On Wed, 24 Jun 2020 at 12:44, Will Deacon [off-list ref] wrote:

On Tue, Jun 23, 2020 at 09:44:11PM -0700, Kees Cook wrote:

quoted

On Tue, Jun 23, 2020 at 08:31:42PM -0700, 'Fangrui Song' via Clang Built Linux wrote:

quoted

On 2020-06-23, Kees Cook wrote:

quoted

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 <redacted>
---
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in arch/arm64/boot/Makefile look both insufficient and out of date.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.

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