Re: [PATCH v6 2/4] mm/slub: only zero the requested size of buffer for kzalloc

[Feng Tang <feng.tang@intel.com>]

On Tue, Sep 27, 2022 at 03:11:24AM +0800, Andrey Konovalov wrote:
> On Tue, Sep 13, 2022 at 8:54 AM Feng Tang <feng.tang@intel.com> wrote:
> >
> 
> Hi Feng,
> 
> > kzalloc/kmalloc will round up the request size to a fixed size
> > (mostly power of 2), so the allocated memory could be more than
> > requested. Currently kzalloc family APIs will zero all the
> > allocated memory.
> >
> > To detect out-of-bound usage of the extra allocated memory, only
> > zero the requested part, so that sanity check could be added to
> > the extra space later.
> 
> I still don't like the idea of only zeroing the requested memory and
> not the whole object. Considering potential info-leak vulnerabilities.
> 
> Can we only do this when SLAB_DEBUG is enabled?

Good point! will add slub_debug_orig_size(s) check. 

> > Performance wise, smaller zeroing length also brings shorter
> > execution time, as shown from test data on various server/desktop
> > platforms.
> >
> > For kzalloc users who will call ksize() later and utilize this
> > extra space, please be aware that the space is not zeroed any
> > more.
> 
> CC Kees
 
Thanks for adding Kees, who provided review from security point
of review.

> >
> > Signed-off-by: Feng Tang <feng.tang@intel.com>
> > ---
> >  mm/slab.c |  7 ++++---
> >  mm/slab.h |  5 +++--
> >  mm/slub.c | 10 +++++++---
[...]
> > @@ -730,7 +730,8 @@ static inline struct kmem_cache *slab_pre_alloc_hook(struct kmem_cache *s,
> >
> >  static inline void slab_post_alloc_hook(struct kmem_cache *s,
> >                                         struct obj_cgroup *objcg, gfp_t flags,
> > -                                       size_t size, void **p, bool init)
> > +                                       size_t size, void **p, bool init,
> > +                                       unsigned int orig_size)
> >  {
> >         size_t i;
> >
> > @@ -746,7 +747,7 @@ static inline void slab_post_alloc_hook(struct kmem_cache *s,
> >         for (i = 0; i < size; i++) {
> >                 p[i] = kasan_slab_alloc(s, p[i], flags, init);
> >                 if (p[i] && init && !kasan_has_integrated_init())
> > -                       memset(p[i], 0, s->object_size);
> > +                       memset(p[i], 0, orig_size);
> 
> Note that when KASAN is enabled and has integrated init, it will
> initialize the whole object, which leads to an inconsistency with this
> change.

Do you mean for kzalloc() only? or there is some kasan check newly added?

I'm not familiar with kasan code, and during development, I usually
enabled KASAN and KFENCE configs and did catch some bugs, while 0Day
bot also reported some. And with latest v6 patchset, I haven't seen
kasan/kfence failed cases.

And for generic slub objects, when slub_debug is enabled, the object
data area could be already modified like in init_object() 

	if (s->flags & __OBJECT_POISON) {
		memset(p, POISON_FREE, s->object_size - 1);
		p[s->object_size - 1] = POISON_END;
	}

slub-redzone check actually splitis it into 2 regions [0, orig_size-1],
and [orig_size, object_size-1], and adds different sanity check to
them.

Anyway, I'll go check the latest linux-next tree.

Thanks,
Feng