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

[Kees Cook]

On Mon, Jun 29, 2020 at 05:08:03PM +0200, Ard Biesheuvel wrote:
> On Mon, 29 Jun 2020 at 17:06, 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
> > +
> >  #
> >  # 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
> >
> 
> 
> Looks fine
> 
> Acked-by: Ard Biesheuvel <ardb@xxxxxxxxxx>

Thanks!

> if you want to keep it with the set, or I can take it as a EFI fix.

My current plan is to just keep it all together.

-- 
Kees Cook