From: Kees Cook <keescook@chromium.org>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Nathan Chancellor <nathan@kernel.org>,
Will Deacon <will@kernel.org>,
linux-kernel@vger.kernel.org,
linux-security-module@vger.kernel.org,
"Gustavo A . R . Silva" <gustavoars@kernel.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [PATCH] hardening: Default to INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO
Date: Tue, 14 Sep 2021 12:09:26 -0700 [thread overview]
Message-ID: <202109141207.BA9EAD8@keescook> (raw)
In-Reply-To: <CAKwvOdnrO7X8h-g9Pn8RmfJhqj2zn3HJwpQ0p2EONNtFF0w-uA@mail.gmail.com>
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@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@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.
>
> 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
next prev parent reply other threads:[~2021-09-14 19:09 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-14 10:28 [PATCH] hardening: Default to INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT_ZERO Will Deacon
2021-09-14 15:58 ` Nathan Chancellor
2021-09-14 17:21 ` Kees Cook
2021-09-14 18:53 ` Nick Desaulniers
2021-09-14 19:09 ` Kees Cook [this message]
2021-09-14 19:14 ` Kees Cook
2021-09-14 19:22 ` Nick Desaulniers
2021-09-14 19:36 ` Nathan Chancellor
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