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

On Tue, Sep 14, 2021 at 11:53:38AM -0700, Nick Desaulniers wrote:

`On Tue, Sep 14, 2021 at 10:21 AM Kees Cook [off-list ref] wrote:

quoted

On Tue, Sep 14, 2021 at 08:58:12AM -0700, Nathan Chancellor wrote:

quoted

On 9/14/2021 3:28 AM, Will Deacon wrote:

quoted

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 <redacted>
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.

quoted

---

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.

So then would that mean that CC_HAS_AUTO_VAR_INIT_ZERO_WITH_ENABLE
evaluates to true then, in your patch below?

No, I exclude it based on the results from
CC_HAS_AUTO_VAR_INIT_ZERO_WITHOUT_ENABLE.

here:

quoted

+       def_bool !CC_HAS_AUTO_VAR_INIT_ZERO_WITHOUT_ENABLE && \

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?

That is a bit more sensible, yes. :) Let me try that...

-- 
Kees Cook