[PATCH] hardening: Default to INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO

[Nick Desaulniers]

`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?

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?

> 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



-- 
Thanks,
~Nick Desaulniers