Re: [PATCH 3/4] gfp: mm: introduce __GFP_NOINIT

[Alexander Potapenko <glider@google.com>]

From: Kees Cook <keescook@chromium.org>
Date: Wed, May 8, 2019 at 9:16 PM
To: Alexander Potapenko
Cc: Andrew Morton, Christoph Lameter, Kees Cook, Laura Abbott,
Linux-MM, linux-security-module, Kernel Hardening, Masahiro Yamada,
James Morris, Serge E. Hallyn, Nick Desaulniers, Kostya Serebryany,
Dmitry Vyukov, Sandeep Patil, Randy Dunlap, Jann Horn, Mark Rutland

> On Wed, May 8, 2019 at 8:38 AM Alexander Potapenko <glider@google.com> wrote:
> > When passed to an allocator (either pagealloc or SL[AOU]B), __GFP_NOINIT
> > tells it to not initialize the requested memory if the init_on_alloc
> > boot option is enabled. This can be useful in the cases newly allocated
> > memory is going to be initialized by the caller right away.
> >
> > __GFP_NOINIT doesn't affect init_on_free behavior, except for SLOB,
> > where init_on_free implies init_on_alloc.
> >
> > __GFP_NOINIT basically defeats the hardening against information leaks
> > provided by init_on_alloc, so one should use it with caution.
> >
> > This patch also adds __GFP_NOINIT to alloc_pages() calls in SL[AOU]B.
> > Doing so is safe, because the heap allocators initialize the pages they
> > receive before passing memory to the callers.
> >
> > Slowdown for the initialization features compared to init_on_free=0,
> > init_on_alloc=0:
> >
> > hackbench, init_on_free=1:  +6.84% sys time (st.err 0.74%)
> > hackbench, init_on_alloc=1: +7.25% sys time (st.err 0.72%)
> >
> > Linux build with -j12, init_on_free=1:  +8.52% wall time (st.err 0.42%)
> > Linux build with -j12, init_on_free=1:  +24.31% sys time (st.err 0.47%)
> > Linux build with -j12, init_on_alloc=1: -0.16% wall time (st.err 0.40%)
> > Linux build with -j12, init_on_alloc=1: +1.24% sys time (st.err 0.39%)
> >
> > The slowdown for init_on_free=0, init_on_alloc=0 compared to the
> > baseline is within the standard error.
> >
> > Signed-off-by: Alexander Potapenko <glider@google.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
> > Cc: James Morris <jmorris@namei.org>
> > Cc: "Serge E. Hallyn" <serge@hallyn.com>
> > Cc: Nick Desaulniers <ndesaulniers@google.com>
> > Cc: Kostya Serebryany <kcc@google.com>
> > Cc: Dmitry Vyukov <dvyukov@google.com>
> > Cc: Kees Cook <keescook@chromium.org>
> > Cc: Sandeep Patil <sspatil@android.com>
> > Cc: Laura Abbott <labbott@redhat.com>
> > Cc: Randy Dunlap <rdunlap@infradead.org>
> > Cc: Jann Horn <jannh@google.com>
> > Cc: Mark Rutland <mark.rutland@arm.com>
> > Cc: linux-mm@kvack.org
> > Cc: linux-security-module@vger.kernel.org
> > Cc: kernel-hardening@lists.openwall.com
> > ---
> >  include/linux/gfp.h | 6 +++++-
> >  include/linux/mm.h  | 2 +-
> >  kernel/kexec_core.c | 2 +-
> >  mm/slab.c           | 2 +-
> >  mm/slob.c           | 3 ++-
> >  mm/slub.c           | 1 +
> >  6 files changed, 11 insertions(+), 5 deletions(-)
> >
> > diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> > index fdab7de7490d..66d7f5604fe2 100644
> > --- a/include/linux/gfp.h
> > +++ b/include/linux/gfp.h
> > @@ -44,6 +44,7 @@ struct vm_area_struct;
> >  #else
> >  #define ___GFP_NOLOCKDEP       0
> >  #endif
> > +#define ___GFP_NOINIT          0x1000000u
>
> I mentioned this in the other patch, but I think this needs to be
> moved ahead of GFP_NOLOCKDEP and adjust the values for GFP_NOLOCKDEP
> and to leave the IS_ENABLED() test in __GFP_BITS_SHIFT alone.
Do we really need this blinking GFP_NOLOCKDEP bit at all?
This approach doesn't scale, we can't even have a second feature that
has a bit depending on the config settings.
Cannot we just fix the number of bits instead?

> >  /* If the above are modified, __GFP_BITS_SHIFT may need updating */
> >
> >  /*
> > @@ -208,16 +209,19 @@ struct vm_area_struct;
> >   * %__GFP_COMP address compound page metadata.
> >   *
> >   * %__GFP_ZERO returns a zeroed page on success.
> > + *
> > + * %__GFP_NOINIT requests non-initialized memory from the underlying allocator.
> >   */
> >  #define __GFP_NOWARN   ((__force gfp_t)___GFP_NOWARN)
> >  #define __GFP_COMP     ((__force gfp_t)___GFP_COMP)
> >  #define __GFP_ZERO     ((__force gfp_t)___GFP_ZERO)
> > +#define __GFP_NOINIT   ((__force gfp_t)___GFP_NOINIT)
> >
> >  /* Disable lockdep for GFP context tracking */
> >  #define __GFP_NOLOCKDEP ((__force gfp_t)___GFP_NOLOCKDEP)
> >
> >  /* Room for N __GFP_FOO bits */
> > -#define __GFP_BITS_SHIFT (23 + IS_ENABLED(CONFIG_LOCKDEP))
> > +#define __GFP_BITS_SHIFT (25)
>
> AIUI, this will break non-CONFIG_LOCKDEP kernels: it should just be:
>
> -#define __GFP_BITS_SHIFT (23 + IS_ENABLED(CONFIG_LOCKDEP))
> +#define __GFP_BITS_SHIFT (24 + IS_ENABLED(CONFIG_LOCKDEP))
>
> >  #define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))
> >
> >  /**
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index ee1a1092679c..8ab152750eb4 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -2618,7 +2618,7 @@ DECLARE_STATIC_KEY_FALSE(init_on_alloc);
> >  static inline bool want_init_on_alloc(gfp_t flags)
> >  {
> >         if (static_branch_unlikely(&init_on_alloc))
> > -               return true;
> > +               return !(flags & __GFP_NOINIT);
> >         return flags & __GFP_ZERO;
>
> What do you think about renaming __GFP_NOINIT to __GFP_NO_AUTOINIT or something?
>
> Regardless, yes, this is nice.
>
> --
> Kees Cook



-- 
Alexander Potapenko
Software Engineer

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