fs: relax mount_setattr() permission checks

When we added mount_setattr() I added additional checks compared to the
legacy do_reconfigure_mnt() and do_change_type() helpers used by regular
mount(2). If that mount had a parent then verify that the caller and the
mount namespace the mount is attached to match and if not make sure that
it's an anonymous mount.

The real rootfs falls into neither category. It is neither an anoymous
mount because it is obviously attached to the initial mount namespace
but it also obviously doesn't have a parent mount. So that means legacy
mount(2) allows changing mount properties on the real rootfs but
mount_setattr(2) blocks this. I never thought much about this but of
course someone on this planet of earth changes properties on the real
rootfs as can be seen in [1].

Since util-linux finally switched to the new mount api in 2.39 not so
long ago it also relies on mount_setattr() and that surfaced this issue
when Fedora 39 finally switched to it. Fix this.

Link: https://bugzilla.redhat.com/show_bug.cgi?id=2256843
Link: https://lore.kernel.org/r/20240206-vfs-mount-rootfs-v1-1-19b335eee133@kernel.orgReviewed-by: Jan Kara <jack@suse.cz>
Reported-by: Karel Zak <kzak@redhat.com>
Cc: stable@vger.kernel.org # v5.12+
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/namespace.c

@@ -4472,10 +4472,15 @@ static int do_mount_setattr(struct path *path, struct mount_kattr *kattr)
 	/*
 	 * If this is an attached mount make sure it's located in the callers
 	 * mount namespace. If it's not don't let the caller interact with it.
-	 * If this is a detached mount make sure it has an anonymous mount
-	 * namespace attached to it, i.e. we've created it via OPEN_TREE_CLONE.
+	 *
+	 * If this mount doesn't have a parent it's most often simply a
+	 * detached mount with an anonymous mount namespace. IOW, something
+	 * that's simply not attached yet. But there are apparently also users
+	 * that do change mount properties on the rootfs itself. That obviously
+	 * neither has a parent nor is it a detached mount so we cannot
+	 * unconditionally check for detached mounts.
 	 */
-	if (!(mnt_has_parent(mnt) ? check_mnt(mnt) : is_anon_ns(mnt->mnt_ns)))
+	if ((mnt_has_parent(mnt) || !is_anon_ns(mnt->mnt_ns)) && !check_mnt(mnt))
 		goto out;
 
 	/*