From: Sahara <keun-o.p...@digital14.com>
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.ve...@digital14.com>
Reported-by: Timo Simola <timo.sim...@digital14.com>
Signed-off-by: Sahara <keun-o.p...@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
