In the Linux kernel, the following vulnerability has been resolved:
ovl: don't allow datadir only
In theory overlayfs could support upper layer directly referring to a data layer, but there's no current use case for this.
Originally, when data-only layers were introduced, this wasn't allowed, only introduced by the "datadir+" feature, but without actually handling this case, resulting in an Oops.
Fix by disallowing datadir without lowerdir.