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

[Vlastimil Babka <vbabka@suse.cz>]

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@syzkaller.appspotmail.com
> Signed-off-by: Jann Horn <jannh@google.com>

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.

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.

> +}
> +#endif /* CONFIG_SLUB_RCU_DEBUG */
> +
>  #ifdef CONFIG_KASAN_GENERIC
>  void ___cache_free(struct kmem_cache *cache, void *x, unsigned long addr)
>  {
>  	do_slab_free(cache, virt_to_slab(x), x, x, 1, addr);
>  }
>  #endif
>