[$] KAISER: hiding the kernel from user space

By corbet Since the beginning, Linux has mapped the kernel’s memory into the address
space of every running process. There are solid performance reasons for
doing this, and the processor’s memory-management unit can ordinarily be
trusted to prevent user space from accessing that memory. More recently,
though, some more subtle security issues related to this mapping have come
to light, leading to the rapid development of a new patch set that ends this
longstanding practice for the x86 architecture.

From: LWN