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

[Dave Martin]

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 in arch/arm64/boot/Makefile look both insufficient and out of date.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