[PATCH v3 1/2] initmem: introduce CONFIG_INIT_ALL_MEMORY and CONFIG_INIT_ALL_STACK
Alexander Potapenko
glider at google.com
Tue Apr 9 09:02:58 UTC 2019
On Tue, Apr 9, 2019 at 10:38 AM Masahiro Yamada
<yamada.masahiro at socionext.com> wrote:
>
> On Tue, Apr 9, 2019 at 2:04 AM Alexander Potapenko <glider at google.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 at google.com>
> > Cc: Masahiro Yamada <yamada.masahiro at socionext.com>
> > Cc: James Morris <jmorris at namei.org>
> > Cc: "Serge E. Hallyn" <serge at hallyn.com>
> > Cc: Nick Desaulniers <ndesaulniers at google.com>
> > Cc: Kostya Serebryany <kcc at google.com>
> > Cc: Dmitry Vyukov <dvyukov at google.com>
> > Cc: Kees Cook <keescook at chromium.org>
> > Cc: Sandeep Patil <sspatil at android.com>
> > Cc: linux-security-module at vger.kernel.org
> > Cc: linux-kbuild at vger.kernel.org
> > Cc: kernel-hardening at lists.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
Google Germany GmbH
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