[PATCH v6 07/29] efi/libstub: Disable -mbranch-protection

[Kees Cook]

In preparation for adding --orphan-handling=warn to more architectures,
disable -mbranch-protection, as EFI does not yet support it[1].  This was
noticed due to it producing unwanted .note.gnu.property sections (prefixed
with .init due to the objcopy build step).

However, we must also work around a bug in Clang where the section is
still emitted for code-less object files[2], so also remove the section
during the objcopy.

[1] https://lore.kernel.org/lkml/CAMj1kXHck12juGi=E=P4hWP_8vQhQ+-x3vBMc3TGeRWdQ-XkxQ@xxxxxxxxxxxxxx
[2] https://bugs.llvm.org/show_bug.cgi?id=46480

Cc: Arvind Sankar <nivedita@xxxxxxxxxxxx>
Cc: Atish Patra <atish.patra@xxxxxxx>
Cc: linux-efi@xxxxxxxxxxxxxxx
Acked-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 drivers/firmware/efi/libstub/Makefile | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
index 5eefd60917df..0c911e391d75 100644
--- a/drivers/firmware/efi/libstub/Makefile
+++ b/drivers/firmware/efi/libstub/Makefile
@@ -18,7 +18,8 @@ cflags-$(CONFIG_X86)		+= -m$(BITS) -D__KERNEL__ \
 # arm64 uses the full KBUILD_CFLAGS so it's necessary to explicitly
 # disable the stackleak plugin
 cflags-$(CONFIG_ARM64)		:= $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS)) \
-				   -fpie $(DISABLE_STACKLEAK_PLUGIN)
+				   -fpie $(DISABLE_STACKLEAK_PLUGIN) \
+				   $(call cc-option,-mbranch-protection=none)
 cflags-$(CONFIG_ARM)		:= $(subst $(CC_FLAGS_FTRACE),,$(KBUILD_CFLAGS)) \
 				   -fno-builtin -fpic \
 				   $(call cc-option,-mno-single-pic-base)
@@ -66,6 +67,12 @@ lib-$(CONFIG_X86)		+= x86-stub.o
 CFLAGS_arm32-stub.o		:= -DTEXT_OFFSET=$(TEXT_OFFSET)
 CFLAGS_arm64-stub.o		:= -DTEXT_OFFSET=$(TEXT_OFFSET)
 
+# Even when -mbranch-protection=none is set, Clang will generate a
+# .note.gnu.property for code-less object files (like lib/ctype.c),
+# so work around this by explicitly removing the unwanted section.
+# https://bugs.llvm.org/show_bug.cgi?id=46480
+STUBCOPY_FLAGS-y		+= --remove-section=.note.gnu.property
+
 #
 # For x86, bootloaders like systemd-boot or grub-efi do not zero-initialize the
 # .bss section, so the .bss section of the EFI stub needs to be included in the
-- 
2.25.1