Re: [PATCH v6 2/2] slub: Introduce CONFIG_SLUB_RCU_DEBUG

[Jann Horn]

On Wed, Aug 7, 2024 at 11:26 PM Vlastimil Babka <vbabka@xxxxxxx> wrote:
> On 8/2/24 22:31, Jann Horn wrote:
> > Currently, KASAN is unable to catch use-after-free in SLAB_TYPESAFE_BY_RCU
> > slabs because use-after-free is allowed within the RCU grace period by
> > design.
> >
> > Add a SLUB debugging feature which RCU-delays every individual
> > kmem_cache_free() before either actually freeing the object or handing it
> > off to KASAN, and change KASAN to poison freed objects as normal when this
> > option is enabled.
> >
> > For now I've configured Kconfig.debug to default-enable this feature in the
> > KASAN GENERIC and SW_TAGS modes; I'm not enabling it by default in HW_TAGS
> > mode because I'm not sure if it might have unwanted performance degradation
> > effects there.
> >
> > Note that this is mostly useful with KASAN in the quarantine-based GENERIC
> > mode; SLAB_TYPESAFE_BY_RCU slabs are basically always also slabs with a
> > ->ctor, and KASAN's assign_tag() currently has to assign fixed tags for
> > those, reducing the effectiveness of SW_TAGS/HW_TAGS mode.
> > (A possible future extension of this work would be to also let SLUB call
> > the ->ctor() on every allocation instead of only when the slab page is
> > allocated; then tag-based modes would be able to assign new tags on every
> > reallocation.)
> >
> > Tested-by: syzbot+263726e59eab6b442723@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
>
> Actually, wait a second...
>
> > +#ifdef CONFIG_SLUB_RCU_DEBUG
> > +static void slab_free_after_rcu_debug(struct rcu_head *rcu_head)
> > +{
> > +     struct rcu_delayed_free *delayed_free =
> > +                     container_of(rcu_head, struct rcu_delayed_free, head);
> > +     void *object = delayed_free->object;
> > +     struct slab *slab = virt_to_slab(object);
> > +     struct kmem_cache *s;
> > +
> > +     if (WARN_ON(is_kfence_address(object)))
> > +             return;
> > +
> > +     /* find the object and the cache again */
> > +     if (WARN_ON(!slab))
> > +             return;
> > +     s = slab->slab_cache;
> > +     if (WARN_ON(!(s->flags & SLAB_TYPESAFE_BY_RCU)))
> > +             return;
> > +
> > +     /* resume freeing */
> > +     if (!slab_free_hook(s, object, slab_want_init_on_free(s), true))
> > +             return;
> > +     do_slab_free(s, slab, object, object, 1, _THIS_IP_);
> > +     kfree(delayed_free);
>
> This will leak memory of delayed_free when slab_free_hook() returns false
> (such as because of KASAN quarantine), the kfree() needs to happen always.
> Even in the WARN_ON cases but that's somewhat less critical.

... oh. Indeed. I guess really I can just move the kfree(delayed_free)
up before the first bailout, we're not accessing anything in it after
loading the ->object member...

> Thanks to David Sterba for making me look again, as he's been asking me
> about recent OOMs in -next with heavy kmalloc-32 cache usage (delayed_free
> is 24 bytes) and CONFIG_SLUB_RCU_DEBUG was so far almost certainly confirmed.

Nice catch...

I guess I'll get to send a v7 of the series.