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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