[cfe-dev] [RFC] Implementing Randstruct in Clang

[Connor Kuehl via cfe-dev]

This RFC accompanies the patchset on Phabricator: 
https://reviews.llvm.org/D59254

The Randstruct feature is a compile-time hardening technique that 
randomizes the field layout for designated structures of a code base. 
Admittedly, this is mostly useful for closed-source releases of code 
(since the randomization seed would be available for public and open 
source applications).

Why implement it? This patch set enhances Clang’s feature parity with 
that of GCC which already has the Randstruct feature. There's a growing 
interest in compiling projects such as the Linux kernel with Clang. The 
Linux kernel is a good example of a large code base which uses 
randomized field order in certain structures to help thwart attacks that 
depend on structure layouts in memory.

This patch set is a from-scratch reimplementation of the Randstruct 
feature that was originally ported to GCC. The patches for this 
implementation in GCC can be found here:

     https://www.openwall.com/lists/kernel-hardening/2017/04/06/14.

This feature identifies structures for randomization in two ways. The 
first method targets structures that are manually marked with the new 
“randomize_layout” attribute. The second is an optional feature that 
will automatically select and randomize structures that are found to 
consist entirely of function pointers. This automatic selection feature 
can be extended to include other vulnerable structure types that are 
safe to randomize as they are identified. You can also opt a specific 
structure out of this feature with the “no_randomize_layout” attribute. 
Automatic structure selection is enabled with the “-randstruct-auto” 
compiler flag. By default, Randstruct seeds on the empty string, but a 
seed can be supplied with the “-randstruct-seed=” command line argument.

Clang Randstruct is the sum total of an undergraduate computer science 
capstone team’s effort.

Portland State University Clang Randstruct Capstone Team (Fall 
2018-Winter 2019):

Co-authored-by: Cole Nixon <nixontcole at gmail.com>
Co-authored-by: Connor Kuehl <cipkuehl at gmail.com>
Co-authored-by: James Foster <jafosterja at gmail.com>
Co-authored-by: Jeff Takahashi <jeffrey.takahashi at gmail.com>
Co-authored-by: Jordan Cantrell <jordan.cantrell at mail.com>
Co-authored-by: Nikk Forbus <nicholas.forbus at gmail.com>
Co-authored-by: Tim Pugh <nwtpugh at gmail.com>