security: check if shared memory belongs to current user and is only …

…read/writeable for them

Fixes luizalabs#33

shared_memory_dict/dict.py

@@ -1,4 +1,5 @@
 import logging
+import os
 import sys
 import warnings
 from contextlib import contextmanager
@@ -159,11 +160,25 @@ def _get_or_create_memory_block(
         try:
             return SharedMemory(name=name)
         except FileNotFoundError:
+            self.check_security(name)
             shm = SharedMemory(name=name, create=True, size=size)
             data = self._serializer.dumps({})
             shm.buf[: len(data)] = data
             return shm
 
+    def check_security(self, name: str) -> None:
+        """Check if shared memory belongs to the current user and is only read+writeable for us"""
+        if os.name == 'nt':
+            return
+
+        if '/' in name:
+            raise TypeError('Name must not contain "/".')
+
+        shm_file = os.path.join('/dev/shm', name)
+        stat = os.stat(shm_file)
+        if stat.st_uid != os.getuid() or stat.st_gid != os.getgid() or stat.st_mode != 0o100600:
+            os.unlink(shm_file)
+
     def _save_memory(self, db: Dict[str, Any]) -> None:
         data = self._serializer.dumps(db)
         try: