randstruct: Reorganize Kconfigs and attribute macros

In preparation for Clang supporting randstruct, reorganize the Kconfigs,
move the attribute macros, and generalize the feature to be named
CONFIG_RANDSTRUCT for on/off, CONFIG_RANDSTRUCT_FULL for the full
randomization mode, and CONFIG_RANDSTRUCT_PERFORMANCE for the cache-line
sized mode.

Cc: linux-hardening@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220503205503.3054173-4-keescook@chromium.org

Documentation/kbuild/reproducible-builds.rst

@@ -99,10 +99,9 @@ unreproducible parts can be treated as sources:
 Structure randomisation
 -----------------------
 
-If you enable ``CONFIG_GCC_PLUGIN_RANDSTRUCT``, you will need to
-pre-generate the random seed in
-``scripts/gcc-plugins/randomize_layout_seed.h`` so the same value
-is used in rebuilds.
+If you enable ``CONFIG_RANDSTRUCT``, you will need to pre-generate
+the random seed in ``scripts/gcc-plugins/randomize_layout_seed.h``
+so the same value is used in rebuilds.
 
 Debug info conflicts
 --------------------

arch/riscv/Kconfig

@@ -468,7 +468,7 @@ config CC_HAVE_STACKPROTECTOR_TLS
 
 config STACKPROTECTOR_PER_TASK
 	def_bool y
-	depends on !GCC_PLUGIN_RANDSTRUCT
+	depends on !RANDSTRUCT
 	depends on STACKPROTECTOR && CC_HAVE_STACKPROTECTOR_TLS
 
 config PHYS_RAM_BASE_FIXED

arch/x86/mm/pti.c

@@ -540,7 +540,7 @@ static inline bool pti_kernel_image_global_ok(void)
 	 * cases where RANDSTRUCT is in use to help keep the layout a
 	 * secret.
 	 */
-	if (IS_ENABLED(CONFIG_GCC_PLUGIN_RANDSTRUCT))
+	if (IS_ENABLED(CONFIG_RANDSTRUCT))
 		return false;
 
 	return true;

include/linux/compiler-gcc.h

@@ -66,14 +66,6 @@
 		__builtin_unreachable();	\
 	} while (0)
 
-#if defined(RANDSTRUCT_PLUGIN) && !defined(__CHECKER__)
-#define __randomize_layout __attribute__((randomize_layout))
-#define __no_randomize_layout __attribute__((no_randomize_layout))
-/* This anon struct can add padding, so only enable it under randstruct. */
-#define randomized_struct_fields_start	struct {
-#define randomized_struct_fields_end	} __randomize_layout;
-#endif
-
 /*
  * GCC 'asm goto' miscompiles certain code sequences:
  *

include/linux/compiler_types.h

@@ -242,15 +242,15 @@ struct ftrace_likely_data {
 # define __latent_entropy
 #endif
 
-#ifndef __randomize_layout
+#if defined(RANDSTRUCT) && !defined(__CHECKER__)
+# define __randomize_layout __designated_init __attribute__((randomize_layout))
+# define __no_randomize_layout __attribute__((no_randomize_layout))
+/* This anon struct can add padding, so only enable it under randstruct. */
+# define randomized_struct_fields_start	struct {
+# define randomized_struct_fields_end	} __randomize_layout;
+#else
 # define __randomize_layout __designated_init
-#endif
-
-#ifndef __no_randomize_layout
 # define __no_randomize_layout
-#endif
-
-#ifndef randomized_struct_fields_start
 # define randomized_struct_fields_start
 # define randomized_struct_fields_end
 #endif

include/linux/vermagic.h

@@ -32,11 +32,11 @@
 #else
 #define MODULE_VERMAGIC_MODVERSIONS ""
 #endif
-#ifdef RANDSTRUCT_PLUGIN
+#ifdef RANDSTRUCT
 #include <generated/randomize_layout_hash.h>
-#define MODULE_RANDSTRUCT_PLUGIN "RANDSTRUCT_PLUGIN_" RANDSTRUCT_HASHED_SEED
+#define MODULE_RANDSTRUCT "RANDSTRUCT_" RANDSTRUCT_HASHED_SEED
 #else
-#define MODULE_RANDSTRUCT_PLUGIN
+#define MODULE_RANDSTRUCT
 #endif
 
 #define VERMAGIC_STRING 						\
@@ -44,6 +44,6 @@
 	MODULE_VERMAGIC_SMP MODULE_VERMAGIC_PREEMPT 			\
 	MODULE_VERMAGIC_MODULE_UNLOAD MODULE_VERMAGIC_MODVERSIONS	\
 	MODULE_ARCH_VERMAGIC						\
-	MODULE_RANDSTRUCT_PLUGIN
+	MODULE_RANDSTRUCT
 
 #endif /* _LINUX_VERMAGIC_H */

kernel/panic.c

@@ -48,7 +48,7 @@ unsigned int __read_mostly sysctl_oops_all_cpu_backtrace;
 
 int panic_on_oops = CONFIG_PANIC_ON_OOPS_VALUE;
 static unsigned long tainted_mask =
-	IS_ENABLED(CONFIG_GCC_PLUGIN_RANDSTRUCT) ? (1 << TAINT_RANDSTRUCT) : 0;
+	IS_ENABLED(CONFIG_RANDSTRUCT) ? (1 << TAINT_RANDSTRUCT) : 0;
 static int pause_on_oops;
 static int pause_on_oops_flag;
 static DEFINE_SPINLOCK(pause_on_oops_lock);

scripts/Makefile.gcc-plugins

@@ -24,8 +24,8 @@ gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_STRUCTLEAK)		\
 
 gcc-plugin-$(CONFIG_GCC_PLUGIN_RANDSTRUCT)	+= randomize_layout_plugin.so
 gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_RANDSTRUCT)		\
-		+= -DRANDSTRUCT_PLUGIN
-gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_RANDSTRUCT_PERFORMANCE)	\
+		+= -DRANDSTRUCT
+gcc-plugin-cflags-$(CONFIG_RANDSTRUCT_PERFORMANCE)		\
 		+= -fplugin-arg-randomize_layout_plugin-performance-mode
 
 gcc-plugin-$(CONFIG_GCC_PLUGIN_STACKLEAK)	+= stackleak_plugin.so

scripts/gcc-plugins/Kconfig

@@ -46,44 +46,6 @@ config GCC_PLUGIN_LATENT_ENTROPY
 	   * https://grsecurity.net/
 	   * https://pax.grsecurity.net/
 
-config GCC_PLUGIN_RANDSTRUCT
-	bool "Randomize layout of sensitive kernel structures"
-	select MODVERSIONS if MODULES
-	help
-	  If you say Y here, the layouts of structures that are entirely
-	  function pointers (and have not been manually annotated with
-	  __no_randomize_layout), or structures that have been explicitly
-	  marked with __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-plugins/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.
-
-	  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	# do not reduce test coverage
-	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 GCC_PLUGIN_ARM_SSP_PER_TASK
 	bool
 	depends on GCC_PLUGINS && ARM

security/Kconfig.hardening

@@ -266,4 +266,66 @@ config ZERO_CALL_USED_REGS
 
 endmenu
 
+choice
+	prompt "Randomize layout of sensitive kernel structures"
+	default RANDSTRUCT_FULL if COMPILE_TEST && GCC_PLUGINS
+	default RANDSTRUCT_NONE
+	help
+	  If you enable this, the layouts of structures that are entirely
+	  function pointers (and have not been manually annotated with
+	  __no_randomize_layout), or structures that have been explicitly
+	  marked with __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/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".
+
+	config RANDSTRUCT_NONE
+		bool "Disable structure layout randomization"
+		help
+		  Build normally: no structure layout randomization.
+
+	config RANDSTRUCT_FULL
+		bool "Fully randomize structure layout"
+		depends on GCC_PLUGINS
+		select MODVERSIONS if MODULES
+		help
+		  Fully randomize the member layout of sensitive
+		  structures as much as possible, which may have both a
+		  memory size and performance impact.
+
+	config RANDSTRUCT_PERFORMANCE
+		bool "Limit randomization of structure layout to cache-lines"
+		depends on GCC_PLUGINS
+		select MODVERSIONS if MODULES
+		help
+		  Randomization of sensitive kernel structures will make a
+		  best effort at restricting randomization to cacheline-sized
+		  groups of members. It will further not randomize bitfields
+		  in structures. This reduces the performance hit of RANDSTRUCT
+		  at the cost of weakened randomization.
+endchoice
+
+config RANDSTRUCT
+	def_bool !RANDSTRUCT_NONE
+
+config GCC_PLUGIN_RANDSTRUCT
+	def_bool GCC_PLUGINS && RANDSTRUCT
+	help
+	  Use GCC plugin to randomize structure layout.
+
+	  This plugin was ported from grsecurity/PaX. More
+	  information at:
+	   * https://grsecurity.net/
+	   * https://pax.grsecurity.net/
+
 endmenu