[mod] hardening "calculator plugin" / limit execution time to 50 ms

The execution of the function for the calculation is outsourced to a process
whose runtime is limited to 50 milliseconds.

Related:

- [1] https://github.com/searxng/searxng/pull/3377#issuecomment-2067977375

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/plugins/calculator.py

@@ -4,10 +4,13 @@
 
 import ast
 import operator
+from multiprocessing import Process, Queue
 
 from flask_babel import gettext
 from searx import settings
 
+from searx.plugins import logger
+
 name = "Basic Calculator"
 description = gettext("Calculate mathematical expressions via the search bar")
 default_on = False
@@ -15,6 +18,8 @@ default_on = False
 preference_section = 'general'
 plugin_id = 'calculator'
 
+logger = logger.getChild(plugin_id)
+
 operators = {
     ast.Add: operator.add,
     ast.Sub: operator.sub,
@@ -51,6 +56,30 @@ def _eval(node):
     raise TypeError(node)
 
 
+def timeout_func(timeout, func, *args, **kwargs):
+
+    def handler(q: Queue, func, args, **kwargs):  # pylint:disable=invalid-name
+        try:
+            q.put(func(*args, **kwargs))
+        except:
+            q.put(None)
+            raise
+
+    que = Queue()
+    p = Process(target=handler, args=(que, func, args), kwargs=kwargs)
+    p.start()
+    p.join(timeout=timeout)
+    ret_val = None
+    if not p.is_alive():
+        ret_val = que.get()
+    else:
+        logger.debug("terminate function after timeout is exceeded")
+        p.terminate()
+    p.join()
+    p.close()
+    return ret_val
+
+
 def post_search(_request, search):
     # don't run on public instances due to possible attack surfaces
     if settings['server']['public_instance']:
@@ -74,13 +103,15 @@ def post_search(_request, search):
 
     # in python, powers are calculated via **
     query_py_formatted = query.replace("^", "**")
-    try:
-        result = str(_eval_expr(query_py_formatted))
+
+    # Prevent the runtime from being longer than 50 ms
+    result = timeout_func(0.05, _eval_expr, query_py_formatted)
+    if result is None:
+        return True
+    result = str(result)
+
     if result != query:
         search.result_container.answers['calculate'] = {'answer': f"{query} = {result}"}
-    except (TypeError, SyntaxError, ArithmeticError):
-        pass
-
     return True