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

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Date:   Wed, 24 Jun 2020 12:46:32 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Will Deacon <will@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Fangrui Song <maskray@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        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 ML <x86@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 3/9] efi/libstub: Remove .note.gnu.property

On Wed, 24 Jun 2020 at 12:44, Will Deacon <will@...nel.org> wrote:
>
> On Tue, Jun 23, 2020 at 09:44:11PM -0700, Kees Cook wrote:
> > 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.
>
> For the kernel Image, how do we remove these sections? The objcopy flags
> in arch/arm64/boot/Makefile look both insufficient and out of date. My
> vmlinux ends up with both a ".notes" and a ".init.note.gnu.property"
> segment.
>

The latter is the fault of the libstub make rules, that prepend .init
to all section names.

I'm not sure if there is a point to having PAC and/or BTI in the EFI
stub, given that it runs under the control of the firmware, with its
memory mappings and PAC configuration etc.