linux-kernel - Re: [PATCH] hardening: Default to INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT

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Date:   Tue, 14 Sep 2021 12:22:24 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] hardening: Default to INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO

On Tue, Sep 14, 2021 at 12:14 PM Kees Cook <keescook@...omium.org> wrote:
>
> On Tue, Sep 14, 2021 at 11:53:38AM -0700, Nick Desaulniers wrote:
> > Rather than create 2 new kconfigs with 1 new invocation of the
> > compiler via cc-option, how about just adding an `ifdef
> > CONFIG_CC_IS_CLANG` guard around adding the obnoxious flag to
> > `KBUILD_CFLAGS` in the top level Makefile?
>
> v2:

LGTM
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>

I think the formal patch should have Will's Suggested-by on it.

>
> diff --git a/Makefile b/Makefile
> index 34a0afc3a8eb..72d165ffabdb 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -831,12 +831,12 @@ endif
>
>  # Initialize all stack variables with a zero value.
>  ifdef CONFIG_INIT_STACK_ALL_ZERO
> -# Future support for zero initialization is still being debated, see
> -# https://bugs.llvm.org/show_bug.cgi?id=45497. These flags are subject to being
> -# renamed or dropped.
>  KBUILD_CFLAGS  += -ftrivial-auto-var-init=zero
> +ifdef CONFIG_CC_IS_CLANG
> +# https://bugs.llvm.org/show_bug.cgi?id=45497
>  KBUILD_CFLAGS  += -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
>  endif
> +endif
>
>  # While VLAs have been removed, GCC produces unreachable stack probes
>  # for the randomize_kstack_offset feature. Disable it for all compilers.
> diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
> index 90cbaff86e13..ded17b8abce2 100644
> --- a/security/Kconfig.hardening
> +++ b/security/Kconfig.hardening
> @@ -23,13 +23,16 @@ config CC_HAS_AUTO_VAR_INIT_PATTERN
>         def_bool $(cc-option,-ftrivial-auto-var-init=pattern)
>
>  config CC_HAS_AUTO_VAR_INIT_ZERO
> +       # GCC ignores the -enable flag, so we can test for the feature with
> +       # a single invocation using the flag, but drop it as appropriate in
> +       # the Makefile, depending on the presence of Clang.
>         def_bool $(cc-option,-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang)
>
>  choice
>         prompt "Initialize kernel stack variables at function entry"
>         default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS
>         default INIT_STACK_ALL_PATTERN if COMPILE_TEST && CC_HAS_AUTO_VAR_INIT_PATTERN
> -       default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_PATTERN
> +       default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO
>         default INIT_STACK_NONE
>         help
>           This option enables initialization of stack variables at
>
> --
> Kees Cook



-- 
Thanks,
~Nick Desaulniers