Re: [PATCH v7 1/2] mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options

[Alexander Potapenko <glider@google.com>]

On Fri, Jun 21, 2019 at 9:09 AM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Mon 17-06-19 17:10:49, Alexander Potapenko wrote:
> > The new options are needed to prevent possible information leaks and
> > make control-flow bugs that depend on uninitialized values more
> > deterministic.
> >
> > init_on_alloc=1 makes the kernel initialize newly allocated pages and heap
> > objects with zeroes. Initialization is done at allocation time at the
> > places where checks for __GFP_ZERO are performed.
> >
> > init_on_free=1 makes the kernel initialize freed pages and heap objects
> > with zeroes upon their deletion. This helps to ensure sensitive data
> > doesn't leak via use-after-free accesses.
> >
> > Both init_on_alloc=1 and init_on_free=1 guarantee that the allocator
> > returns zeroed memory. The two exceptions are slab caches with
> > constructors and SLAB_TYPESAFE_BY_RCU flag. Those are never
> > zero-initialized to preserve their semantics.
> >
> > Both init_on_alloc and init_on_free default to zero, but those defaults
> > can be overridden with CONFIG_INIT_ON_ALLOC_DEFAULT_ON and
> > CONFIG_INIT_ON_FREE_DEFAULT_ON.
> >
> > Slowdown for the new features compared to init_on_free=0,
> > init_on_alloc=0:
> >
> > hackbench, init_on_free=1:  +7.62% sys time (st.err 0.74%)
> > hackbench, init_on_alloc=1: +7.75% sys time (st.err 2.14%)
> >
> > Linux build with -j12, init_on_free=1:  +8.38% wall time (st.err 0.39%)
> > Linux build with -j12, init_on_free=1:  +24.42% sys time (st.err 0.52%)
> > Linux build with -j12, init_on_alloc=1: -0.13% wall time (st.err 0.42%)
> > Linux build with -j12, init_on_alloc=1: +0.57% sys time (st.err 0.40%)
> >
> > The slowdown for init_on_free=0, init_on_alloc=0 compared to the
> > baseline is within the standard error.
> >
> > The new features are also going to pave the way for hardware memory
> > tagging (e.g. arm64's MTE), which will require both on_alloc and on_free
> > hooks to set the tags for heap objects. With MTE, tagging will have the
> > same cost as memory initialization.
> >
> > Although init_on_free is rather costly, there are paranoid use-cases where
> > in-memory data lifetime is desired to be minimized. There are various
> > arguments for/against the realism of the associated threat models, but
> > given that we'll need the infrastructre for MTE anyway, and there are
> > people who want wipe-on-free behavior no matter what the performance cost,
> > it seems reasonable to include it in this series.
>
> Thanks for reworking the original implemenation. This looks much better!
>
> > Signed-off-by: Alexander Potapenko <glider@google.com>
> > Acked-by: Kees Cook <keescook@chromium.org>
> > To: Andrew Morton <akpm@linux-foundation.org>
> > To: Christoph Lameter <cl@linux.com>
> > To: Kees Cook <keescook@chromium.org>
> > Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
> > Cc: Michal Hocko <mhocko@kernel.org>
> > 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: 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: Marco Elver <elver@google.com>
> > Cc: linux-mm@kvack.org
> > Cc: linux-security-module@vger.kernel.org
> > Cc: kernel-hardening@lists.openwall.com
>
> Acked-by: Michal Hocko <mhocko@suse.cz> # page allocator parts.
>
> kmalloc based parts look good to me as well but I am not sure I fill
> qualified to give my ack there without much more digging and I do not
> have much time for that now.
>
> [...]
> > diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> > index fd5c95ff9251..2f75dd0d0d81 100644
> > --- a/kernel/kexec_core.c
> > +++ b/kernel/kexec_core.c
> > @@ -315,7 +315,7 @@ static struct page *kimage_alloc_pages(gfp_t gfp_mask, unsigned int order)
> >               arch_kexec_post_alloc_pages(page_address(pages), count,
> >                                           gfp_mask);
> >
> > -             if (gfp_mask & __GFP_ZERO)
> > +             if (want_init_on_alloc(gfp_mask))
> >                       for (i = 0; i < count; i++)
> >                               clear_highpage(pages + i);
> >       }
>
> I am not really sure I follow here. Why do we want to handle
> want_init_on_alloc here? The allocated memory comes from the page
> allocator and so it will get zeroed there. arch_kexec_post_alloc_pages
> might touch the content there but is there any actual risk of any kind
> of leak?
You're right, we don't want to initialize this memory if init_on_alloc is on.
We need something along the lines of:
  if (!static_branch_unlikely(&init_on_alloc))
    if (gfp_mask & __GFP_ZERO)
      // clear the pages

Another option would be to disable initialization in alloc_pages() using a flag.
>
> > diff --git a/mm/dmapool.c b/mm/dmapool.c
> > index 8c94c89a6f7e..e164012d3491 100644
> > --- a/mm/dmapool.c
> > +++ b/mm/dmapool.c
> > @@ -378,7 +378,7 @@ void *dma_pool_alloc(struct dma_pool *pool, gfp_t mem_flags,
> >  #endif
> >       spin_unlock_irqrestore(&pool->lock, flags);
> >
> > -     if (mem_flags & __GFP_ZERO)
> > +     if (want_init_on_alloc(mem_flags))
> >               memset(retval, 0, pool->size);
> >
> >       return retval;
>
> Don't you miss dma_pool_free and want_init_on_free?
Agreed.
I'll fix this and add tests for DMA pools as well.
> --
> Michal Hocko
> SUSE Labs



-- 
Alexander Potapenko
Software Engineer

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