Additional hardening options

[Paul Eggleton <bluelightning@bluelightning.org>]

Hi folks

I've been looking into a couple of compiler flags for hardening that I think we 
might want to consider enabling by default in security-flags.inc:


1) -fstack-clash-protection

This option was introduced to gcc 8.x and provides protection against the 
stack clash vulnerability:

https://securingsoftware.blogspot.com/2017/12/stack-clash-vulnerability.html

It has been enabled in some Linux distributions already (e.g. Ubuntu, Fedora).


2) -z noexecstack (or alternative mitigations)

gcc will enable an executable stack under a few different circumstances - see 
here for details

https://wiki.gentoo.org/wiki/Hardened/GNU_stack_quickstart

I've written a check that we could add to insane.bbclass that warns/errors on 
binaries with an executable stack. Does this seem reasonable to have?
The other possibility is we add -Wl,-z,noexecstack to LDFLAGS and then see 
what breaks, but unfortunately issues are likely only going to show up when 
the program crashes at runtime, and also it will stop the aforementioned check 
from working.


Any opinions?

Thanks
Paul