Re: [PATCH v5 2/4] mm/slub: only zero the requested size of buffer for kzalloc

[Feng Tang]

On Sun, Sep 11, 2022 at 07:11:18AM +0800, Andrey Konovalov wrote:
> On Wed, Sep 7, 2022 at 9:10 AM Feng Tang <feng.tang@xxxxxxxxx> wrote:
> >
> > 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.
> >
> > For kzalloc users who will call ksize() later and utilize this
> > extra space, please be aware that the space is not zeroed any
> > more.
> >
> > Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
> > ---
> >  mm/slab.c | 6 +++---
> >  mm/slab.h | 9 +++++++--
> >  mm/slub.c | 6 +++---
> >  3 files changed, 13 insertions(+), 8 deletions(-)
> >
> > diff --git a/mm/slab.c b/mm/slab.c
> > index a5486ff8362a..73ecaa7066e1 100644
> > --- a/mm/slab.c
> > +++ b/mm/slab.c
> > @@ -3253,7 +3253,7 @@ slab_alloc_node(struct kmem_cache *cachep, struct list_lru *lru, gfp_t flags,
> >         init = slab_want_init_on_alloc(flags, cachep);
> >
> >  out:
> > -       slab_post_alloc_hook(cachep, objcg, flags, 1, &objp, init);
> > +       slab_post_alloc_hook(cachep, objcg, flags, 1, &objp, init, 0);
> >         return objp;
> >  }
> >
> > @@ -3506,13 +3506,13 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
> >          * Done outside of the IRQ disabled section.
> >          */
> >         slab_post_alloc_hook(s, objcg, flags, size, p,
> > -                               slab_want_init_on_alloc(flags, s));
> > +                               slab_want_init_on_alloc(flags, s), 0);
> >         /* FIXME: Trace call missing. Christoph would like a bulk variant */
> >         return size;
> >  error:
> >         local_irq_enable();
> >         cache_alloc_debugcheck_after_bulk(s, flags, i, p, _RET_IP_);
> > -       slab_post_alloc_hook(s, objcg, flags, i, p, false);
> > +       slab_post_alloc_hook(s, objcg, flags, i, p, false, 0);
> >         kmem_cache_free_bulk(s, i, p);
> >         return 0;
> >  }
> > diff --git a/mm/slab.h b/mm/slab.h
> > index d0ef9dd44b71..20f9e2a9814f 100644
> > --- a/mm/slab.h
> > +++ b/mm/slab.h
> > @@ -730,12 +730,17 @@ 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;
> >
> >         flags &= gfp_allowed_mask;
> >
> > +       /* If original request size(kmalloc) is not set, use object_size */
> > +       if (!orig_size)
> > +               orig_size = s->object_size;
> > +
> >         /*
> >          * As memory initialization might be integrated into KASAN,
> >          * kasan_slab_alloc and initialization memset must be
> > @@ -746,7 +751,7 @@ 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);
> 
> Arguably, with slab_want_init_on_alloc(), all allocated memory should
> be zeroed to prevent possibility of info-leaks, even unused paddings.
> Perhaps, Alexander can give his opinion here.

Initially, I thought about only zero the requested part(orig_size)
when slub_debug is enabled for that slab. But from the profiling,
zeroing 4096+1 bytes and zeroing 8192 bytes, has obvious difference
in execution time (about 10 us vs 18 us).

Semantics wise, requesting 'A' bytes being zeroed and expecting
'A+B' zeroed bytes is not very valid, IMHO

Also this 2/4 patch is also a preparation for 4/4 of redzone
extension, without it, the redzone initialization will be
overridden by the zeroing.

Thanks,
Feng

> Thanks!
>