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

[Andrey Konovalov]

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.

Thanks!


>                 kmemleak_alloc_recursive(p[i], s->object_size, 1,
>                                          s->flags, flags);
>         }
> diff --git a/mm/slub.c b/mm/slub.c
> index effd994438e6..f523601d3fcf 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3376,7 +3376,7 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s, struct list_l
>         init = slab_want_init_on_alloc(gfpflags, s);
>
>  out:
> -       slab_post_alloc_hook(s, objcg, gfpflags, 1, &object, init);
> +       slab_post_alloc_hook(s, objcg, gfpflags, 1, &object, init, orig_size);
>
>         return object;
>  }
> @@ -3833,11 +3833,11 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
>          * Done outside of the IRQ disabled fastpath loop.
>          */
>         slab_post_alloc_hook(s, objcg, flags, size, p,
> -                               slab_want_init_on_alloc(flags, s));
> +                               slab_want_init_on_alloc(flags, s), 0);
>         return i;
>  error:
>         slub_put_cpu_ptr(s->cpu_slab);
> -       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;
>  }
> --
> 2.34.1
>
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