linux-kernel - Re: [PATCH 3/3] kbuild: Implement Clang's stack initialization

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Message-ID: <CAK7LNAQpm5Tz_Fo0rSq00emB7BLnfYB-+mwXd9GiMKAqKCfkrA@mail.gmail.com>
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