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SubjectRe: [PATCH v4 05/17] ctype: Work around Clang -mbranch-protection=none bug
On Mon, Jun 29, 2020 at 8:06 AM Kees Cook <keescook@chromium.org> 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@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

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@google.com>

> +
> #
> # 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

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