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Message-ID: <CAG_fn=VceGkQPuJ45ffmy-9rRdx515z10N97FApeZR9YrXSHVA@mail.gmail.com>
Date: Tue, 25 Jun 2019 17:42:45 +0200
From: Alexander Potapenko <glider@...gle.com>
To: Kees Cook <keescook@...omium.org>
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 Memory Management List <linux-mm@...ck.org>,
linux-security-module <linux-security-module@...r.kernel.org>,
Kernel Hardening <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 21, 2019 at 3:37 AM Kees Cook <keescook@...omium.org> wrote:
>
> 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 *));
In the cases we support s->offset is always zero (we don't initialize
slabs with ctors or RCU), but using its value is a sane
generalization.
> > >
> > > - 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?
As noted in another thread, using "size" is incorrect, because it may
overwrite the redzone after the object.
I'll send a patch to fix that.
Overwriting the metadata indeed shouldn't make sense in the allocation case.
> --
> Kees Cook
--
Alexander Potapenko
Software Engineer
Google Germany GmbH
Erika-Mann-Straße, 33
80636 München
Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg
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