some of which use eval,
+    // all of which run in the System Principal context.
+    nsAutoString jsConfigPref;
+    Preferences::GetString("general.config.filename", jsConfigPref);
+    if (!jsConfigPref.IsEmpty()) {
+      MOZ_LOG(sCSMLog, LogLevel::Debug,
+              ("Allowing a javascript load of %s because of "
+               "general.config.filename",
+               aFilename));
+      return true;
+    }
+  }
+
+  if (XRE_IsE10sParentProcess() &&
+      !StaticPrefs::extensions_webextensions_remote()) {
+    MOZ_LOG(sCSMLog, LogLevel::Debug,
+            ("Allowing a javascript load of %s because the web extension "
+             "process is disabled.",
+             aFilename));
+    return true;
+  }
+
   NS_ConvertUTF8toUTF16 filenameU(aFilename);
   if (StringBeginsWith(filenameU, NS_LITERAL_STRING("chrome://"))) {
     // If it's a chrome:// url, allow it