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

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Message-ID: <202006232143.66828CD3@keescook>
Date:   Tue, 23 Jun 2020 21:44:11 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Fangrui Song <maskray@...gle.com>
Cc:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Peter Collingbourne <pcc@...gle.com>,
        James Morse <james.morse@....com>,
        Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Arnd Bergmann <arnd@...db.de>, x86@...nel.org,
        clang-built-linux@...glegroups.com, linux-arch@...r.kernel.org,
        linux-efi@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/9] efi/libstub: Remove .note.gnu.property

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@...omium.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