Re: [PATCH] mm: slub: re-initialize randomized freelist sequence in calculate_sizes

[Kees Cook <keescook@chromium.org>]

On Sat, Aug 08, 2020 at 01:50:30PM +0400, kpark3469@gmail.com wrote:
> From: Sahara <keun-o.park@digital14.com>

Hi!

> 
> Slab cache flags are exported to sysfs and are allowed to get modified
> from userspace. Some of those may call calculate_sizes function because
> the changed flag can take an effect on slab object size and layout,
> which means kmem_cache may have different order and objects.
> The freelist pointer corruption occurs if some slab flags are modified
> while CONFIG_SLAB_FREELIST_RANDOM is turned on.
> 
>  $ echo 0 > /sys/kernel/slab/zs_handle/store_user
>  $ echo 0 > /sys/kernel/slab/zspage/store_user
>  $ mkswap /dev/block/zram0
>  $ swapon /dev/block/zram0 -p 32758
> 
>  =============================================================================
>  BUG zs_handle (Not tainted): Freepointer corrupt

The problems here are actually larger than just the freelist pointers,
so this was actually solved by just making these parameters not writable
at runtime:

https://git.kernel.org/linus/ad38b5b1131e2a0e5c46724847da2e1eba31fb68

I wonder if perhaps the above patch needs to be explicitly sent to the
-stable trees?

-Kees

>  -----------------------------------------------------------------------------
> 
>  Disabling lock debugging due to kernel taint
>  INFO: Slab 0xffffffbf29603600 objects=102 used=102 fp=0x0000000000000000 flags=0x0200
>  INFO: Object 0xffffffca580d8d78 @offset=3448 fp=0xffffffca580d8ed0
> 
>  Redzone 00000000f3cddd6c: bb bb bb bb bb bb bb bb                          ........
>  Object 0000000082d5d74e: 6b 6b 6b 6b 6b 6b 6b a5                          kkkkkkk.
>  Redzone 000000008fd80359: bb bb bb bb bb bb bb bb                          ........
>  Padding 00000000c7f56047: 5a 5a 5a 5a 5a 5a 5a 5a                          ZZZZZZZZ
> 
> In this example, an Android device tries to use zram as a swap and to
> turn off store_user in order to reduce the slub object size.
> When calculate_sizes is called in kmem_cache_open, size, order and
> objects for zs_handle is:
>  size:360, order:0, objects:22
> However, if the SLAB_STORE_USER bit is cleared in store_user_store:
>  size: 56, order:1, objects:73
> 
> All the size, order, and objects is changed by calculate_sizes(), but
> the size of the random_seq array is still old value(22). As a result,
> out-of-bound array access can occur at shuffle_freelist() when slab
> allocation is requested.
> 
> This patch fixes the problem by re-allocating the random_seq array
> with re-calculated correct objects value.
> 
> Fixes: 210e7a43fa905 ("mm: SLUB freelist randomization")
> Reported-by: Ari-Pekka Verta <ari-pekka.verta@digital14.com>
> Reported-by: Timo Simola <timo.simola@digital14.com>
> Signed-off-by: Sahara <keun-o.park@digital14.com>
> ---
>  mm/slub.c | 23 ++++++++++++++++-------
>  1 file changed, 16 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index f226d66408ee..be1e4d6682b8 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3704,7 +3704,22 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
>  	if (oo_objects(s->oo) > oo_objects(s->max))
>  		s->max = s->oo;
>  
> -	return !!oo_objects(s->oo);
> +	if (!oo_objects(s->oo))
> +		return 0;
> +
> +	/*
> +	 * Initialize the pre-computed randomized freelist if slab is up.
> +	 * If the randomized freelist random_seq is already initialized,
> +	 * free and re-initialize it with re-calculated value.
> +	 */
> +	if (slab_state >= UP) {
> +		if (s->random_seq)
> +			cache_random_seq_destroy(s);
> +		if (init_cache_random_seq(s))
> +			return 0;
> +	}
> +
> +	return 1;
>  }
>  
>  static int kmem_cache_open(struct kmem_cache *s, slab_flags_t flags)
> @@ -3748,12 +3763,6 @@ static int kmem_cache_open(struct kmem_cache *s, slab_flags_t flags)
>  	s->remote_node_defrag_ratio = 1000;
>  #endif
>  
> -	/* Initialize the pre-computed randomized freelist if slab is up */
> -	if (slab_state >= UP) {
> -		if (init_cache_random_seq(s))
> -			goto error;
> -	}
> -
>  	if (!init_kmem_cache_nodes(s))
>  		goto error;
>  
> -- 
> 2.17.1
> 

-- 
Kees Cook