[$] Page-table hardening with memory protection keys

Attacks on the kernel can take many forms; one popular exploitation path isto find a way to overwrite some memory with attacker-supplied data. If theright memory can be targeted, one well-targeted stray write is all that isneeded to take control of the system. Since the system's page tablesregulate access to memory, they are an attractive target for this type ofattack. This patch set from Kevin Brodsky is an attempt to protect page tables (and,eventually, other data structures) using the "memory protection keys"feature provided by a number of CPU architectures.