x86: Use -mindirect-branch-cs-prefix for RETPOLINE builds

In order to further enable commit:

  bbe2df3f6b6d ("x86/alternative: Try inline spectre_v2=retpoline,amd")

add the new GCC flag -mindirect-branch-cs-prefix:

  https://gcc.gnu.org/g:2196a681d7810ad8b227bf983f38ba716620545e
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102952
  https://bugs.llvm.org/show_bug.cgi?id=52323

to RETPOLINE=y builds. This should allow fully inlining retpoline,amd
for GCC builds.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lkml.kernel.org/r/20211119165630.276205624@infradead.org

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 2f40de5d82a2c362b699f5211a0fcb4d25d0fa2d..c38b6577c103da44f775a40a69506ae409c30c3f 100644 (file)
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -14,6 +14,7 @@ endif
 
 ifdef CONFIG_CC_IS_GCC
 RETPOLINE_CFLAGS       := $(call cc-option,-mindirect-branch=thunk-extern -mindirect-branch-register)
+RETPOLINE_CFLAGS       += $(call cc-option,-mindirect-branch-cs-prefix)
 RETPOLINE_VDSO_CFLAGS  := $(call cc-option,-mindirect-branch=thunk-inline -mindirect-branch-register)
 endif
 ifdef CONFIG_CC_IS_CLANG