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

[Will Deacon]

On Wed, Jun 24, 2020 at 12:46:32PM +0200, Ard Biesheuvel wrote:
> On Wed, 24 Jun 2020 at 12:44, Will Deacon <will@xxxxxxxxxx> 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:
> > > > 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 in arch/arm64/boot/Makefile look both insufficient and out of date.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.

Hmm. I tried adding -mbranch-protection=none to arm64 cflags for the stub,
but I still see this note in vmlinux. It looks like it comes in via the
stub copy of lib-ctype.o, but I don't know why that would force the
note. The cflags look ok to me [1] and I confirmed that the note is
being generated by the compiler.

> 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.

Agreed, I just can't figure out how to get rid of the note.

Will

[1] -mlittle-endian -DKASAN_SHADOW_SCALE_SHIFT=3 -Qunused-arguments -Wall -Wundef -Werror=strict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -fshort-wchar -fno-PIE -Werror=implicit-function-declaration -Werror=implicit-int -Wno-format-security -std=gnu89 --target=aarch64-linux-gnu --prefix=/usr/local/google/home/willdeacon/bin/ --gcc-toolchain=/usr/local/google/home/willdeacon -no-integrated-as -Werror=unknown-warning-option -mgeneral-regs-only -DCONFIG_CC_HAS_K_CONSTRAINT=1 -fno-asynchronous-unwind-tables -mbranch-protection=pac-ret+leaf+bti -Wa,-march=armv8.3-a -DKASAN_SHADOW_SCALE_SHIFT=3 -fno-delete-null-pointer-checks -Wno-address-of-packed-member -O2 -Wframe-larger-than=2048 -fstack-protector-strong -Wno-format-invalid-specifier -Wno-gnu -mno-global-merge -Wno-unused-const-variable -fno-omit-frame-pointer -fno-optimize-sibling-calls -g -Wdeclaration-after-statement -Wvla -Wno-pointer-sign -Wno-array-bounds -fno-strict-overflow -fno-merge-all-constants -fno-stack-check -Werror=date-time -Werror=incompatible-pointer-types -fmacro-prefix-map=./= -Wno-initializer-overrides -Wno-format -Wno-sign-compare -Wno-format-zero-length -Wno-tautological-constant-out-of-range-compare -fpie -mbranch-protection=none -I./scripts/dtc/libfdt -Os -DDISABLE_BRANCH_PROFILING -include ./drivers/firmware/efi/libstub/hidden.h -D__NO_FORTIFY -ffreestanding -fno-stack-protector -fno-addrsig -D__DISABLE_EXPORTS    -DKBUILD_MODFILE='"drivers/firmware/efi/libstub/lib-ctype"' -DKBUILD_BASENAME='"lib_ctype"' -DKBUILD_MODNAME='"lib_ctype"' -c -o drivers/firmware/efi/libstub/lib-ctype.o lib/ctype.c