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

Sun Jun 14 18:43:16 UTC 2020

On Sun, Jun 14, 2020 at 07:16:33PM +0200, Alexander Potapenko wrote:
> 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?

There isn't a strong policy -- it's mostly a question of "how painful
will it be for end-users to change this?" Given the users of this are
the Clang folks, and it's a very new option, I would prefer we just
change it and carry on.

That said, I'd also like at some point do:

 choice
 	prompt "Initialize kernel stack variables at function entry"
 	default GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if COMPILE_TEST && GCC_PLUGINS
-	default INIT_STACK_ALL_ZERO if COMPILE_TEST && CC_HAS_AUTO_VAR_INIT_ZERO
+	default INIT_STACK_ALL_ZERO if CC_HAS_AUTO_VAR_INIT
 	default INIT_STACK_NONE
 	help
 	  This option enables initialization of stack variables at

But not at this time...

-- 
Kees Cook