Re: [PATCH] slub: Actually fix freelist pointer vs redzoning

[Marco Elver]

On Fri, 9 Oct 2020 at 01:34, Kees Cook <keescook@xxxxxxxxxxxx> 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 their freelist pointer written beyond
> s->object_size, causing the redzone to corrupt the freelist pointer.
> This was very visible with "slub_debug=ZF":
>
> BUG test (Tainted: G    B            ): 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 there appear to be no such small-sized caches in the kernel
> currently.)
>
> Reported-by: Marco Elver <elver@xxxxxxxxxx>
> Link: https://lore.kernel.org/linux-mm/20200807160627.GA1420741@xxxxxxxxxxxxxxxx/
> Fixes: 89b83f282d8b (slub: avoid redzone when choosing freepointer location)
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  mm/slub.c | 17 +++++------------
>  1 file changed, 5 insertions(+), 12 deletions(-)

Tested-by: Marco Elver <elver@xxxxxxxxxx>

Thank you!

> diff --git a/mm/slub.c b/mm/slub.c
> index 68c02b2eecd9..979f5da26992 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3641,7 +3641,6 @@ 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;
>
>         /*
> @@ -3650,13 +3649,6 @@ 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
>         /*
> @@ -3682,7 +3674,7 @@ 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;
>
> @@ -3694,7 +3686,8 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
>                  * 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
> @@ -3703,13 +3696,13 @@ 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
>