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Date: Thu, 10 Nov 2022 16:48:35 +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 3/3] mm/slub: extend redzone check to extra allocated
kmalloc space than requested
On 10/21/22 05:24, Feng Tang wrote:
> kmalloc will round up the request size to a fixed size (mostly power
> of 2), so there could be a extra space than what is requested, whose
> size is the actual buffer size minus original request size.
>
> To better detect out of bound access or abuse of this space, add
> redzone sanity check for it.
>
> In current kernel, some kmalloc user already knows the existence of
> the space and utilizes it after calling 'ksize()' to know the real
> size of the allocated buffer. So we skip the sanity check for objects
> which have been called with ksize(), as treating them as legitimate
> users.
Hm so once Kees's effort is finished and all ksize() users behave correctly,
we can drop all that skip_orig_size_check() code, right?
> In some cases, the free pointer could be saved inside the latter
> part of object data area, which may overlap the redzone part(for
> small sizes of kmalloc objects). As suggested by Hyeonggon Yoo,
> force the free pointer to be in meta data area when kmalloc redzone
> debug is enabled, to make all kmalloc objects covered by redzone
> check.
>
> Suggested-by: Vlastimil Babka <vbabka@...e.cz>
> Signed-off-by: Feng Tang <feng.tang@...el.com>
> Acked-by: Hyeonggon Yoo <42.hyeyoo@...il.com>
Looks fine, but a suggestion below:
> ---
> mm/slab.h | 4 ++++
> mm/slab_common.c | 4 ++++
> mm/slub.c | 51 ++++++++++++++++++++++++++++++++++++++++++++----
> 3 files changed, 55 insertions(+), 4 deletions(-)
>
> diff --git a/mm/slab.h b/mm/slab.h
> index 8b4ee02fc14a..1dd773afd0c4 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -885,4 +885,8 @@ void __check_heap_object(const void *ptr, unsigned long n,
> }
> #endif
>
> +#ifdef CONFIG_SLUB_DEBUG
> +void skip_orig_size_check(struct kmem_cache *s, const void *object);
> +#endif
> +
> #endif /* MM_SLAB_H */
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 33b1886b06eb..0bb4625f10a2 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -1037,6 +1037,10 @@ size_t __ksize(const void *object)
> return folio_size(folio);
> }
>
> +#ifdef CONFIG_SLUB_DEBUG
> + skip_orig_size_check(folio_slab(folio)->slab_cache, object);
> +#endif
> +
> return slab_ksize(folio_slab(folio)->slab_cache);
> }
>
> diff --git a/mm/slub.c b/mm/slub.c
> index adff7553b54e..76581da6b9df 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -829,6 +829,17 @@ static inline void set_orig_size(struct kmem_cache *s,
> if (!slub_debug_orig_size(s))
> return;
>
> +#ifdef CONFIG_KASAN_GENERIC
> + /*
> + * KASAN could save its free meta data in object's data area at
> + * offset 0, if the size is larger than 'orig_size', it will
> + * overlap the data redzone in [orig_size+1, object_size], and
> + * the check should be skipped.
> + */
> + if (kasan_metadata_size(s, true) > orig_size)
> + orig_size = s->object_size;
> +#endif
> +
> p += get_info_end(s);
> p += sizeof(struct track) * 2;
>
> @@ -848,6 +859,11 @@ static inline unsigned int get_orig_size(struct kmem_cache *s, void *object)
> return *(unsigned int *)p;
> }
>
> +void skip_orig_size_check(struct kmem_cache *s, const void *object)
> +{
> + set_orig_size(s, (void *)object, s->object_size);
> +}
> +
> static void slab_bug(struct kmem_cache *s, char *fmt, ...)
> {
> struct va_format vaf;
> @@ -966,13 +982,27 @@ static __printf(3, 4) void slab_err(struct kmem_cache *s, struct slab *slab,
> static void init_object(struct kmem_cache *s, void *object, u8 val)
> {
> u8 *p = kasan_reset_tag(object);
> + unsigned int orig_size = s->object_size;
>
> - if (s->flags & SLAB_RED_ZONE)
> + if (s->flags & SLAB_RED_ZONE) {
> memset(p - s->red_left_pad, val, s->red_left_pad);
>
> + if (slub_debug_orig_size(s) && val == SLUB_RED_ACTIVE) {
> + orig_size = get_orig_size(s, object);
> +
> + /*
> + * Redzone the extra allocated space by kmalloc
> + * than requested.
> + */
> + if (orig_size < s->object_size)
> + memset(p + orig_size, val,
> + s->object_size - orig_size);
Wondering if we can remove this if - memset and instead below:
> + }
> + }
> +
> if (s->flags & __OBJECT_POISON) {
> - memset(p, POISON_FREE, s->object_size - 1);
> - p[s->object_size - 1] = POISON_END;
> + memset(p, POISON_FREE, orig_size - 1);
> + p[orig_size - 1] = POISON_END;
> }
>
> if (s->flags & SLAB_RED_ZONE)
This continues by:
memset(p + s->object_size, val, s->inuse - s->object_size);
Instead we could do this, no?
memset(p + orig_size, val, s->inuse - orig_size);
> @@ -1120,6 +1150,7 @@ static int check_object(struct kmem_cache *s, struct slab *slab,
> {
> u8 *p = object;
> u8 *endobject = object + s->object_size;
> + unsigned int orig_size;
>
> if (s->flags & SLAB_RED_ZONE) {
> if (!check_bytes_and_report(s, slab, object, "Left Redzone",
> @@ -1129,6 +1160,17 @@ static int check_object(struct kmem_cache *s, struct slab *slab,
> if (!check_bytes_and_report(s, slab, object, "Right Redzone",
> endobject, val, s->inuse - s->object_size))
> return 0;
> +
> + if (slub_debug_orig_size(s) && val == SLUB_RED_ACTIVE) {
> + orig_size = get_orig_size(s, object);
> +
> + if (s->object_size > orig_size &&
> + !check_bytes_and_report(s, slab, object,
> + "kmalloc Redzone", p + orig_size,
> + val, s->object_size - orig_size)) {
> + return 0;
> + }
> + }
> } else {
> if ((s->flags & SLAB_POISON) && s->object_size < s->inuse) {
> check_bytes_and_report(s, slab, p, "Alignment padding",
> @@ -4206,7 +4248,8 @@ static int calculate_sizes(struct kmem_cache *s)
> */
> s->inuse = size;
>
> - if ((flags & (SLAB_TYPESAFE_BY_RCU | SLAB_POISON)) ||
> + if (slub_debug_orig_size(s) ||
> + (flags & (SLAB_TYPESAFE_BY_RCU | SLAB_POISON)) ||
> ((flags & SLAB_RED_ZONE) && s->object_size < sizeof(void *)) ||
> s->ctor) {
> /*
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