Access control to shared virtual address space within a single logical partition is provided. The access control includes: associating, by a hypervisor of the data processing system, a memory protection key with a portion of a single logical partition's virtual address space being shared by multiple entities, the key preventing access by one of the multiple entities to that portion of the virtual address space, and allowing access by another of the entities to that portion of the virtual address space; and locking by the hypervisor the memory protection key from modification by the one entity, wherein the locking prevents the one entity from modifying the key and thereby gaining access to the portion of the single logical partition's virtual address space with the associated memory protection key. In one embodiment, the one entity is the single logical partition itself, and the another entity is a partition adjunct.