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Date: Thu, 7 Mar 2019 09:11:54 -0800
From: Kees Cook <keescook@...omium.org>
To: Alexander Potapenko <glider@...gle.com>
Cc: Masahiro Yamada <yamada.masahiro@...ionext.com>, James Morris <jmorris@...ei.org>, 
	"Serge E. Hallyn" <serge@...lyn.com>, 
	linux-security-module <linux-security-module@...r.kernel.org>, 
	linux-kbuild <linux-kbuild@...r.kernel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, 
	Kostya Serebryany <kcc@...gle.com>, Dmitry Vyukov <dvyukov@...gle.com>, Sandeep Patil <sspatil@...roid.com>, 
	Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 1/1] RFC: initmem: introduce CONFIG_INIT_ALL_MEMORY and CONFIG_INIT_ALL_STACK

On Thu, Mar 7, 2019 at 5:35 AM Alexander Potapenko <glider@...gle.com> wrote:
>
> This patch is a part of a bigger initiative to allow initializing
> heap/stack memory in the Linux kernels by default.
> The rationale behind doing so is to reduce the severity of bugs caused
> by using uninitialized memory.
>
> CONFIG_INIT_ALL_MEMORY is going to be an umbrella config for options
> that force heap and stack initialization.

We have some level of control over the heap portions already with the
poisoning options, but they currently require command-line enabling.

For the slab allocator:
CONFIG_SLUB_DEBUG=y
and boot with "slub_debug=P"

For the page allocator:
CONFIG_PAGE_POISONING=y
CONFIG_PAGE_POISONING_NO_SANITY=y
CONFIG_PAGE_POISONING_ZERO=y
and boot with "page_poison=1"

So I think it would make sense to wire up the boot defaults and
continue some of the benchmarking work to improve the performance hit.

> CONFIG_INIT_ALL_STACK turns on stack initialization based on the
> -ftrivial-auto-var-init Clang flag.
>
> -ftrivial-auto-var-init is a Clang flag that provides trivial
> initializers for uninitialized local variables, variable fields and
> padding.
>
> It has three possible values:
>   pattern - uninitialized locals are filled with a fixed pattern
>     (mostly 0xAA on 64-bit platforms, see https://reviews.llvm.org/D54604
>     for more details) likely to cause crashes when uninitialized value is
>     used;
>   zero (it's still debated whether this flag makes it to the official
>     Clang release) - uninitialized locals are filled with zeroes;
>   uninitialized (default) - uninitialized locals are left intact.

This should get wired up to CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL in
the GCC case.

> The proposed config builds the kernel with
> -ftrivial-auto-var-init=pattern.
>
> Developers have the possibility to opt-out of this feature on a
> per-file (by using the INIT_ALL_MEMORY_ Makefile prefix) or per-variable
> (by using __attribute__((uninitialized))) basis.

I would like a clean way to offer a paranoid "true init all memory"
config that will ignore the attribute. Also, BYREF_ALL needs to gain
the attribute knowledge.

> Signed-off-by: Alexander Potapenko <glider@...gle.com>
> Cc: Masahiro Yamada <yamada.masahiro@...ionext.com>
> Cc: James Morris <jmorris@...ei.org>
> Cc: "Serge E. Hallyn" <serge@...lyn.com>
> Cc: Nick Desaulniers <ndesaulniers@...gle.com>
> Cc: Kostya Serebryany <kcc@...gle.com>
> Cc: Dmitry Vyukov <dvyukov@...gle.com>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Sandeep Patil <sspatil@...roid.com>
> Cc: linux-security-module@...r.kernel.org
> Cc: linux-kbuild@...r.kernel.org
>
> ---
>  Makefile                 |  3 ++-
>  scripts/Makefile.initmem | 17 +++++++++++++++++
>  scripts/Makefile.lib     |  6 ++++++
>  security/Kconfig         |  1 +
>  security/Kconfig.initmem | 22 ++++++++++++++++++++++
>  5 files changed, 48 insertions(+), 1 deletion(-)
>  create mode 100644 scripts/Makefile.initmem
>  create mode 100644 security/Kconfig.initmem
>
> diff --git a/Makefile b/Makefile
> index f070e0d65186..028ca37878fd 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -448,7 +448,7 @@ export HOSTCXX KBUILD_HOSTCXXFLAGS LDFLAGS_MODULE CHECK CHECKFLAGS
>
>  export KBUILD_CPPFLAGS NOSTDINC_FLAGS LINUXINCLUDE OBJCOPYFLAGS KBUILD_LDFLAGS
>  export KBUILD_CFLAGS CFLAGS_KERNEL CFLAGS_MODULE
> -export CFLAGS_KASAN CFLAGS_KASAN_NOSANITIZE CFLAGS_UBSAN
> +export CFLAGS_KASAN CFLAGS_KASAN_NOSANITIZE CFLAGS_UBSAN CFLAGS_INITMEM
>  export KBUILD_AFLAGS AFLAGS_KERNEL AFLAGS_MODULE
>  export KBUILD_AFLAGS_MODULE KBUILD_CFLAGS_MODULE KBUILD_LDFLAGS_MODULE
>  export KBUILD_AFLAGS_KERNEL KBUILD_CFLAGS_KERNEL
> @@ -840,6 +840,7 @@ KBUILD_ARFLAGS := $(call ar-option,D)
>  include scripts/Makefile.kasan
>  include scripts/Makefile.extrawarn
>  include scripts/Makefile.ubsan
> +include scripts/Makefile.initmem
>
>  # Add any arch overrides and user supplied CPPFLAGS, AFLAGS and CFLAGS as the
>  # last assignments
> diff --git a/scripts/Makefile.initmem b/scripts/Makefile.initmem
> new file mode 100644
> index 000000000000..f49be398f2c1
> --- /dev/null
> +++ b/scripts/Makefile.initmem
> @@ -0,0 +1,17 @@
> +ifdef CONFIG_INIT_ALL_MEMORY
> +
> +# Clang's -ftrivial-auto-var-init=pattern flag initializes the
> +# uninitialized parts of local variables (including fields and padding)
> +# with a fixed pattern (0xAA in most cases).
> +ifdef CONFIG_INIT_ALL_STACK
> + CFLAGS_INITMEM := -ftrivial-auto-var-init=pattern
> +endif
> +
> +ifeq ($(call cc-option, $(CFLAGS_INITMEM) -Werror),)
> +   ifneq ($(CONFIG_COMPILE_TEST),y)
> +        $(warning Cannot use CONFIG_INIT_ALL_MEMORY: \
> +            -ftrivial-auto-var-init is not supported by compiler)
> +   endif
> +endif

This should be done in the Kconfig (see how stack protector is
handled), rather than the Makefile. See below.

> +
> +endif
> diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
> index 12b88d09c3a4..53d18fd15c79 100644
> --- a/scripts/Makefile.lib
> +++ b/scripts/Makefile.lib
> @@ -131,6 +131,12 @@ _c_flags += $(if $(patsubst n%,, \
>                 $(CFLAGS_UBSAN))
>  endif
>
> +ifeq ($(CONFIG_INIT_ALL_MEMORY),y)
> +_c_flags += $(if $(patsubst n%,, \
> +               $(INIT_ALL_MEMORY_$(basetarget).o)$(INIT_ALL_MEMORY)y), \
> +               $(CFLAGS_INITMEM))
> +endif
> +
>  ifeq ($(CONFIG_KCOV),y)
>  _c_flags += $(if $(patsubst n%,, \
>         $(KCOV_INSTRUMENT_$(basetarget).o)$(KCOV_INSTRUMENT)$(CONFIG_KCOV_INSTRUMENT_ALL)), \
> diff --git a/security/Kconfig b/security/Kconfig
> index e4fe2f3c2c65..cc12a39424dd 100644
> --- a/security/Kconfig
> +++ b/security/Kconfig
> @@ -230,6 +230,7 @@ config STATIC_USERMODEHELPER_PATH
>           If you wish for all usermode helper programs to be disabled,
>           specify an empty string here (i.e. "").
>
> +source "security/Kconfig.initmem"
>  source "security/selinux/Kconfig"
>  source "security/smack/Kconfig"
>  source "security/tomoyo/Kconfig"
> diff --git a/security/Kconfig.initmem b/security/Kconfig.initmem
> new file mode 100644
> index 000000000000..5ac3cf3e7f88
> --- /dev/null
> +++ b/security/Kconfig.initmem
> @@ -0,0 +1,22 @@
> +menu "Initialize all memory"
> +
> +config INIT_ALL_MEMORY
> +       bool "Initialize all memory"
> +       default n
> +       help
> +         Enforce memory initialization to mitigate infoleaks and make
> +         the control-flow bugs depending on uninitialized values more
> +         deterministic.

See my notes about enabling page/slab poisoning via this option (which
should probably be a separate HEAP option).

> +
> +if INIT_ALL_MEMORY
> +
> +config INIT_ALL_STACK
> +       bool "Initialize all stack"
> +       depends on INIT_ALL_MEMORY

For example of doing this in Kconfig:

config CC_HAS_AUTO_VAR_INIT
        def_bool $(cc-option,-ftrivial-auto-var-init=pattern)

config INIT_ALL_STACK
        bool "Initialize all stack"
        depends on CC_HAS_AUTO_VAR_INIT || HAVE_GCC_PLUGINS
        select GCC_PLUGINS if !CC_HAS_AUTO_VAR_INIT
        select GCC_PLUGIN_STRUCTLEAK if !CC_HAS_AUTO_VAR_INIT
        select GCC_PLUGIN_STRUCTLEAK_BYREF_ALL if !CC_HAS_AUTO_VAR_INIT

probably GCC_PLUGINS needs to be "default y" so this select mess is
less crazy. I already started work on reorganizing the Kconfig there:
https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=kspp/gcc-plugin/structleak&id=81a56f6dcd20325607d6008f4bb560c96f4c821a

> +       default y
> +       help
> +         Initialize uninitialized stack data with a 0xAA pattern.
> +         This config option only supports Clang builds at the moment.
> +
> +endif # INIT_ALL_MEMORY
> +endmenu
> --
> 2.21.0.352.gf09ad66450-goog
>

-- 
Kees Cook

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