(cc Thomas and linux-mm)
On Sat, 8 Aug 2020 13:50:30 +0400 kpark3469@xxxxxxxxx wrote:
> From: Sahara <keun-o.park@xxxxxxxxxxxxx>
>
> 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
> -----------------------------------------------------------------------------
>
> 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@xxxxxxxxxxxxx>
> Reported-by: Timo Simola <timo.simola@xxxxxxxxxxxxx>
> Signed-off-by: Sahara <keun-o.park@xxxxxxxxxxxxx>
> ---
> 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
