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Date: Thu, 20 Jun 2019 18:37:30 -0700
From: Kees Cook <keescook@...omium.org>
To: Alexander Potapenko <glider@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Christoph Lameter <cl@...ux.com>,
Masahiro Yamada <yamada.masahiro@...ionext.com>,
Michal Hocko <mhocko@...nel.org>, James Morris <jmorris@...ei.org>,
"Serge E. Hallyn" <serge@...lyn.com>,
Nick Desaulniers <ndesaulniers@...gle.com>,
Kostya Serebryany <kcc@...gle.com>,
Dmitry Vyukov <dvyukov@...gle.com>,
Sandeep Patil <sspatil@...roid.com>,
Laura Abbott <labbott@...hat.com>,
Randy Dunlap <rdunlap@...radead.org>, Jann Horn <jannh@...gle.com>,
Mark Rutland <mark.rutland@....com>, Marco Elver <elver@...gle.com>,
linux-mm@...ck.org, linux-security-module@...r.kernel.org,
kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH v6 1/3] mm: security: introduce init_on_alloc=1 and
init_on_free=1 boot options
On Fri, Jun 07, 2019 at 08:42:27AM -0700, Kees Cook wrote:
> On Thu, Jun 06, 2019 at 06:48:43PM +0200, Alexander Potapenko wrote:
> > [...]
> > diff --git a/mm/slub.c b/mm/slub.c
> > index cd04dbd2b5d0..9c4a8b9a955c 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > [...]
> > @@ -2741,8 +2758,14 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s,
> > prefetch_freepointer(s, next_object);
> > stat(s, ALLOC_FASTPATH);
> > }
> > + /*
> > + * If the object has been wiped upon free, make sure it's fully
> > + * initialized by zeroing out freelist pointer.
> > + */
> > + if (unlikely(slab_want_init_on_free(s)) && object)
> > + *(void **)object = NULL;
In looking at metadata again, I noticed that I don't think this is
correct, as it needs to be using s->offset to find the location of the
freelist pointer:
memset(object + s->offset, 0, sizeof(void *));
> >
> > - if (unlikely(gfpflags & __GFP_ZERO) && object)
> > + if (unlikely(slab_want_init_on_alloc(gfpflags, s)) && object)
> > memset(object, 0, s->object_size);
init_on_alloc is using "object_size" but init_on_free is using "size". I
assume the "alloc" wipe is smaller because metadata was just written
for the allocation?
--
Kees Cook
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