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 08:58:12 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org
Cc:     linux-security-module@...r.kernel.org,
        Kees Cook <keescook@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        "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 9/14/2021 3:28 AM, Will Deacon wrote:
> CC_HAS_AUTO_VAR_INIT_ZERO requires a supported set of compiler options
> distinct from those needed by CC_HAS_AUTO_VAR_INIT_PATTERN, Fix up
> the Kconfig dependency for INIT_STACK_ALL_ZERO to test for the former
> instead of the latter, as these are the options passed by the top-level
> Makefile.
> 
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Nathan Chancellor <nathan@...nel.org>
> Cc: Nick Desaulniers <ndesaulniers@...gle.com>
> Cc: Gustavo A. R. Silva <gustavoars@...nel.org>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Fixes: dcb7c0b9461c ("hardening: Clarify Kconfig text for auto-var-init")
> Signed-off-by: Will Deacon <will@...nel.org>

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

One comment below.

> ---
> 
> I just noticed this while reading the code and I suspect it doesn't really
> matter in practice.
> 
>   security/Kconfig.hardening | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
> index 90cbaff86e13..341e2fdcba94 100644
> --- a/security/Kconfig.hardening
> +++ b/security/Kconfig.hardening
> @@ -29,7 +29,7 @@ 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
> 

While I think this change is correct in and of itself, 
CONFIG_INIT_STACK_ALL_ZERO is broken with GCC 12.x, as 
CONFIG_CC_HAS_AUTO_VAR_INIT_ZERO won't be set even though GCC now 
supports -ftrivial-auto-var-init=zero because GCC does not implement the 
-enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang 
flag for obvious reasons ;) the cc-option call probably needs to be 
adjusted.

Cheers,
Nathan