Re: [PATCH v5 11/14] Add start_renaming_two_dentries()

Stephen Smalley
2025-11-10T16:09:09Z

On Wed, Nov 5, 2025 at 7:56 PM NeilBrown <neilb@ownmail.net> wrote:
>
> From: NeilBrown <neil@brown.name>
>
> A few callers want to lock for a rename and already have both dentries.
> Also debugfs does want to perform a lookup but doesn't want permission
> checking, so start_renaming_dentry() cannot be used.
>
> This patch introduces start_renaming_two_dentries() which is given both
> dentries. debugfs performs one lookup itself. As it will only continue
> with a negative dentry and as those cannot be renamed or unlinked, it is
> safe to do the lookup before getting the rename locks.
>
> overlayfs uses start_renaming_two_dentries() in three places and selinux
> uses it twice in sel_make_policy_nodes().
>
> In sel_make_policy_nodes() we now lock for rename twice instead of just
> once so the combined operation is no longer atomic w.r.t the parent
> directory locks. As selinux_state.policy_mutex is held across the whole
> operation this does open up any interesting races.
>
> Reviewed-by: Amir Goldstein <amir73il@gmail.com>
> Signed-off-by: NeilBrown <neil@brown.name>
>
> ---
> changes since v3:
> added missing assignment to rd.mnt_idmap in ovl_cleanup_and_whiteout
> ---

> diff --git a/security/selinux/selinuxfs.c b/security/selinux/selinuxfs.c
> index 232e087bce3e..a224ef9bb831 100644
> --- a/security/selinux/selinuxfs.c
> +++ b/security/selinux/selinuxfs.c
> @@ -539,22 +540,30 @@ static int sel_make_policy_nodes(struct selinux_fs_info *fsi,
> if (ret)
> goto out;
>
> - lock_rename(tmp_parent, fsi->sb->s_root);
> + rd.old_parent = tmp_parent;
> + rd.new_parent = fsi->sb->s_root;
>
> /* booleans */
> - d_exchange(tmp_bool_dir, fsi->bool_dir);
> + ret = start_renaming_two_dentries(&rd, tmp_bool_dir, fsi->bool_dir);
> + if (!ret) {
> + d_exchange(tmp_bool_dir, fsi->bool_dir);

I would recommend an immediate goto out if ret != 0; we don't want to
silently fall through and possibly reset ret on the next
start_renaming_two_dentries() call, thereby ultimately returning 0 to
the caller and acting as if nothing bad happened.

>
> - swap(fsi->bool_num, bool_num);
> - swap(fsi->bool_pending_names, bool_names);
> - swap(fsi->bool_pending_values, bool_values);
> + swap(fsi->bool_num, bool_num);
> + swap(fsi->bool_pending_names, bool_names);
> + swap(fsi->bool_pending_values, bool_values);
>
> - fsi->bool_dir = tmp_bool_dir;
> + fsi->bool_dir = tmp_bool_dir;
> + end_renaming(&rd);
> + }
>
> /* classes */
> - d_exchange(tmp_class_dir, fsi->class_dir);
> - fsi->class_dir = tmp_class_dir;
> + ret = start_renaming_two_dentries(&rd, tmp_class_dir, fsi->class_dir);
> + if (ret == 0) {
> + d_exchange(tmp_class_dir, fsi->class_dir);
> + fsi->class_dir = tmp_class_dir;
>
> - unlock_rename(tmp_parent, fsi->sb->s_root);
> + end_renaming(&rd);
> + }
>
> out:
> sel_remove_old_bool_data(bool_num, bool_names, bool_values);
> --
> 2.50.0.107.gf914562f5916.dirty
>