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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id b10sm8840318pfb.110.2020.06.14.11.43.17 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 14 Jun 2020 11:43:17 -0700 (PDT) Date: Sun, 14 Jun 2020 11:43:16 -0700 From: Kees Cook To: Alexander Potapenko Cc: Masahiro Yamada , James Morris , Maciej =?utf-8?Q?=C5=BBenczykowski?= , Nick Desaulniers , linux-security-module , LKML Subject: Re: [PATCH] [RFC] security: allow using Clang's zero initialization for stack variables Message-ID: <202006141140.81AAA907D@keescook> References: <20200614144534.237035-1-glider@google.com> <202006141000.B93DF245@keescook> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-security-module@vger.kernel.org Precedence: bulk List-ID: On Sun, Jun 14, 2020 at 07:16:33PM +0200, Alexander Potapenko wrote: > On Sun, Jun 14, 2020 at 7:04 PM Kees Cook wrote: > > > > 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 > > > Cc: Nick Desaulniers > > > Signed-off-by: Alexander Potapenko > > > --- > > > 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