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

[Thomas Garnier <thgarnie@google.com>]

 I am not super familiar with the sysfs setup here but the random
sequence should work as expected. One minor comment below.

Reviewed-by: Thomas Garnier <thgarnie@chromium.org>

On Wed, Aug 19, 2020 at 1:26 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
>
> (cc Thomas and linux-mm)
>
> On Sat,  8 Aug 2020 13:50:30 +0400 kpark3469@gmail.com wrote:
>
> > From: Sahara <keun-o.park@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.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);

kfree(NULL) is a noop, so you don't need to check s->random_seq.

> > +             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



--
Thomas