On Fri, May 30, 2025 at 5:20 AM Mickaël Salaün [off-list ref] wrote:
[...]

quoted

If we update path_parent in this patchset with choose_mountpoint(),
and use it in Landlock, we will close this race condition, right?

choose_mountpoint() is currently private, but if we add a new filesystem
helper, I think the right approach would be to expose follow_dotdot(),
updating its arguments with public types.  This way the intermediates
mount points will not be exposed, RCU optimization will be leveraged,
and usage of this new helper will be simplified.

I think it is easier to add a helper similar to follow_dotdot(), but not with
nameidata. follow_dotdot() touches so many things in nameidata, so it
is better to keep it as-is. I am having the following:

/**
 * path_parent - Find the parent of path
 * @path: input and output path.
 * @root: root of the path walk, do not go beyond this root. If @root is
 *        zero'ed, walk all the way to real root.
 *
 * Given a path, find the parent path. Replace @path with the parent path.
 * If we were already at the real root or a disconnected root, @path is
 * not changed.
 *
 * Returns:
 *  true  - if @path is updated to its parent.
 *  false - if @path is already the root (real root or @root).
 */
bool path_parent(struct path *path, const struct path *root)
{
        struct dentry *parent;

        if (path_equal(path, root))
                return false;

        if (unlikely(path->dentry == path->mnt->mnt_root)) {
                struct path p;

                if (!choose_mountpoint(real_mount(path->mnt), root, &p))
                        return false;
                path_put(path);
                *path = p;
                return true;
        }

        if (unlikely(IS_ROOT(path->dentry)))
                return false;

        parent = dget_parent(path->dentry);
        if (unlikely(!path_connected(path->mnt, parent))) {
                dput(parent);
                return false;
        }
        dput(path->dentry);
        path->dentry = parent;
        return true;
}
EXPORT_SYMBOL_GPL(path_parent);

And for Landlock, it is simply:

                if (path_parent(&walker_path, &root))
                        continue;

                if (unlikely(IS_ROOT(walker_path.dentry))) {
                        /*
                         * Stops at disconnected or real root directories.
                         * Only allows access to internal filesystems
                         * (e.g. nsfs, which is reachable through
                         * /proc/<pid>/ns/<namespace>).
                         */
                        if (walker_path.mnt->mnt_flags & MNT_INTERNAL) {
                                allowed_parent1 = true;
                                allowed_parent2 = true;
                        }
                        break;
                }

Does this look right?

Thanks,
Song