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From: Andrew Morton <akpm@linux-foundation.org>
To: Kees Cook <keescook@chromium.org>
Cc: Marco Elver <elver@google.com>, "Lin, Zhenpeng" <zplin@psu.edu>,
	stable@vger.kernel.org, Vlastimil Babka <vbabka@suse.cz>,
	Christoph Lameter <cl@linux.com>,
	Pekka Enberg <penberg@kernel.org>,
	David Rientjes <rientjes@google.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Roman Gushchin <guro@fb.com>,
	linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
	linux-mm@kvack.org, "Lin, Zhenpeng" <zplin@psu.edu>
Subject: Re: [PATCH v4 3/3] mm/slub: Actually fix freelist pointer vs redzoning
Date: Tue, 8 Jun 2021 13:56:33 -0700	[thread overview]
Message-ID: <20210608135633.167bd07cf8011a792a128976@linux-foundation.org> (raw)
In-Reply-To: <20210608183955.280836-4-keescook@chromium.org>

On Tue,  8 Jun 2021 11:39:55 -0700 Kees Cook <keescook@chromium.org> wrote:

> It turns out that SLUB redzoning ("slub_debug=Z") checks from
> s->object_size rather than from s->inuse (which is normally bumped
> to make room for the freelist pointer), so a cache created with an
> object size less than 24 would have the freelist pointer written beyond
> s->object_size, causing the redzone to be corrupted by the freelist
> pointer. This was very visible with "slub_debug=ZF":
> 
> BUG test (Tainted: G    B            ): Right Redzone overwritten
> -----------------------------------------------------------------------------
> 
> INFO: 0xffff957ead1c05de-0xffff957ead1c05df @offset=1502. First byte 0x1a instead of 0xbb
> INFO: Slab 0xffffef3950b47000 objects=170 used=170 fp=0x0000000000000000 flags=0x8000000000000200
> INFO: Object 0xffff957ead1c05d8 @offset=1496 fp=0xffff957ead1c0620
> 
> Redzone  (____ptrval____): bb bb bb bb bb bb bb bb               ........
> Object   (____ptrval____): 00 00 00 00 00 f6 f4 a5               ........
> Redzone  (____ptrval____): 40 1d e8 1a aa                        @....
> Padding  (____ptrval____): 00 00 00 00 00 00 00 00               ........
> 
> Adjust the offset to stay within s->object_size.
> 
> (Note that no caches of in this size range are known to exist in the
> kernel currently.)

We already have
https://lkml.kernel.org/r/6746FEEA-FD69-4792-8DDA-C78F5FE7DA02@psu.edu.
Is this patch better?

> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3689,7 +3689,6 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
>  {
>  	slab_flags_t flags = s->flags;
>  	unsigned int size = s->object_size;
> -	unsigned int freepointer_area;
>  	unsigned int order;
>  
>  	/*
> @@ -3698,13 +3697,6 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
>  	 * the possible location of the free pointer.
>  	 */
>  	size = ALIGN(size, sizeof(void *));
> -	/*
> -	 * This is the area of the object where a freepointer can be
> -	 * safely written. If redzoning adds more to the inuse size, we
> -	 * can't use that portion for writing the freepointer, so
> -	 * s->offset must be limited within this for the general case.
> -	 */
> -	freepointer_area = size;
>  
>  #ifdef CONFIG_SLUB_DEBUG
>  	/*
> @@ -3730,7 +3722,7 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
>  
>  	/*
>  	 * With that we have determined the number of bytes in actual use
> -	 * by the object. This is the potential offset to the free pointer.
> +	 * by the object and redzoning.
>  	 */
>  	s->inuse = size;
>  
> @@ -3753,13 +3745,13 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
>  		 */
>  		s->offset = size;
>  		size += sizeof(void *);
> -	} else if (freepointer_area > sizeof(void *)) {
> +	} else {
>  		/*
>  		 * Store freelist pointer near middle of object to keep
>  		 * it away from the edges of the object to avoid small
>  		 * sized over/underflows from neighboring allocations.
>  		 */
> -		s->offset = ALIGN(freepointer_area / 2, sizeof(void *));
> +		s->offset = ALIGN_DOWN(s->object_size / 2, sizeof(void *));
>  	}
>  
>  #ifdef CONFIG_SLUB_DEBUG
> -- 
> 2.25.1

  reply	other threads:[~2021-06-08 20:56 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-08 18:39 [PATCH v4 0/3] " Kees Cook
2021-06-08 18:39 ` [PATCH v4 1/3] mm/slub: Clarify verification reporting Kees Cook
2021-06-08 18:39 ` [PATCH v4 2/3] mm/slub: Fix redzoning for small allocations Kees Cook
2021-06-11  9:13   ` Vlastimil Babka
2021-06-08 18:39 ` [PATCH v4 3/3] mm/slub: Actually fix freelist pointer vs redzoning Kees Cook
2021-06-08 20:56   ` Andrew Morton [this message]
2021-06-08 23:11     ` Kees Cook
2021-06-08 20:53 ` [PATCH v4 0/3] " Andrew Morton
2021-06-08 23:08   ` Kees Cook

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