[PATCH] [RFC] security: allow using Clang's zero initialization for stack variables

[Alexander Potapenko]

On Sun, Jun 14, 2020 at 7:04 PM Kees Cook <keescook at chromium.org> wrote:
>
> On Sun, Jun 14, 2020 at 04:45:34PM +0200, glider at google.com wrote:
> > In addition to -ftrivial-auto-var-init=pattern (used by
> > CONFIG_INIT_STACK_ALL now) Clang also supports zero initialization for
> > locals enabled by -ftrivial-auto-var-init=zero.
> > The future of this flag is still being debated, see
> > https://bugs.llvm.org/show_bug.cgi?id=45497
> > Right now it is guarded by another flag,
> > -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang,
> > which means it may not be supported by future Clang releases.
> > Another possible resolution is that -ftrivial-auto-var-init=zero will
> > persist (as certain users have already started depending on it), but the
> > name of the guard flag will change.
> >
> > In the meantime, zero initialization has proven itself as a good
> > production mitigation measure against uninitialized locals. Unlike
> > pattern initialization, which has a higher chance of triggering existing
> > bugs, zero initialization provides safe defaults for strings, pointers,
> > indexes, and sizes. On the other hand, pattern initialization remains
> > safer for return values.
> > Performance-wise, the difference between pattern and zero initialization
> > is usually negligible, although the generated code for zero
> > initialization is more compact.
> >
> > The proposed config, CONFIG_USE_CLANG_ZERO_INITIALIZATION, makes
> > CONFIG_INIT_STACK_ALL use zero initialization if the corresponding flags
> > are supported by Clang.
> >
> > Cc: Kees Cook <keescook at chromium.org>
> > Cc: Nick Desaulniers <ndesaulniers at google.com>
> > Signed-off-by: Alexander Potapenko <glider at google.com>
> > ---
> >  Makefile                   | 15 ++++++++++++++-
> >  security/Kconfig.hardening | 16 ++++++++++++++++
> >  2 files changed, 30 insertions(+), 1 deletion(-)
> >
> > diff --git a/Makefile b/Makefile
> > index fd31992bf918..2860bad7e39a 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -802,9 +802,22 @@ KBUILD_CFLAGS    += -fomit-frame-pointer
> >  endif
> >  endif
> >
> > -# Initialize all stack variables with a pattern, if desired.
> > +# Initialize all stack variables, if desired.
> >  ifdef CONFIG_INIT_STACK_ALL
> > +
> > +# Use pattern initialization by default.
> > +ifndef CONFIG_USE_CLANG_ZERO_INITIALIZATION
> >  KBUILD_CFLAGS        += -ftrivial-auto-var-init=pattern
> > +else
> > +
> > +ifdef CONFIG_CC_HAS_AUTO_VAR_ZERO_INIT
> > +# 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 -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
> > +endif
> > +
> > +endif
> >  endif
>
> I'd prefer this be split instead of built as a nested if (i.e. entirely
> control section via the Kconfig -- see below).
>
> >
> >  DEBUG_CFLAGS := $(call cc-option, -fno-var-tracking-assignments)
> > diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
> > index af4c979b38ee..299d27c6d78c 100644
> > --- a/security/Kconfig.hardening
> > +++ b/security/Kconfig.hardening
> > @@ -22,6 +22,9 @@ menu "Memory initialization"
> >  config CC_HAS_AUTO_VAR_INIT
> >       def_bool $(cc-option,-ftrivial-auto-var-init=pattern)
> >
> > +config CC_HAS_AUTO_VAR_ZERO_INIT
> > +     def_bool $(cc-option,-ftrivial-auto-var-init=zero -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang)
> > +
>
> I'd like to be more specific here. Let's rename CC_HAS_AUTO_VAR_INIT to
> CC_HAS_AUTO_VAR_INIT_PATTERN, and change the other to
> CC_HAS_AUTO_VAR_INIT_ZERO (they then both match the word order of the
> option, and the thing that changes is the last word).
>
> >  choice
> >       prompt "Initialize kernel stack variables at function entry"
> >       default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS
> > @@ -100,6 +103,19 @@ choice
> >
> >  endchoice
> >
> > +config USE_CLANG_ZERO_INITIALIZATION
> > +     bool "Use Clang's zero initialization for local variables"
> > +     depends on CC_HAS_AUTO_VAR_ZERO_INIT
> > +     depends on INIT_STACK_ALL
> > +     help
> > +       If set, uses zeros instead of 0xAA to initialize local variables in
> > +       INIT_STACK_ALL. Zeroing the stack provides safer defaults for strings,
> > +       pointers, indexes, and sizes. The downsides are less-safe defaults for
> > +       return values, and exposing fewer bugs where the underlying code
> > +       relies on zero initialization.
> > +       The corresponding flag isn't officially supported by Clang and may
> > +       sooner or later go away or get renamed.
> > +
>
> Similarly, I'd like to rename INIT_STACK_ALL to INIT_STACK_ALL_PATTERN
> and then add INIT_STACK_ALL_ZERO.

What are the policies regarding keeping the existing config flags?
Don't we need to preserve INIT_STACK_ALL?