Re: [PATCH v6 1/3] 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 3:37 AM Kees Cook <keescook@chromium.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

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