[PATCH] [RFC] security: allow using Clang's zero initialization for stack variables

[Kees Cook]

On Sun, Jun 14, 2020 at 04:45:34PM +0200, glider at google.com wrote:
> In addition to -ftrivial-auto-var-init=pattern (used by
> CONFIG_INIT_STACK_ALL now) Clang also supports zero initialization for
> locals enabled by -ftrivial-auto-var-init=zero.
> The future of this flag is still being debated, see
> https://bugs.llvm.org/show_bug.cgi?id=45497
> Right now it is guarded by another flag,
> -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang,
> which means it may not be supported by future Clang releases.
> Another possible resolution is that -ftrivial-auto-var-init=zero will
> persist (as certain users have already started depending on it), but the
> name of the guard flag will change.
> 
> In the meantime, zero initialization has proven itself as a good
> production mitigation measure against uninitialized locals. Unlike
> pattern initialization, which has a higher chance of triggering existing
> bugs, zero initialization provides safe defaults for strings, pointers,
> indexes, and sizes. On the other hand, pattern initialization remains
> safer for return values.
> Performance-wise, the difference between pattern and zero initialization
> is usually negligible, although the generated code for zero
> initialization is more compact.
> 
> The proposed config, CONFIG_USE_CLANG_ZERO_INITIALIZATION, makes
> CONFIG_INIT_STACK_ALL use zero initialization if the corresponding flags
> are supported by Clang.
> 
> Cc: Kees Cook <keescook at chromium.org>
> Cc: Nick Desaulniers <ndesaulniers at google.com>
> Signed-off-by: Alexander Potapenko <glider at google.com>
> ---
>  Makefile                   | 15 ++++++++++++++-
>  security/Kconfig.hardening | 16 ++++++++++++++++
>  2 files changed, 30 insertions(+), 1 deletion(-)
> 
> diff --git a/Makefile b/Makefile
> index fd31992bf918..2860bad7e39a 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -802,9 +802,22 @@ KBUILD_CFLAGS	+= -fomit-frame-pointer
>  endif
>  endif
>  
> -# Initialize all stack variables with a pattern, if desired.
> +# Initialize all stack variables, if desired.
>  ifdef CONFIG_INIT_STACK_ALL
> +
> +# Use pattern initialization by default.
> +ifndef CONFIG_USE_CLANG_ZERO_INITIALIZATION
>  KBUILD_CFLAGS	+= -ftrivial-auto-var-init=pattern
> +else
> +
> +ifdef CONFIG_CC_HAS_AUTO_VAR_ZERO_INIT
> +# 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 -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
> +endif
> +
> +endif
>  endif

I'd prefer this be split instead of built as a nested if (i.e. entirely
control section via the Kconfig -- see below).

>  
>  DEBUG_CFLAGS	:= $(call cc-option, -fno-var-tracking-assignments)
> diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
> index af4c979b38ee..299d27c6d78c 100644
> --- a/security/Kconfig.hardening
> +++ b/security/Kconfig.hardening
> @@ -22,6 +22,9 @@ menu "Memory initialization"
>  config CC_HAS_AUTO_VAR_INIT
>  	def_bool $(cc-option,-ftrivial-auto-var-init=pattern)
>  
> +config CC_HAS_AUTO_VAR_ZERO_INIT
> +	def_bool $(cc-option,-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang)
> +

I'd like to be more specific here. Let's rename CC_HAS_AUTO_VAR_INIT to
CC_HAS_AUTO_VAR_INIT_PATTERN, and change the other to
CC_HAS_AUTO_VAR_INIT_ZERO (they then both match the word order of the
option, and the thing that changes is the last word).

>  choice
>  	prompt "Initialize kernel stack variables at function entry"
>  	default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS
> @@ -100,6 +103,19 @@ choice
>  
>  endchoice
>  
> +config USE_CLANG_ZERO_INITIALIZATION
> +	bool "Use Clang's zero initialization for local variables"
> +	depends on CC_HAS_AUTO_VAR_ZERO_INIT
> +	depends on INIT_STACK_ALL
> +	help
> +	  If set, uses zeros instead of 0xAA to initialize local variables in
> +	  INIT_STACK_ALL. Zeroing the stack provides safer defaults for strings,
> +	  pointers, indexes, and sizes. The downsides are less-safe defaults for
> +	  return values, and exposing fewer bugs where the underlying code
> +	  relies on zero initialization.
> +	  The corresponding flag isn't officially supported by Clang and may
> +	  sooner or later go away or get renamed.
> +

Similarly, I'd like to rename INIT_STACK_ALL to INIT_STACK_ALL_PATTERN
and then add INIT_STACK_ALL_ZERO.

>  config GCC_PLUGIN_STRUCTLEAK_VERBOSE
>  	bool "Report forcefully initialized variables"
>  	depends on GCC_PLUGIN_STRUCTLEAK
> -- 
> 2.27.0.290.gba653c62da-goog
> 

But yes, let's get this change implemented. :)

-- 
Kees Cook