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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id f23sm244869pja.8.2020.06.29.08.06.21 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 29 Jun 2020 08:06:21 -0700 (PDT) Date: Mon, 29 Jun 2020 08:06:20 -0700 From: Kees Cook To: Ard Biesheuvel Cc: Will Deacon , Dave Martin , clang-built-linux , Catalin Marinas , Mark Rutland , Peter Collingbourne , James Morse , Borislav Petkov , Thomas Gleixner , Ingo Molnar , Russell King , Masahiro Yamada , Arvind Sankar , Nick Desaulniers , Nathan Chancellor , Arnd Bergmann , X86 ML , linux-arch , linux-efi , Linux ARM , Linux Kernel Mailing List Subject: Re: [PATCH v4 05/17] ctype: Work around Clang -mbranch-protection=none bug Message-ID: <202006290806.3BDE2A8@keescook> References: <20200629061840.4065483-1-keescook@chromium.org> <20200629061840.4065483-6-keescook@chromium.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-efi-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-efi@vger.kernel.org On Mon, Jun 29, 2020 at 10:15:47AM +0200, Ard Biesheuvel wrote: > On Mon, 29 Jun 2020 at 08:18, Kees Cook 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 > > Cc: Will Deacon > > Cc: Dave Martin > > Cc: clang-built-linux@googlegroups.com > > Signed-off-by: Kees Cook > > --- > > 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 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 Cc: Arvind Sankar Cc: Atish Patra Cc: linux-efi@vger.kernel.org Signed-off-by: Kees Cook 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