On Wed, Jun 24, 2020 at 12:26:47PM +0100, Will Deacon wrote: > 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 in arch/arm64/boot/Makefile look both insufficient and out of date.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. Because this section is generated by the linker itself I think you might have to send it to /DISCARD/ in the link, or strip it explicitly after linking. Cheers ---Dave
