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

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Date:   Wed, 08 Mar 2023 19:05:18 +0300
From:   Evgeniy Baskov <baskov@...ras.ru>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Borislav Petkov <bp@...en8.de>, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexey Khoroshilov <khoroshilov@...ras.ru>,
        Peter Jones <pjones@...hat.com>,
        "Limonciello, Mario" <mario.limonciello@....com>,
        joeyli <jlee@...e.com>, lvc-project@...uxtesting.org,
        x86@...nel.org, linux-efi@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v4 07/26] x86/build: Check W^X of vmlinux during build

On 2023-03-08 12:34, Ard Biesheuvel wrote:
> On Thu, 15 Dec 2022 at 13:38, Evgeniy Baskov <baskov@...ras.ru> 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@....com>
>> Tested-by: Peter Jones <pjones@...hat.com>
>> Signed-off-by: Evgeniy Baskov <baskov@...ras.ru>
>> ---
>>  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)

Thank you for suggestion! I will fix it.