randstruct: Enable Clang support

Clang 15 will support randstruct via the -frandomize-layout-seed-file=...
option. Update the Kconfig and Makefile to recognize this feature.

Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: linux-kbuild@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220503205503.3054173-7-keescook@chromium.org

diff --git a/scripts/Makefile.randstruct b/scripts/Makefile.randstruct
index 4d741e6..24e283e 100644 (file)
--- a/scripts/Makefile.randstruct
+++ b/scripts/Makefile.randstruct
@@ -7,6 +7,9 @@ randstruct-cflags-y     \
        += -fplugin=$(objtree)/scripts/gcc-plugins/randomize_layout_plugin.so
 randstruct-cflags-$(CONFIG_RANDSTRUCT_PERFORMANCE)             \
        += -fplugin-arg-randomize_layout_plugin-performance-mode
+else
+randstruct-cflags-y    \
+       += -frandomize-layout-seed-file=$(objtree)/scripts/basic/randstruct.seed
 endif
 
 export RANDSTRUCT_CFLAGS := $(randstruct-cflags-y)

diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
index 0277ba5..bd2aabb 100644 (file)
--- a/security/Kconfig.hardening
+++ b/security/Kconfig.hardening
@@ -266,9 +266,12 @@ config ZERO_CALL_USED_REGS
 
 endmenu
 
+config CC_HAS_RANDSTRUCT
+       def_bool $(cc-option,-frandomize-layout-seed-file=/dev/null)
+
 choice
        prompt "Randomize layout of sensitive kernel structures"
-       default RANDSTRUCT_FULL if COMPILE_TEST && GCC_PLUGINS
+       default RANDSTRUCT_FULL if COMPILE_TEST && (GCC_PLUGINS || CC_HAS_RANDSTRUCT)
        default RANDSTRUCT_NONE
        help
          If you enable this, the layouts of structures that are entirely
@@ -297,13 +300,20 @@ choice
 
        config RANDSTRUCT_FULL
                bool "Fully randomize structure layout"
-               depends on GCC_PLUGINS
+               depends on CC_HAS_RANDSTRUCT || 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.
 
+                 One difference between the Clang and GCC plugin
+                 implementations is the handling of bitfields. The GCC
+                 plugin treats them as fully separate variables,
+                 introducing sometimes significant padding. Clang tries
+                 to keep adjacent bitfields together, but with their bit
+                 ordering randomized.
+
        config RANDSTRUCT_PERFORMANCE
                bool "Limit randomization of structure layout to cache-lines"
                depends on GCC_PLUGINS