On Mon, Jun 29, 2020 at 8:06 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote: > > On Mon, Jun 29, 2020 at 10:15:47AM +0200, Ard Biesheuvel wrote: > > On Mon, 29 Jun 2020 at 08:18, Kees Cook <keescook@xxxxxxxxxxxx> 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@xxxxxxxxxxxxxx > > > [2] https://bugs.llvm.org/show_bug.cgi?id=46480 > > > > > > Cc: Ard Biesheuvel <ardb@xxxxxxxxxx> > > > Cc: Will Deacon <will@xxxxxxxxxx> > > > Cc: Dave Martin <Dave.Martin@xxxxxxx> > > > Cc: clang-built-linux@xxxxxxxxxxxxxxxx > > > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> > > > --- > > > 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@xxxxxxxxxxxx> > 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@xxxxxxxxxxxxxx > [2] https://bugs.llvm.org/show_bug.cgi?id=46480 > > Cc: Ard Biesheuvel <ardb@xxxxxxxxxx> > Cc: Arvind Sankar <nivedita@xxxxxxxxxxxx> > Cc: Atish Patra <atish.patra@xxxxxxx> > Cc: linux-efi@xxxxxxxxxxxxxxx > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> > > 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 This is definitely better than the empty function. Though a patch is posted for fixing this in LLVM. Assuming that lands before this, we might not actually need this workaround? arch/arm64/Kconfig 1625 config ARM64_BTI_KERNEL ... 1633 # https://reviews.llvm.org/rGb8ae3fdfa579dbf366b1bb1cbfdbf8c51db7fa55 1634 depends on !CC_IS_CLANG || CLANG_VERSION >= 100001 So if Daniel's patch lands AND is backported into the clang 10.0.1 release, then we might not need to carry this workaround? Either way, Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> > + > # > # 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 -- Thanks, ~Nick Desaulniers
