Re: [PATCH v4 07/26] x86/build: Check W^X of vmlinux during build

[Ard Biesheuvel]

On Thu, 15 Dec 2022 at 13:38, Evgeniy Baskov <baskov@xxxxxxxxx> wrote:
>
> Check if there are simultaneously writable and executable
> program segments in vmlinux ELF image and fail build if there are any.
>
> This would prevent accidental introduction of RWX segments.
>
> Tested-by: Mario Limonciello <mario.limonciello@xxxxxxx>
> Tested-by: Peter Jones <pjones@xxxxxxxxxx>
> Signed-off-by: Evgeniy Baskov <baskov@xxxxxxxxx>
> ---
>  arch/x86/boot/compressed/Makefile | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index 1acff356d97a..4dcab38f5a38 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -112,11 +112,17 @@ vmlinux-objs-$(CONFIG_EFI) += $(obj)/efi.o
>  vmlinux-objs-$(CONFIG_EFI_MIXED) += $(obj)/efi_mixed.o
>  vmlinux-objs-$(CONFIG_EFI_STUB) += $(objtree)/drivers/firmware/efi/libstub/lib.a
>
> +quiet_cmd_wx_check = WXCHK   $<
> +cmd_wx_check = if $(OBJDUMP) -p $< | grep "flags .wx" > /dev/null; \
> +              then (echo >&2 "$<: Simultaneously writable and executable sections are prohibited"; \
> +                    /bin/false); fi
> +
>  $(obj)/vmlinux: $(vmlinux-objs-y) FORCE
>         $(call if_changed,ld)
>
>  OBJCOPYFLAGS_vmlinux.bin :=  -R .comment -S
>  $(obj)/vmlinux.bin: vmlinux FORCE
> +       $(call cmd,wx_check)

This breaks the way we track dependencies between make targets: the
FORCE will result in the check being performed every time, even if
nothing gets rebuilt.

Better to do something like the below (apologies for the alphabet soup)


--- a/arch/x86/boot/compressed/Makefile
+++ b/arch/x86/boot/compressed/Makefile
@@ -112,18 +112,17 @@ vmlinux-objs-$(CONFIG_EFI) += $(obj)/efi.o
 vmlinux-objs-$(CONFIG_EFI_MIXED) += $(obj)/efi_mixed.o
 vmlinux-objs-$(CONFIG_EFI_STUB) +=
$(objtree)/drivers/firmware/efi/libstub/lib.a

-quiet_cmd_wx_check = WXCHK   $<
-cmd_wx_check = if $(OBJDUMP) -p $< | grep "flags .wx" > /dev/null; \
-              then (echo >&2 "$<: Simultaneously writable and
executable sections are prohibited"; \
-                    /bin/false); fi
+quiet_cmd_objcopy_and_wx_check = $(quiet_cmd_objcopy)
+      cmd_objcopy_and_wx_check = if $(OBJDUMP) -p $< | grep "flags
.wx" > /dev/null; then \
+                                       (echo >&2 "$<: Simultaneously
writable and executable sections are prohibited"; \
+                                       /bin/false); else $(cmd_objcopy); fi

 $(obj)/vmlinux: $(vmlinux-objs-y) FORCE
        $(call if_changed,ld)

 OBJCOPYFLAGS_vmlinux.bin :=  -R .comment -S
 $(obj)/vmlinux.bin: vmlinux FORCE
-       $(call cmd,wx_check)
-       $(call if_changed,objcopy)
+       $(call if_changed,objcopy_and_wx_check)