From: Nick Desaulniers <ndesaulniers@google.com> To: Kees Cook <keescook@chromium.org> Cc: Ard Biesheuvel <ardb@kernel.org>, Will Deacon <will@kernel.org>, Dave Martin <Dave.Martin@arm.com>, clang-built-linux <clang-built-linux@googlegroups.com>, Catalin Marinas <catalin.marinas@arm.com>, Mark Rutland <mark.rutland@arm.com>, Peter Collingbourne <pcc@google.com>, James Morse <james.morse@arm.com>, Borislav Petkov <bp@suse.de>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, Russell King <linux@armlinux.org.uk>, Masahiro Yamada <masahiroy@kernel.org>, Arvind Sankar <nivedita@alum.mit.edu>, Nathan Chancellor <natechancellor@gmail.com>, Arnd Bergmann <arnd@arndb.de>, X86 ML <x86@kernel.org>, linux-arch <linux-arch@vger.kernel.org>, linux-efi <linux-efi@vger.kernel.org>, Linux ARM <linux-arm-kernel@lists.infradead.org> Subject: Re: [PATCH v4 05/17] ctype: Work around Clang -mbranch-protection=none bug Date: Mon, 29 Jun 2020 11:02:51 -0700 [thread overview] Message-ID: <CAKwvOd=DMfmvfiEX7KDPLs75SbNz+LAGSwC3V_=LgGH3kjtE=g@mail.gmail.com> (raw) In-Reply-To: <202006290806.3BDE2A8@keescook> 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
WARNING: multiple messages have this Message-ID (diff)
From: Nick Desaulniers <ndesaulniers@google.com> To: Kees Cook <keescook@chromium.org> Cc: Ard Biesheuvel <ardb@kernel.org>, Will Deacon <will@kernel.org>, Dave Martin <Dave.Martin@arm.com>, clang-built-linux <clang-built-linux@googlegroups.com>, Catalin Marinas <catalin.marinas@arm.com>, Mark Rutland <mark.rutland@arm.com>, Peter Collingbourne <pcc@google.com>, James Morse <james.morse@arm.com>, Borislav Petkov <bp@suse.de>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, Russell King <linux@armlinux.org.uk>, Masahiro Yamada <masahiroy@kernel.org>, Arvind Sankar <nivedita@alum.mit.edu>, Nathan Chancellor <natechancellor@gmail.com>, Arnd Bergmann <arnd@arndb.de>, X86 ML <x86@kernel.org>, linux-arch <linux-arch@vger.kernel.org>, linux-efi <linux-efi@vger.kernel.org>, Linux ARM <linux-arm-kernel@lists.infradead.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Daniel Kiss <daniel.kiss@arm.com> Subject: Re: [PATCH v4 05/17] ctype: Work around Clang -mbranch-protection=none bug Date: Mon, 29 Jun 2020 11:02:51 -0700 [thread overview] Message-ID: <CAKwvOd=DMfmvfiEX7KDPLs75SbNz+LAGSwC3V_=LgGH3kjtE=g@mail.gmail.com> (raw) Message-ID: <20200629180251.a5RhWokyvmxcTGRw7JUdatXz3OhyLnfQZ544SwDpt4M@z> (raw) In-Reply-To: <202006290806.3BDE2A8@keescook> 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
next prev parent reply other threads:[~2020-06-29 18:02 UTC|newest] Thread overview: 70+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-06-29 6:18 [PATCH v4 00/17] Warn on orphan section placement Kees Cook 2020-06-29 6:18 ` Kees Cook 2020-06-29 6:18 ` [PATCH v4 01/17] vmlinux.lds.h: Add .gnu.version* to DISCARDS Kees Cook 2020-06-29 6:18 ` Kees Cook 2020-06-29 6:18 ` [PATCH v4 02/17] vmlinux.lds.h: Avoid KASAN and KCSAN's unwanted sections Kees Cook 2020-06-29 6:18 ` Kees Cook 2020-06-29 6:18 ` [PATCH v4 03/17] vmlinux.lds.h: Split ELF_DETAILS from STABS_DEBUG Kees Cook 2020-06-29 6:18 ` Kees Cook 2020-06-29 6:18 ` [PATCH v4 04/17] vmlinux.lds.h: Add .symtab, .strtab, and .shstrtab to ELF_DETAILS Kees Cook 2020-06-29 6:18 ` Kees Cook 2020-06-29 6:18 ` [PATCH v4 05/17] ctype: Work around Clang -mbranch-protection=none bug Kees Cook 2020-06-29 6:18 ` Kees Cook 2020-06-29 8:15 ` Ard Biesheuvel 2020-06-29 8:15 ` Ard Biesheuvel 2020-06-29 14:43 ` Kees Cook 2020-06-29 14:43 ` Kees Cook 2020-06-29 15:06 ` Kees Cook 2020-06-29 15:06 ` Kees Cook 2020-06-29 15:08 ` Ard Biesheuvel 2020-06-29 15:08 ` Ard Biesheuvel 2020-06-29 15:16 ` Kees Cook 2020-06-29 15:16 ` Kees Cook 2020-06-29 18:02 ` Nick Desaulniers [this message] 2020-06-29 18:02 ` Nick Desaulniers 2020-06-29 18:37 ` Kees Cook 2020-06-29 18:37 ` Kees Cook 2020-06-29 6:18 ` [PATCH v4 06/17] efi/libstub: Disable -mbranch-protection Kees Cook 2020-06-29 6:18 ` Kees Cook 2020-06-29 6:18 ` [PATCH v4 07/17] arm64/build: Use common DISCARDS in linker script Kees Cook 2020-06-29 6:18 ` Kees Cook 2020-06-29 6:18 ` [PATCH v4 08/17] arm64/mm: Remove needless section quotes Kees Cook 2020-06-29 6:18 ` Kees Cook 2020-06-29 19:53 ` Nick Desaulniers 2020-06-29 19:53 ` Nick Desaulniers 2020-06-29 20:03 ` Joe Perches 2020-06-29 20:03 ` Joe Perches 2020-06-29 20:29 ` Nick Desaulniers 2020-06-29 20:29 ` Nick Desaulniers 2020-06-29 20:04 ` Kees Cook 2020-06-29 20:04 ` Kees Cook 2020-06-29 20:54 ` Arvind Sankar 2020-06-29 20:54 ` Arvind Sankar 2020-06-29 21:06 ` Arvind Sankar 2020-06-29 21:06 ` Arvind Sankar 2020-06-29 6:18 ` [PATCH v4 09/17] arm64/build: Remove .eh_frame* sections due to unwind tables Kees Cook 2020-06-29 6:18 ` Kees Cook 2020-06-29 6:18 ` [PATCH v4 10/17] arm64/kernel: Remove needless Call Frame Information annotations Kees Cook 2020-06-29 6:18 ` [PATCH v4 11/17] arm64/build: Warn on orphan section placement Kees Cook 2020-06-29 6:18 ` Kees Cook 2020-06-29 6:18 ` [PATCH v4 12/17] arm/build: Refactor linker script headers Kees Cook 2020-06-29 6:18 ` Kees Cook 2020-06-29 6:18 ` [PATCH v4 13/17] arm/build: Explicitly keep .ARM.attributes sections Kees Cook 2020-06-29 6:18 ` [PATCH v4 14/17] arm/build: Warn on orphan section placement Kees Cook 2020-06-29 6:18 ` Kees Cook 2020-06-29 15:54 ` Arvind Sankar 2020-06-29 15:54 ` Arvind Sankar 2020-06-29 18:07 ` Russell King - ARM Linux admin 2020-06-29 18:07 ` Russell King - ARM Linux admin 2020-06-29 18:15 ` Arvind Sankar 2020-06-29 18:15 ` Arvind Sankar 2020-06-29 19:03 ` Russell King - ARM Linux admin 2020-06-29 19:47 ` Nick Desaulniers 2020-06-29 19:47 ` Nick Desaulniers 2020-06-29 6:18 ` [PATCH v4 15/17] arm/boot: " Kees Cook 2020-06-29 6:18 ` Kees Cook 2020-06-29 6:18 ` [PATCH v4 16/17] x86/build: " Kees Cook 2020-06-29 6:18 ` [PATCH v4 17/17] x86/boot: " Kees Cook 2020-06-29 6:18 ` Kees Cook 2020-08-21 16:02 ` [PATCH v4 00/17] " Will Deacon 2020-08-21 17:36 ` Kees Cook
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