Re: [PATCH] hardening: Default to INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO

From: Nathan Chancellor <nathan@kernel.org>
To: Kees Cook <keescook@chromium.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
	Will Deacon <will@kernel.org>,
	linux-kernel@vger.kernel.org,
	linux-security-module@vger.kernel.org,
	"Gustavo A . R . Silva" <gustavoars@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [PATCH] hardening: Default to INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO
Date: Tue, 14 Sep 2021 12:36:37 -0700	[thread overview]
Message-ID: <YUD5xfIDzrQmPE6D@archlinux-ax161> (raw)
In-Reply-To: <202109141214.630BB3A@keescook>

On Tue, Sep 14, 2021 at 12:14:47PM -0700, Kees Cook 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:

Thanks for clarifying! (since I did not actually test before making the
comment...)

Reviewed-by: Nathan Chancellor <nathan@kernel.org>

> 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
> 