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Date: Tue, 9 Apr 2019 11:02:58 +0200
From: Alexander Potapenko <glider@...gle.com>
To: Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc: James Morris <jmorris@...ei.org>, "Serge E. Hallyn" <serge@...lyn.com>, 
	linux-security-module <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>, 
	Sandeep Patil <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 10:38 AM Masahiro Yamada
<yamada.masahiro@...ionext.com> wrote:
>
> 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.
There's CFLAGS_INITMEM in the top Makefile, so I suppose it should be working.
But I agree there's no point in adding extra files for a single build flag.
>
> 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



-- 
Alexander Potapenko
Software Engineer

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