Re: [PATCH] slub: choose the right freelist pointer location when creating small caches

[Kees Cook]

On Sat, Jun 05, 2021 at 01:58:13AM +0000, Lin, Zhenpeng wrote:
> When enabling CONFIG_SLUB_DEBUG and booting with "slub_debug=Z", the
> kernel crashes at creating caches if the object size is smaller
> than 2*sizeof(void*). The problem is due to the wrong calculation
> of freepointer_area. The freelist pointer can be stored in the
> middle of object only if the object size is not smaller than
> 2*sizeof(void*). Otherwise, the freelist pointer will be corrupted by
> SLUB_RED_ZONE.
> 
> Fixes: 3202fa62fb43 ("slub: relocate freelist pointer to middle of object")
> Fixes: 89b83f282d8b ("slub: avoid redzone when choosing freepointer location")
> Signed-off-by: Zhenpeng Lin <zplin@xxxxxxx>
> ---
> mm/slub.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 3f96e099817a..cb23233ee683 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3704,7 +3704,7 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
> 	 * can't use that portion for writing the freepointer, so
> 	 * s->offset must be limited within this for the general case.
> 	 */
> -	freepointer_area = size;
> +	freepointer_area = s->object_size;
> 
> #ifdef CONFIG_SLUB_DEBUG
> 	/*
> @@ -3751,7 +3751,7 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
> 		 */
> 		s->offset = size;
> 		size += sizeof(void *);
> -	} else if (freepointer_area > sizeof(void *)) {
> +	} else if (freepointer_area >= 2 * sizeof(void *)) {
> 		/*
> 		 * Store freelist pointer near middle of object to keep
> 		 * it away from the edges of the object to avoid small
> --
> 2.17.1

NAK, I'd prefer this get cleaned up more completely, especially since
there are no objects of that size in the kernel currently:

https://lore.kernel.org/lkml/20201015033712.1491731-1-keescook@xxxxxxxxxxxx/

-- 
Kees Cook