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

[Souptick Joarder <jrdr.linux@gmail.com>]

On Thu, May 9, 2019 at 6:53 PM Alexander Potapenko <glider@google.com> wrote:
>
> 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.
> > >

Not sure, but I think this patch will clash with Matthew's posted patch series
*Remove 'order' argument from many mm functions*.

> > > 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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