kernel-hardening - Re: Re: [PATCH RFC v2 1/1] gcc-plugins: Add stackleak feature erasing the kernel stack at the end of syscalls

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Date: Tue, 20 Jun 2017 12:11:56 -0700
From: Kees Cook <keescook@...omium.org>
To: Mark Rutland <mark.rutland@....com>
Cc: Laura Abbott <labbott@...hat.com>, 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 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)
...


-Kees


-- 
Kees Cook
Pixel Security

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