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Date: Wed, 21 Jun 2017 08:54:58 -0700
From: Laura Abbott <labbott@...hat.com>
To: Mark Rutland <mark.rutland@....com>, Kees Cook <keescook@...omium.org>
Cc: Alexander Popov <alex.popov@...ux.com>,
 "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
 PaX Team <pageexec@...email.hu>, Brad Spengler <spender@...ecurity.net>,
 Tycho Andersen <tycho@...ker.com>, Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: Re: [PATCH RFC v2 1/1] gcc-plugins: Add
 stackleak feature erasing the kernel stack at the end of syscalls

On 06/21/2017 02:24 AM, Mark Rutland wrote:
> On Tue, Jun 20, 2017 at 12:11:56PM -0700, Kees Cook wrote:
>> On Tue, Jun 20, 2017 at 4:20 AM, Mark Rutland <mark.rutland@....com> wrote:
>>> On Tue, Jun 13, 2017 at 02:51:59PM -0700, Laura Abbott wrote:
>>>> On 06/09/2017 10:28 AM, Kees Cook wrote:
>>>>> It seems like it shouldn't be too hard to add on-user-return erasure
>>>>> code to other architectures too.
>>>>
>>>> I played around getting this to compile for arm64 with a dummy
>>>> stack clearing function. arm64 is doing something special with the
>>>> efistub so it fails to link with
>>>>
>>>> drivers/firmware/efi/libstub/arm-stub.c:45:(.init.text+0x54): relocation
>>>>       truncated to fit: R_AARCH64_CALL26 against undefined symbol `__efistub_track_stack'
>>>>
>>>> The relocation to the .init.text section and appending __efistub happens after
>>>> compilation so the checks in the plugin itself don't work. I haven't come up
>>>> with a solution to not have the plugin run on the stub yet.
>>>
>>> Can we do something like what we do for KCOV, and (only) place the
>>> plugin-invoking flags to into CFLAGS_STACKLEAK, which we can filter out
>>> in scripts/Makefile.lib?
>>
>> That seems fine to me, though plugins have tended to provide a
>> DISABLE_foo_bar_PLUGIN export (like used in ppc for turning off the
>> latent entropy plugin):
>>
>> scripts/Makefile.gcc-plugins:    DISABLE_LATENT_ENTROPY_PLUGIN +=
>> -fplugin-arg-latent_entropy_plugin-disable
>>
>> arch/powerpc/kernel/Makefile:CFLAGS_cputable.o +=
>> $(DISABLE_LATENT_ENTROPY_PLUGIN)
>> arch/powerpc/kernel/Makefile:CFLAGS_prom_init.o +=
>> $(DISABLE_LATENT_ENTROPY_PLUGIN)
>> ...
> 
> Interesting. I hadn't encountered that style before.
> 
> I'd seen the per-makefile config disables:
> 
> drivers/firmware/efi/libstub/Makefile:GCOV_PROFILE                      := n
> drivers/firmware/efi/libstub/Makefile:KASAN_SANITIZE                    := n
> drivers/firmware/efi/libstub/Makefile:UBSAN_SANITIZE                    := n
> drivers/firmware/efi/libstub/Makefile:KCOV_INSTRUMENT                   := n
> 
> ... and per-object config disables:
> 
> arch/arm64/mm/Makefile:KASAN_SANITIZE_physaddr.o        += n
> arch/arm64/mm/Makefile:KASAN_SANITIZE_kasan_init.o      := n
> 
> ... which AFAICT are both handled by the standard scripts/Makefile.lib idiom:
> 
> ifeq ($(CONFIG_KASAN),y)
> _c_flags += $(if $(patsubst n%,, \
>                 $(KASAN_SANITIZE_$(basetarget).o)$(KASAN_SANITIZE)y), \
>                 $(CFLAGS_KASAN))
> endif
> 
> ... it seems a shame for plugins to follow a different pattern.
> 
> Thanks,
> Mark.
> 

That's what I tried to do as a first attempt but I got bogged down
in Makefiles and figuring out how to not pass the plugin flags to
specific files. Agreed it would be good to have the plugins follow
a similar pattern.

Thanks,
Laura

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