Messages in this thread Patch in this message | | | Date | Sun, 14 Jun 2020 10:04:49 -0700 | From | Kees Cook <> | Subject | Re: [PATCH] [RFC] security: allow using Clang's zero initialization for stack variables |
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On Sun, Jun 14, 2020 at 04:45:34PM +0200, glider@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@chromium.org> > Cc: Nick Desaulniers <ndesaulniers@google.com> > Signed-off-by: Alexander Potapenko <glider@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.
> config GCC_PLUGIN_STRUCTLEAK_VERBOSE > bool "Report forcefully initialized variables" > depends on GCC_PLUGIN_STRUCTLEAK > -- > 2.27.0.290.gba653c62da-goog >
But yes, let's get this change implemented. :)
-- Kees Cook
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