Re: [PATCH v7 1/3] mm/slub: only zero requested size of buffer for kzalloc when debug enabled

[Andrey Konovalov]

On Fri, Oct 21, 2022 at 5:24 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 redzone 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 when debug is enabled. (Thanks to Kees Cook's effort to
> sanitize all ksize() user cases [1], this won't be a big issue).
>
> [1]. https://lore.kernel.org/all/20220922031013.2150682-1-keescook@xxxxxxxxxxxx/#r
> Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
> ---
>  mm/slab.c |  7 ++++---
>  mm/slab.h | 18 ++++++++++++++++--
>  mm/slub.c | 10 +++++++---
>  3 files changed, 27 insertions(+), 8 deletions(-)
>
> diff --git a/mm/slab.c b/mm/slab.c
> index a5486ff8362a..4594de0e3d6b 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -3253,7 +3253,8 @@ 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,
> +                               cachep->object_size);
>         return objp;
>  }
>
> @@ -3506,13 +3507,13 @@ 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), s->object_size);
>         /* 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, s->object_size);
>         kmem_cache_free_bulk(s, i, p);
>         return 0;
>  }
> diff --git a/mm/slab.h b/mm/slab.h
> index 0202a8c2f0d2..8b4ee02fc14a 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -720,12 +720,26 @@ 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)
>  {
> +       unsigned int zero_size = s->object_size;
>         size_t i;
>
>         flags &= gfp_allowed_mask;
>
> +       /*
> +        * For kmalloc object, the allocated memory size(object_size) is likely
> +        * larger than the requested size(orig_size). If redzone check is
> +        * enabled for the extra space, don't zero it, as it will be redzoned
> +        * soon. The redzone operation for this extra space could be seen as a
> +        * replacement of current poisoning under certain debug option, and
> +        * won't break other sanity checks.
> +        */
> +       if (kmem_cache_debug_flags(s, SLAB_STORE_USER) &&
> +           (s->flags & SLAB_KMALLOC))
> +               zero_size = orig_size;
> +
>         /*
>          * As memory initialization might be integrated into KASAN,
>          * kasan_slab_alloc and initialization memset must be
> @@ -736,7 +750,7 @@ 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, zero_size);
>                 kmemleak_alloc_recursive(p[i], s->object_size, 1,
>                                          s->flags, flags);
>                 kmsan_slab_alloc(s, p[i], flags);
> diff --git a/mm/slub.c b/mm/slub.c
> index 12354fb8d6e4..17292c2d3eee 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3395,7 +3395,11 @@ 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);
> +       /*
> +        * When init equals 'true', like for kzalloc() family, only
> +        * @orig_size bytes will be zeroed instead of s->object_size
> +        */
> +       slab_post_alloc_hook(s, objcg, gfpflags, 1, &object, init, orig_size);
>
>         return object;
>  }
> @@ -3852,11 +3856,11 @@ 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), s->object_size);
>         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, s->object_size);
>         kmem_cache_free_bulk(s, i, p);
>         return 0;
>  }
> --
> 2.34.1
>

For the KASAN part:

Reviewed-by: Andrey Konovalov <andreyknvl@xxxxxxxxx>

Thanks!