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

[Kees Cook <keescook@chromium.org>]

On Tue, Sep 14, 2021 at 08:58:12AM -0700, Nathan Chancellor wrote:
> 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@chromium.org>
> > Cc: Nathan Chancellor <nathan@kernel.org>
> > Cc: Nick Desaulniers <ndesaulniers@google.com>
> > Cc: Gustavo A. R. Silva <gustavoars@kernel.org>
> > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Fixes: dcb7c0b9461c ("hardening: Clarify Kconfig text for auto-var-init")
> > Signed-off-by: Will Deacon <will@kernel.org>
> 
> Reviewed-by: Nathan Chancellor <nathan@kernel.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.

GCC silently ignores the -enable flag, so things actually work correctly
as-is. But, yes, it makes the command line long and doesn't make sense.
How about we do this instead:

diff --git a/Makefile b/Makefile
index 34a0afc3a8eb..34439deac939 100644
--- a/Makefile
+++ b/Makefile
@@ -831,12 +831,11 @@ 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_HAS_AUTO_VAR_INIT_ZERO_WITH_ENABLE
 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..beea81df3081 100644
--- a/security/Kconfig.hardening
+++ b/security/Kconfig.hardening
@@ -22,14 +22,22 @@ menu "Memory initialization"
 config CC_HAS_AUTO_VAR_INIT_PATTERN
 	def_bool $(cc-option,-ftrivial-auto-var-init=pattern)
 
+config CC_HAS_AUTO_VAR_INIT_ZERO_WITHOUT_ENABLE
+	def_bool $(cc-option,-ftrivial-auto-var-init=zero)
+
+config CC_HAS_AUTO_VAR_INIT_ZERO_WITH_ENABLE
+	# https://bugs.llvm.org/show_bug.cgi?id=45497
+	def_bool !CC_HAS_AUTO_VAR_INIT_ZERO_WITHOUT_ENABLE && \
+		 $(cc-option,-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang)
+
 config CC_HAS_AUTO_VAR_INIT_ZERO
-	def_bool $(cc-option,-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang)
+	def_bool CC_HAS_AUTO_VAR_INIT_ZERO_WITHOUT_ENABLE || CC_HAS_AUTO_VAR_INIT_ZERO_WITH_ENABLE
 
 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