Re: [PATCH v3 3/9] efi/libstub: Remove .note.gnu.property

From: Kees Cook <keescook@chromium.org>
To: Fangrui Song <maskray@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	linux-arch@vger.kernel.org, linux-efi@vger.kernel.org,
	Arnd Bergmann <arnd@arndb.de>,
	Peter Collingbourne <pcc@google.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Masahiro Yamada <masahiroy@kernel.org>,
	x86@kernel.org, Nick Desaulniers <ndesaulniers@google.com>,
	Russell King <linux@armlinux.org.uk>,
	linux-kernel@vger.kernel.org,
	Nathan Chancellor <natechancellor@gmail.com>,
	clang-built-linux@googlegroups.com,
	Arvind Sankar <nivedita@alum.mit.edu>,
	Ingo Molnar <mingo@redhat.com>, James Morse <james.morse@arm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@suse.de>, Will Deacon <will@kernel.org>,
	Ard Biesheuvel <ardb@kernel.org>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v3 3/9] efi/libstub: Remove .note.gnu.property
Date: Tue, 23 Jun 2020 21:44:11 -0700	[thread overview]
Message-ID: <202006232143.66828CD3@keescook> (raw)
In-Reply-To: <20200624033142.cinvg6rbg252j46d@google.com>

On Tue, Jun 23, 2020 at 08:31:42PM -0700, 'Fangrui Song' via Clang Built Linux wrote:
> On 2020-06-23, Kees Cook wrote:
> > In preparation for adding --orphan-handling=warn to more architectures,
> > make sure unwanted sections don't end up appearing under the .init
> > section prefix that libstub adds to itself during objcopy.
> > 
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> > ---
> > drivers/firmware/efi/libstub/Makefile | 3 +++
> > 1 file changed, 3 insertions(+)
> > 
> > diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
> > index 75daaf20374e..9d2d2e784bca 100644
> > --- a/drivers/firmware/efi/libstub/Makefile
> > +++ b/drivers/firmware/efi/libstub/Makefile
> > @@ -66,6 +66,9 @@ lib-$(CONFIG_X86)		+= x86-stub.o
> > CFLAGS_arm32-stub.o		:= -DTEXT_OFFSET=$(TEXT_OFFSET)
> > CFLAGS_arm64-stub.o		:= -DTEXT_OFFSET=$(TEXT_OFFSET)
> > 
> > +# Remove unwanted sections first.
> > +STUBCOPY_FLAGS-y		+= --remove-section=.note.gnu.property
> > +
> > #
> > # For x86, bootloaders like systemd-boot or grub-efi do not zero-initialize the
> > # .bss section, so the .bss section of the EFI stub needs to be included in the
> 
> arch/arm64/Kconfig enables ARM64_PTR_AUTH by default. When the config is on
> 
> ifeq ($(CONFIG_ARM64_BTI_KERNEL),y)
> branch-prot-flags-$(CONFIG_CC_HAS_BRANCH_PROT_PAC_RET_BTI) := -mbranch-protection=pac-ret+leaf+bti
> else
> branch-prot-flags-$(CONFIG_CC_HAS_BRANCH_PROT_PAC_RET) := -mbranch-protection=pac-ret+leaf
> endif
> 
> This option creates .note.gnu.property:
> 
> % readelf -n drivers/firmware/efi/libstub/efi-stub.o
> 
> Displaying notes found in: .note.gnu.property
>   Owner                Data size        Description
>   GNU                  0x00000010       NT_GNU_PROPERTY_TYPE_0
>       Properties: AArch64 feature: PAC
> 
> If .note.gnu.property is not desired in drivers/firmware/efi/libstub, specifying
> -mbranch-protection=none can override -mbranch-protection=pac-ret+leaf

We want to keep the branch protection enabled. But since it's not a
"regular" ELF, we don't need to keep the property that identifies the
feature.

-- 
Kees Cook

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