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Date: Thu, 11 Apr 2019 17:05:13 +0900
From: Masahiro Yamada <yamada.masahiro@...ionext.com>
To: Kees Cook <keescook@...omium.org>
Cc: Alexander Potapenko <glider@...gle.com>,
Nick Desaulniers <ndesaulniers@...gle.com>,
Kostya Serebryany <kcc@...gle.com>, Dmitry Vyukov <dvyukov@...gle.com>,
Sandeep Patil <sspatil@...roid.com>, Laura Abbott <labbott@...hat.com>,
Randy Dunlap <rdunlap@...radead.org>,
Alexander Popov <alpopov@...ecurity.com>,
Michal Marek <michal.lkml@...kovi.net>,
Emese Revfy <re.emese@...il.com>, James Morris <jmorris@...ei.org>,
"Serge E. Hallyn" <serge@...lyn.com>,
Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
linux-security-module@...r.kernel.org,
Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 3/3] kbuild: Implement Clang's stack initialization
On Thu, Apr 11, 2019 at 1:16 AM Kees Cook <keescook@...omium.org> wrote:
>
> CONFIG_INIT_STACK_ALL turns on stack initialization based on
> -ftrivial-auto-var-init in Clang builds and on
> -fplugin-arg-structleak_plugin-byref-all in GCC builds.
Is CONFIG_INIT_STACK_ALL wired up to GCC plugin in any way?
I could not understand it from the code.
>
> -ftrivial-auto-var-init is a Clang flag that provides trivial
> initializers for uninitialized local variables, variable fields and
> padding.
>
> It has three possible values:
> pattern - uninitialized locals are filled with a fixed pattern
> (mostly 0xAA on 64-bit platforms, see https://reviews.llvm.org/D54604
> for more details) likely to cause crashes when uninitialized value is
> used;
> zero (it's still debated whether this flag makes it to the official
> Clang release) - uninitialized locals are filled with zeroes;
> uninitialized (default) - uninitialized locals are left intact.
>
> The proposed config builds the kernel with
> -ftrivial-auto-var-init=pattern when selected.
>
> Developers have the possibility to opt-out of this feature on a
> per-variable basis by using __attribute__((uninitialized)).
>
> Co-developed-by: Alexander Potapenko <glider@...gle.com>
> Signed-off-by: Alexander Potapenko <glider@...gle.com>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> Makefile | 5 +++++
> security/Kconfig.hardening | 14 ++++++++++++++
> 2 files changed, 19 insertions(+)
>
> diff --git a/Makefile b/Makefile
> index c0a34064c574..a7d9c6cd0267 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -745,6 +745,11 @@ KBUILD_CFLAGS += -fomit-frame-pointer
> endif
> endif
>
> +# Initialize all stack variables with a pattern, if desired.
> +ifdef CONFIG_INIT_STACK_ALL
> +KBUILD_CFLAGS += -ftrivial-auto-var-init=pattern
> +endif
> +
> DEBUG_CFLAGS := $(call cc-option, -fno-var-tracking-assignments)
>
> ifdef CONFIG_DEBUG_INFO
> diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
> index 9942d9869864..d744e20140b4 100644
> --- a/security/Kconfig.hardening
> +++ b/security/Kconfig.hardening
> @@ -19,9 +19,13 @@ config GCC_PLUGIN_STRUCTLEAK
>
> menu "Memory initialization"
>
> +config CC_HAS_AUTO_VAR_INIT
> + def_bool $(cc-option,-ftrivial-auto-var-init=pattern)
> +
> choice
> prompt "Initialize kernel stack variables at function entry"
> depends on CC_HAS_AUTO_VAR_INIT || GCC_PLUGINS
> + default INIT_STACK_ALL if CC_HAS_AUTO_VAR_INIT
Why should this be enabled by default?
Ins't it a performance regression
since it inserts instructions in function prologue?
--
Best Regards
Masahiro Yamada
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