linux-kernel - Re: [PATCH v3] ARM: fix atags_to_fdt with stack-protector-strong

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Date:	Thu, 7 Jan 2016 10:40:46 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Arnd Bergmann <arnd@...db.de>,
	Simon Horman <horms+renesas@...ge.net.au>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Magnus Damm <damm+renesas@...nsource.se>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] ARM: fix atags_to_fdt with stack-protector-strong

On Thu, Jan 7, 2016 at 3:30 AM, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
> On Wed, Jan 06, 2016 at 03:36:56PM -0800, Kees Cook wrote:
>> diff --git a/arch/arm/boot/compressed/Makefile b/arch/arm/boot/compressed/Makefile
>> index 3f9a9ebc77c3..d7d2c2981f65 100644
>> --- a/arch/arm/boot/compressed/Makefile
>> +++ b/arch/arm/boot/compressed/Makefile
>> @@ -106,6 +106,14 @@ ORIG_CFLAGS := $(KBUILD_CFLAGS)
>>  KBUILD_CFLAGS = $(subst -pg, , $(ORIG_CFLAGS))
>>  endif
>>
>> +# -fstack-protector-strong triggers protection checks in this code,
>> +# but it is being used too early to link to meaningful stack_chk logic.
>> +CFLAGS_atags_to_fdt.o := $(call cc-option, -fno-stack-protector)
>> +CFLAGS_fdt.o := $(call cc-option, -fno-stack-protector)
>> +CFLAGS_fdt_ro.o := $(call cc-option, -fno-stack-protector)
>> +CFLAGS_fdt_rw.o := $(call cc-option, -fno-stack-protector)
>> +CFLAGS_fdt_wip.o := $(call cc-option, -fno-stack-protector)
>
> This will result in "$(call cc-option, -fno-stack-protector)" being
> called five times when this Makefile is parsed, which seems very
> wasteful.  I'm sure there's better solutions to that - maybe caching
> the value in a variable in a higher level makefile (eg,
> arch/arm/Makefile) ?

Good point; I will adjust this to get a single invocation.

> Also, I suspect that all of the decompressor should be built with
> -fno-stack-protector as we don't have sufficient environment here.
> Maybe it should be placed in the global CFLAGS for the decompressor?

I prefer keeping it disabled in as narrow a range as possible. If
other code gains a level of complexity that it triggers the stack
protector code insertion, I think that's worth examining when it
happens. If this ever becomes an actual burden, then yeah, let's do it
for the whole decompressor, but I think it'd be best to revisit it if
it happens again.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security