Re: [PATCH v6 07/18] khwasan: add tag related helper functions

[Dmitry Vyukov <dvyukov@google.com>]

On Mon, Sep 17, 2018 at 8:59 PM, Andrey Konovalov <andreyknvl@google.com> wrote:
> On Wed, Sep 12, 2018 at 6:21 PM, Dmitry Vyukov <dvyukov@google.com> wrote:
>> On Wed, Aug 29, 2018 at 1:35 PM, Andrey Konovalov <andreyknvl@google.com> wrote:
>
>>> +void *khwasan_preset_slub_tag(struct kmem_cache *cache, const void *addr)
>>
>> Can't we do this in the existing kasan_init_slab_obj() hook? It looks
>> like it should do exactly this -- allow any one-time initialization
>> for objects. We could extend it to accept index and return a new
>> pointer.
>> If that does not work for some reason, I would try to at least unify
>> the hook for slab/slub, e.g. pass idx=-1 from slub and then use
>> random_tag().
>> It also seems that we do preset tag for slab multiple times (from
>> slab_get_obj()). Using kasan_init_slab_obj() should resolve this too
>> (hopefully we don't call it multiple times).
>
> The issue is that SLAB stores freelist as an array of indexes instead
> of using an actual linked list like SLUB. So you can't store the tag
> in the pointer while the object is in the freelist, since there's no
> pointer. And, technically, we don't preset tags for SLAB, we just use
> the id as the tag every time a pointer is used, so perhaps we should
> rename the callback. As to unifying the callbacks, sure, we can do
> that.

As per offline discussion: potentially we can use
kasan_init_slab_obj() if we add tag in kmalloc hook by using
obj_to_idx().