[PATCH] hardening: Default to INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO

[Kees Cook]

On Tue, Sep 14, 2021 at 11:53:38AM -0700, Nick Desaulniers wrote:
> `On Tue, Sep 14, 2021 at 10:21 AM Kees Cook <keescook at chromium.org> wrote:
> >
> > 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 at chromium.org>
> > > > Cc: Nathan Chancellor <nathan at kernel.org>
> > > > Cc: Nick Desaulniers <ndesaulniers at google.com>
> > > > Cc: Gustavo A. R. Silva <gustavoars at kernel.org>
> > > > Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
> > > > Fixes: dcb7c0b9461c ("hardening: Clarify Kconfig text for auto-var-init")
> > > > Signed-off-by: Will Deacon <will at kernel.org>
> > >
> > > Reviewed-by: Nathan Chancellor <nathan at 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.
> 
> 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:
> > +       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