Re: [PATCH v4 05/17] ctype: Work around Clang -mbranch-protection=none bug

Kees Cook
2020-06-29T18:42:43Z

On Mon, Jun 29, 2020 at 10:15:47AM +0200, Ard Biesheuvel wrote:
> On Mon, 29 Jun 2020 at 08:18, Kees Cook <keescook@chromium.org> wrote:
> >
> > In preparation for building efi/libstub with -mbranch-protection=none
> > (EFI does not support branch protection features[1]), add no-op code
> > to work around a Clang bug that emits an unwanted .note.gnu.property
> > section for object files without code[2].
> >
> > [1] https://lore.kernel.org/lkml/CAMj1kXHck12juGi=E=P4hWP_8vQhQ+-x3vBMc3TGeRWdQ-XkxQ@mail.gmail.com
> > [2] https://bugs.llvm.org/show_bug.cgi?id=46480
> >
> > Cc: Ard Biesheuvel <ardb@kernel.org>
> > Cc: Will Deacon <will@kernel.org>
> > Cc: Dave Martin <Dave.Martin@arm.com>
> > Cc: clang-built-linux@googlegroups.com
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> > ---
> > lib/ctype.c | 10 ++++++++++
> > 1 file changed, 10 insertions(+)
> >
> > diff --git a/lib/ctype.c b/lib/ctype.c
> > index c819fe269eb2..21245ed57d90 100644
> > --- a/lib/ctype.c
> > +++ b/lib/ctype.c
> > @@ -36,3 +36,13 @@ _L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L,_L, /* 224-239 */
> > _L,_L,_L,_L,_L,_L,_L,_P,_L,_L,_L,_L,_L,_L,_L,_L}; /* 240-255 */
> >
> > EXPORT_SYMBOL(_ctype);
> > +
> > +/*
> > + * Clang will generate .note.gnu.property sections for object files
> > + * without code, even in the presence of -mbranch-protection=none.
> > + * To work around this, define an unused static function.
> > + * https://bugs.llvm.org/show_bug.cgi?id=46480
> > + */
> > +#ifdef CONFIG_CC_IS_CLANG
> > +void __maybe_unused __clang_needs_code_here(void) { }
> > +#endif
> > --
> > 2.25.1
> >
>
> I take it we don't need this horrible hack if we build the EFI stub
> with branch protections and filter out the .note.gnu.property section
> explicitly?
>
> Sorry to backpedal, but that is probably a better approach after all,
> given that the instructions don't hurt, and we will hopefully be able
> to arm them once UEFI (as well as PE/COFF) gets around to describing
> this in a way that both the firmware and the OS can consume.

How does this look?

commit 051ef0b75a386c3fe2f216d16246468147a48c5b
Author: Kees Cook <keescook@chromium.org>
Date: Tue Jun 23 18:02:56 2020 -0700

efi/libstub: Disable -mbranch-protection

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@mail.gmail.com
[2] https://bugs.llvm.org/show_bug.cgi?id=46480

Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Arvind Sankar <nivedita@alum.mit.edu>
Cc: Atish Patra <atish.patra@wdc.com>
Cc: linux-efi@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>

diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
index 75daaf20374e..f9f1922f8f28 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

--
Kees Cook