Re: [PATCH v4 2/3] mm/slub: Fix redzoning for small allocations

[Vlastimil Babka]

On 6/8/21 8:39 PM, Kees Cook wrote:
> The redzone area for SLUB exists between s->object_size and s->inuse
> (which is at least the word-aligned object_size). If a cache were created
> with an object_size smaller than sizeof(void *), the in-object stored
> freelist pointer would overwrite the redzone (e.g. with boot param
> "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____): f6 f4 a5 40 1d e8          ...@..
> Redzone  (____ptrval____): 1a aa                      ..
> Padding  (____ptrval____): 00 00 00 00 00 00 00 00    ........
> 
> Store the freelist pointer out of line when object_size is smaller than
> sizeof(void *) and redzoning is enabled.
> 
> Additionally remove the "smaller than sizeof(void *)" check under
> CONFIG_DEBUG_VM in kmem_cache_sanity_check() as it is now redundant:
> SLAB and SLOB both handle small sizes.
> 
> (Note that no caches within this size range are known to exist in the
> kernel currently.)
> 
> Fixes: 81819f0fc828 ("SLUB core")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>  mm/slab_common.c | 3 +--
>  mm/slub.c        | 8 +++++---
>  2 files changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index a4a571428c51..7cab77655f11 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -97,8 +97,7 @@ EXPORT_SYMBOL(kmem_cache_size);
>  #ifdef CONFIG_DEBUG_VM
>  static int kmem_cache_sanity_check(const char *name, unsigned int size)
>  {
> -	if (!name || in_interrupt() || size < sizeof(void *) ||
> -		size > KMALLOC_MAX_SIZE) {
> +	if (!name || in_interrupt() || size > KMALLOC_MAX_SIZE) {
>  		pr_err("kmem_cache_create(%s) integrity check failed\n", name);
>  		return -EINVAL;
>  	}
> diff --git a/mm/slub.c b/mm/slub.c
> index f91d9fe7d0d8..f58cfd456548 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3734,15 +3734,17 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
>  	 */
>  	s->inuse = size;
>  
> -	if (((flags & (SLAB_TYPESAFE_BY_RCU | SLAB_POISON)) ||
> -		s->ctor)) {
> +	if ((flags & (SLAB_TYPESAFE_BY_RCU | SLAB_POISON)) ||
> +	    ((flags & SLAB_RED_ZONE) && s->object_size < sizeof(void *)) ||
> +	    s->ctor) {
>  		/*
>  		 * Relocate free pointer after the object if it is not
>  		 * permitted to overwrite the first word of the object on
>  		 * kmem_cache_free.
>  		 *
>  		 * This is the case if we do RCU, have a constructor or
> -		 * destructor or are poisoning the objects.
> +		 * destructor, are poisoning the objects, or are
> +		 * redzoning an object smaller than sizeof(void *).
>  		 *
>  		 * The assumption that s->offset >= s->inuse means free
>  		 * pointer is outside of the object is used in the
>