Donate to e Foundation | Murena handsets with /e/OS | Own a part of Murena! Learn more

Commit 313dd1b6 authored by Kees Cook's avatar Kees Cook
Browse files

gcc-plugins: Add the randstruct plugin 



This randstruct plugin is modified from Brad Spengler/PaX Team's code
in the last public patch of grsecurity/PaX based on my understanding
of the code. Changes or omissions from the original code are mine and
don't reflect the original grsecurity/PaX code.

The randstruct GCC plugin randomizes the layout of selected structures
at compile time, as a probabilistic defense against attacks that need to
know the layout of structures within the kernel. This is most useful for
"in-house" kernel builds where neither the randomization seed nor other
build artifacts are made available to an attacker. While less useful for
distribution kernels (where the randomization seed must be exposed for
third party kernel module builds), it still has some value there since now
all kernel builds would need to be tracked by an attacker.

In more performance sensitive scenarios, GCC_PLUGIN_RANDSTRUCT_PERFORMANCE
can be selected to make a best effort to restrict randomization to
cacheline-sized groups of elements, and will not randomize bitfields. This
comes at the cost of reduced randomization.

Two annotations are defined,__randomize_layout and __no_randomize_layout,
which respectively tell the plugin to either randomize or not to
randomize instances of the struct in question. Follow-on patches enable
the auto-detection logic for selecting structures for randomization
that contain only function pointers. It is disabled here to assist with
bisection.

Since any randomized structs must be initialized using designated
initializers, __randomize_layout includes the __designated_init annotation
even when the plugin is disabled so that all builds will require
the needed initialization. (With the plugin enabled, annotations for
automatically chosen structures are marked as well.)

The main differences between this implemenation and grsecurity are:
- disable automatic struct selection (to be enabled in follow-up patch)
- add designated_init attribute at runtime and for manual marking
- clarify debugging output to differentiate bad cast warnings
- add whitelisting infrastructure
- support gcc 7's DECL_ALIGN and DECL_MODE changes (Laura Abbott)
- raise minimum required GCC version to 4.7

Earlier versions of this patch series were ported by Michael Leibowitz.

Signed-off-by: default avatarKees Cook <keescook@chromium.org>
parent 0aa5e49c

Documentation/dontdiff

+2 −0
Original line number Diff line number Diff line
@@ -207,6 +207,8 @@ r200_reg_safe.h 
r300_reg_safe.h

r420_reg_safe.h

r600_reg_safe.h

randomize_layout_hash.h

randomize_layout_seed.h

recordmcount

relocs

rlim_names.h

arch/Kconfig

+39 −0
Original line number Diff line number Diff line
@@ -443,6 +443,45 @@ config GCC_PLUGIN_STRUCTLEAK_VERBOSE 
	  initialized. Since not all existing initializers are detected

	  by the plugin, this can produce false positive warnings.



config GCC_PLUGIN_RANDSTRUCT

	bool "Randomize layout of sensitive kernel structures"

	depends on GCC_PLUGINS

	select MODVERSIONS if MODULES

	help

	  If you say Y here, the layouts of structures explicitly

	  marked by __randomize_layout will be randomized at

	  compile-time.  This can introduce the requirement of an

	  additional information exposure vulnerability for exploits

	  targeting these structure types.



	  Enabling this feature will introduce some performance impact,

	  slightly increase memory usage, and prevent the use of forensic

	  tools like Volatility against the system (unless the kernel

	  source tree isn't cleaned after kernel installation).



	  The seed used for compilation is located at

	  scripts/gcc-plgins/randomize_layout_seed.h.  It remains after

	  a make clean to allow for external modules to be compiled with

	  the existing seed and will be removed by a make mrproper or

	  make distclean.



	  Note that the implementation requires gcc 4.7 or newer.



	  This plugin was ported from grsecurity/PaX. More information at:

	   * https://grsecurity.net/

	   * https://pax.grsecurity.net/



config GCC_PLUGIN_RANDSTRUCT_PERFORMANCE

	bool "Use cacheline-aware structure randomization"

	depends on GCC_PLUGIN_RANDSTRUCT

	depends on !COMPILE_TEST

	help

	  If you say Y here, the RANDSTRUCT randomization will make a

	  best effort at restricting randomization to cacheline-sized

	  groups of elements.  It will further not randomize bitfields

	  in structures.  This reduces the performance hit of RANDSTRUCT

	  at the cost of weakened randomization.



config HAVE_CC_STACKPROTECTOR

	bool

	help

include/linux/compiler-gcc.h

+5 −0
Original line number Diff line number Diff line
@@ -223,6 +223,11 @@ 
/* Mark a function definition as prohibited from being cloned. */

#define __noclone	__attribute__((__noclone__, __optimize__("no-tracer")))



#ifdef RANDSTRUCT_PLUGIN

#define __randomize_layout __attribute__((randomize_layout))

#define __no_randomize_layout __attribute__((no_randomize_layout))

#endif



#endif /* GCC_VERSION >= 40500 */



#if GCC_VERSION >= 40600

include/linux/compiler.h

+8 −0
Original line number Diff line number Diff line
@@ -448,6 +448,14 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s 
# define __latent_entropy

#endif



#ifndef __randomize_layout

# define __randomize_layout __designated_init

#endif



#ifndef __no_randomize_layout

# define __no_randomize_layout

#endif



/*

 * Tell gcc if a function is cold. The compiler will assume any path

 * directly leading to the call is unlikely.

include/linux/vermagic.h

+8 −1
Original line number Diff line number Diff line
@@ -24,10 +24,17 @@ 
#ifndef MODULE_ARCH_VERMAGIC

#define MODULE_ARCH_VERMAGIC ""

#endif

#ifdef RANDSTRUCT_PLUGIN

#include <generated/randomize_layout_hash.h>

#define MODULE_RANDSTRUCT_PLUGIN "RANDSTRUCT_PLUGIN_" RANDSTRUCT_HASHED_SEED

#else

#define MODULE_RANDSTRUCT_PLUGIN

#endif



#define VERMAGIC_STRING 						\

	UTS_RELEASE " "							\

	MODULE_VERMAGIC_SMP MODULE_VERMAGIC_PREEMPT 			\

	MODULE_VERMAGIC_MODULE_UNLOAD MODULE_VERMAGIC_MODVERSIONS	\

	MODULE_ARCH_VERMAGIC

	MODULE_ARCH_VERMAGIC						\

	MODULE_RANDSTRUCT_PLUGIN