[RFC v2] Introduce rare_write() infrastructure

On Wed, Mar 29, 2017 at 11:15:52AM -0700, Kees Cook wrote:

This is take 2 of an RFC series to demonstrate a possible infrastructure
for the "write rarely" memory storage type in the kernel (patch 1). The
intent is to further reduce the internal attack surface of the kernel
by making more variables read-only while "at rest". This is heavily
based on the "__read_only" portion of the KERNEXEC infrastructure from
PaX/grsecurity, though I tried to adjust it to be more in line with
the upstream discussions around the APIs.

How much data are we talking about here?

As part of the series I've included both x86 support (patch 4), exactly
as done in PaX, and ARM support (patches 5-7), similar to what is done in
grsecurity but without support for earlier ARM CPUs. Both are lightly
tested by me, though they need a bit more work, especially ARM as it is
missing the correct domain marking for kernel modules.

The implementation as it stands on ARM is going to gobble up
multiples of 1MB of RAM as you need it to be section aligned due to
using domains, so if we're talking about a small amount of data,
this is very inefficient.  That also only works for non-LPAE as LPAE
is unable to use that method.

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