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

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Date:   Thu, 11 Apr 2019 10:07:14 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
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 <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 1:06 AM Masahiro Yamada
<yamada.masahiro@...ionext.com> wrote:
>
> 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.

No, it's only available under Clang. Clang is all-or-nothing, and the
GCC plugin has a degrees up to "all passed by reference" which isn't
truly "all" (i.e. Clang will initialize variables that aren't passed
by reference and trigger a compiler warning about being
uninitialized.)

> >  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?

There are very few users of Clang right now (mainly Android), so I
figured it'd be nice to start Clang builds from a "protected by
default" here, especially given Linus's thoughts on making this always
happen[1]. I don't want to do it for GCC yet, since that would likely
come as a huge surprise to everyone else. :) But I'm happy to change
this, of course.

-Kees

[1] https://lkml.kernel.org/r/CA+55aFykZL+cSBJjBBts7ebEFfyGPdMzTmLSxKnT_29=j942dA@mail.gmail.com

--
Kees Cook