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Date: Tue, 9 Apr 2019 17:37:12 +0900
From: Masahiro Yamada <yamada.masahiro@...ionext.com>
To: Alexander Potapenko <glider@...gle.com>
Cc: James Morris <jmorris@...ei.org>, "Serge E. Hallyn" <serge@...lyn.com>,
        linux-security-module@...r.kernel.org,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>, Dmitry Vyukov <dvyukov@...gle.com>,
        Kees Cook <keescook@...omium.org>, sspatil@...roid.com,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v3 1/2] initmem: introduce CONFIG_INIT_ALL_MEMORY and CONFIG_INIT_ALL_STACK

On Tue, Apr 9, 2019 at 2:04 AM Alexander Potapenko <glider@...gle.com> wrote:
>
> CONFIG_INIT_ALL_MEMORY is going to be an umbrella config for options
> that force heap and stack initialization.
> The rationale behind doing so is to reduce the severity of bugs caused
> by using uninitialized memory.
>
> CONFIG_INIT_ALL_STACK turns on stack initialization based on
> -ftrivial-auto-var-init in Clang builds and on
> -fplugin-arg-structleak_plugin-byref-all in GCC builds.
>
> -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.
>
> 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-variable basis by using __attribute__((uninitialized)).
>
> For GCC builds, CONFIG_INIT_ALL_STACK is simply wired up to
> CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL. No opt-out is possible at the
> moment.
>
> 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
> Cc: kernel-hardening@...ts.openwall.com
> ---
>  v2:
>   - addressed Kees Cook's comments: added GCC support
>  v3: addressed Masahiro Yamada's comments:
>   - dropped per-file opt-out mechanism
>   - fixed GCC_PLUGINS dependencies
> ---
>  Makefile                 |  3 ++-
>  scripts/Makefile.initmem | 10 ++++++++++
>  security/Kconfig         |  1 +
>  security/Kconfig.initmem | 29 +++++++++++++++++++++++++++++
>  4 files changed, 42 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..a6253d78fe35
> --- /dev/null
> +++ b/scripts/Makefile.initmem
> @@ -0,0 +1,10 @@
> +ifdef CONFIG_INIT_ALL_STACK
> +
> +# 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_CC_HAS_AUTO_VAR_INIT
> +  CFLAGS_INITMEM := -ftrivial-auto-var-init=pattern
> +endif
> +
> +endif

Now that you dropped per-file control,
why is this necessary?
Also, CFLAGS_INITMEM is not wired up to anything,
so this patch has no effect.


I think you can add the following to top Makefile, and that's it.

ifdef CONFIG_AUTO_VAR_INIT
KBUILD_CFLAGS += -ftrivial-auto-var-init=pattern
endif




> 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..5e49a55382ad
> --- /dev/null
> +++ b/security/Kconfig.initmem
> @@ -0,0 +1,29 @@
> +menu "Initialize all memory"
> +
> +config CC_HAS_AUTO_VAR_INIT
> +       def_bool $(cc-option,-ftrivial-auto-var-init=pattern)
> +
> +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.
> +
> +if INIT_ALL_MEMORY
> +
> +config INIT_ALL_STACK
> +       bool "Initialize all stack"
> +       depends on INIT_ALL_MEMORY
> +       depends on CC_HAS_AUTO_VAR_INIT || (HAVE_GCC_PLUGINS && PLUGIN_HOSTCC != "")
> +       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
> +       default y
> +       help
> +         Initialize uninitialized stack data with a fixed pattern
> +         (0x00 in GCC, 0xAA in Clang).
> +


I am not sure about the benefit of these umbrella CONFIGs
since these make 'depends on' and 'select' complicated.
I will leave it to Kees, though.



> +endif # INIT_ALL_MEMORY
> +endmenu
> --
> 2.21.0.392.gf8f6787159e-goog
>


-- 
Best Regards
Masahiro Yamada

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