Re: [PATCH 1/1] RFC: initmem: introduce CONFIG_INIT_ALL_MEMORY and CONFIG_INIT_ALL_STACK

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On Thu, Mar 7, 2019 at 7:37 PM Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote:
>
> On Thu, Mar 7, 2019 at 5:35 AM Alexander Potapenko <glider@xxxxxxxxxx> 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.
> >
> > 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
>
> Hopefully the disagreements there about "defining a new dialect of
> C++" don't apply to us in the kernel.  We'd like to have this feature;
> our dialect of C is already quite far from what ISO has specified,
> both through the explicit use of `-f` compiler flags and use of most
> GNU C extensions.  We already have our own dialect of C for the
> kernel, it's too late for that.  But maybe I should go bikeshed in
> cfe-dev...
>
> Thanks for this patch.  I'll take a closer look once Kees' feedback
> has been addressed.
I've sent out the updated version.
> >     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-file (by using the INIT_ALL_MEMORY_ Makefile prefix) or per-variable
> > (by using __attribute__((uninitialized))) basis.
> >
> > Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
> > Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
> > Cc: James Morris <jmorris@xxxxxxxxx>
> > Cc: "Serge E. Hallyn" <serge@xxxxxxxxxx>
> > Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> > Cc: Kostya Serebryany <kcc@xxxxxxxxxx>
> > Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > Cc: Sandeep Patil <sspatil@xxxxxxxxxxx>
> > Cc: linux-security-module@xxxxxxxxxxxxxxx
> > Cc: linux-kbuild@xxxxxxxxxxxxxxx
> >
> > ---
> >  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
> > +
> > +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.
> > +
> > +if INIT_ALL_MEMORY
> > +
> > +config INIT_ALL_STACK
> > +       bool "Initialize all stack"
> > +       depends on INIT_ALL_MEMORY
> > +       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
> >
>
>
> --
> Thanks,
> ~Nick Desaulniers



-- 
Alexander Potapenko
Software Engineer

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