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Date: Wed, 9 Nov 2022 15:28:19 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Feng Tang <feng.tang@...el.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Christoph Lameter <cl@...ux.com>,
Pekka Enberg <penberg@...nel.org>,
David Rientjes <rientjes@...gle.com>,
Joonsoo Kim <iamjoonsoo.kim@....com>,
Roman Gushchin <roman.gushchin@...ux.dev>,
Hyeonggon Yoo <42.hyeyoo@...il.com>,
Dmitry Vyukov <dvyukov@...gle.com>,
Andrey Konovalov <andreyknvl@...il.com>,
Kees Cook <keescook@...omium.org>
Cc: Dave Hansen <dave.hansen@...el.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com
Subject: Re: [PATCH v7 1/3] mm/slub: only zero requested size of buffer for
kzalloc when debug enabled
On 10/21/22 05:24, Feng Tang 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@chromium.org/#r
> Signed-off-by: Feng Tang <feng.tang@...el.com>
> ---
> 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) &&
Shouldn't we check SLAB_RED_ZONE instead? Otherwise a debugging could be
specified so that SLAB_RED_ZONE is set but 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
s/will be/might be/ because it depends on the debugging?
> + */
> + 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;
> }
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