On Wednesday, October 02, 2013 1:32 PM, Bird, Tim wrote:

A significant difficulty is that the kernel memory for ARM is currently mapped by
section (that is, using 1MB sections). This means that small memory areas
cannot be individually re-mapped RO on page boundaries. If the kernel has code
which must be writable for some reason, then with that current mapping, at least
a 1MB section would be used for that writable code.

FYI - Sony has patches to support 4KB mappings for the kernel, for ARM,
which they've been using internally. These might be useful
for this feature.

; Summary: Overwrite detection for kernel text and read-only data

; Proposer: Tim Bird, Sony Mobile

== Description ==
In embedded systems, drivers or other subsystems can easily (mistakenly)
overwrite kernel text or kernel read-only area. It can be very difficult to debug
who is overwriting kernel.

It would be nice to have some mechanism to detect kernel overwriting or corruption
by setting a write-protect attribute in page tables for kernel text or read-only data.
x86 already has this feature in the form of CONFIG_DEBUG_RODATA. This project
would consist of providing support for this feature in ARM, as well as possibly
creating exceptions for code which needs to modify the kernel text at runtime,
such as Kprobe or ftrace.

A significant difficulty is that the kernel memory for ARM is currently mapped by
section (that is, using 1MB sections). This means that small memory areas
cannot be individually re-mapped RO on page boundaries. If the kernel has code
which must be writable for some reason, then with that current mapping, at least
a 1MB section would be used for that writable code. Likely, the linking sections
of the source would have to be modified to support RO/RW attributes, to coalesce
the sections into correct categories.


== Related work ==
* [http://www.slideshare.net/prabindh/arm-memory-protection-techniques ARM Linux Embedded memory protection techniques]
** Presentation by Prabindh Sundareson in May, 2013 about the status of ARM memory protection features.
* Russel King comments on kernel memory and RO mapping: http://www.spinics.net/lists/arm-kernel/msg120951.html
* The implementation of CONFIG_DEBUG_RODATA in x86 may have some useful information.

== Scope ==
Unknown

== Contractor Candidates ==
None yet.

== Comments ==

[[Category:Project proposals 2013]]