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

On Mon, Nov 10, 2025 at 11:08 AM Stephen Smalley
<[email protected]> wrote:
>
> On Wed, Nov 5, 2025 at 7:56 PM NeilBrown <[email protected]> wrote:
> >
> > From: NeilBrown <[email protected]>
> >
> > 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.

Also, I assume you mean "does NOT open up any interesting races" above.

> >
> > Reviewed-by: Amir Goldstein <[email protected]>
> > Signed-off-by: NeilBrown <[email protected]>
> >
> > ---
> > 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
> >