nixos/security/wrapper: ensure the tmpfs is not world writeable

The /run/wrapper directory is a tmpfs. Unfortunately, it's mounted with
its root directory has the standard (for tmpfs) mode: 1777 (world writeable,
sticky -- the standard mode of shared temporary directories). This means that
every user can create new files and subdirectories there, but can't
move/delete/rename files that belong to other users.

nixos/modules/security/wrappers/default.nix

@@ -169,7 +169,7 @@ in
 
     boot.specialFileSystems.${parentWrapperDir} = {
       fsType = "tmpfs";
-      options = [ "nodev" ];
+      options = [ "nodev" "mode=755" ];
     };
 
     # Make sure our wrapperDir exports to the PATH env variable when
@@ -195,6 +195,8 @@ in
           # programs to be wrapped.
           WRAPPER_PATH=${config.system.path}/bin:${config.system.path}/sbin
 
+          chmod 755 "${parentWrapperDir}"
+
           # We want to place the tmpdirs for the wrappers to the parent dir.
           wrapperDir=$(mktemp --directory --tmpdir="${parentWrapperDir}" wrappers.XXXXXXXXXX)
           chmod a+rx $wrapperDir
@@ -205,6 +207,9 @@ in
             # Atomically replace the symlink
             # See https://axialcorps.com/2013/07/03/atomically-replacing-files-and-directories/
             old=$(readlink -f ${wrapperDir})
+            if [ -e ${wrapperDir}-tmp ]; then
+              rm --force --recursive ${wrapperDir}-tmp
+            fi
             ln --symbolic --force --no-dereference $wrapperDir ${wrapperDir}-tmp
             mv --no-target-directory ${wrapperDir}-tmp ${wrapperDir}
             rm --force --recursive $old