Re: [PATCH v7 3/3] mm/slub: extend redzone check to extra allocated kmalloc space than requested

[Vlastimil Babka <vbabka@suse.cz>]

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@suse.cz>
> Signed-off-by: Feng Tang <feng.tang@intel.com>
> Acked-by: Hyeonggon Yoo <42.hyeyoo@gmail.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) {
>  		/*