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Date: Tue, 3 Oct 2017 08:51:12 -0700
From: Kees Cook <keescook@...omium.org>
To: Mark Rutland <mark.rutland@....com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Masahiro Yamada <yamada.masahiro@...ionext.com>, Michal Marek <mmarek@...e.com>, 
	Yoshinori Sato <ysato@...rs.sourceforge.jp>, Rich Felker <dalias@...c.org>, 
	Ingo Molnar <mingo@...nel.org>, "David S. Miller" <davem@...emloft.net>, 
	Nicholas Piggin <npiggin@...il.com>, Al Viro <viro@...iv.linux.org.uk>, 
	Laura Abbott <labbott@...hat.com>, linux-kbuild <linux-kbuild@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, linux-sh <linux-sh@...r.kernel.org>, 
	"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 0/3] Makefile: Introduce CONFIG_CC_STACKPROTECTOR_AUTO

On Tue, Oct 3, 2017 at 3:04 AM, Mark Rutland <mark.rutland@....com> wrote:
> Hi Kees,
>
> On Mon, Oct 02, 2017 at 12:20:04PM -0700, Kees Cook wrote:
>> As described in the final patch:
>>
>> Nearly all modern compilers support a stack-protector option, and nearly
>> all modern distributions enable the kernel stack-protector, so enabling
>> this by default in kernel builds would make sense. However, Kconfig does
>> not have knowledge of available compiler features, so it isn't safe to
>> force on, as this would unconditionally break builds for the compilers
>> or architectures that don't have support. Instead, this introduces a new
>> option, CONFIG_CC_STACKPROTECTOR_AUTO, which attempts to discover the best
>> possible stack-protector available, and will allow builds to proceed even
>> if the compiler doesn't support any stack-protector.
>>
>> This option is made the default so that kernels built with modern
>> compilers will be protected-by-default against stack buffer overflows,
>> avoiding things like the recent BlueBorne attack. Selection of a specific
>> stack-protector option remains available, including disabling it.
>
> I gave this a spin atop of v4.14-rc3 with a few arm64 toolchains I had
> installed:
>
> * Linaro 17.08 GCC 7.1    // strong
> * Linaro 17.05 GCC 6.1    // strong
> * Linaro 15.08 GCC 5.1    // strong
> * Linaro 14.09 GCC 4.9    // strong
> * Linaro 13.06 GCC 4.8    // none
> * Linaro 13.01 GCC 4.7    // none
>
> AFAICT, the detection is correct, and arm64 toolchains only gained stack
> protector support in GCC 4.9. I manually tested GCC 4.8 and 4.7, and
> got:
>
>   warning: -fstack-protector not supported for this target [enabled by default]
>
> ... so that looks good to me.
>
> One thing I noticed was taht even when the build system detects no
> support for stack-protector, it still passes -DCONFIG_CC_STACKPROTECTOR
> to the toolchain. Is that expected?

Oops, that's a mistake. I had a think-o in the Makefile logic. I will
send a follow-up to fix it.

-Kees

-- 
Kees Cook
Pixel Security

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