resolve the full ffmpeg path under granular root

frigate/test/test_ffmpeg_path_resolution.py

@@ -216,6 +216,13 @@ class TestConfigFfmpegRootGuardSymlinks(unittest.TestCase):
         os.symlink(evil, self.link)
         self.assertEqual(self._resolve({"FRIGATE_ROOT_SERVICES": "frigate"}), BUNDLED)
 
+    def test_bin_linked_into_a_writable_tree_is_refused(self) -> None:
+        evil = os.path.join(self.writable, "evil")
+        os.makedirs(evil)
+        os.symlink(evil, os.path.join(self.root_owned, "bin"))
+        os.symlink(self.root_owned, self.link)
+        self.assertEqual(self._resolve({"FRIGATE_ROOT_SERVICES": "frigate"}), BUNDLED)
+
     def test_unprivileged_service_keeps_the_path_as_written(self) -> None:
         os.symlink(self.root_owned, self.link)
         self.assertEqual(self._resolve({}), f"{self.link}/bin/ffmpeg")

frigate/util/config.py

@@ -125,9 +125,9 @@ def resolve_ffmpeg_path(
     else:
         # return the resolved form so a link in the path cannot be retargeted
         # after the check
-        real = os.path.realpath(path)
+        real = os.path.realpath(f"{path}/bin/{binary}")
         if is_valid_ffmpeg_path(real) and not is_runtime_user_writable(real):
-            return f"{real}/bin/{binary}"
+            return real
 
         _warn_ignored_ffmpeg_path(path, service)
         version = DEFAULT_FFMPEG_VERSION