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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Tue, 31 Jul 2018 17:03:48 +0200
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Subject: Re: [PATCH v4 13/17] khwasan: add hooks implementation
To:     Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc:     Andrey Konovalov <andreyknvl@google.com>,
        vincenzo.frascino@arm.com, Alexander Potapenko <glider@google.com>,
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On Tue, Jul 31, 2018 at 4:50 PM, Andrey Ryabinin
<aryabinin@virtuozzo.com> wrote:
>
>
> On 07/31/2018 04:05 PM, Andrey Konovalov wrote:
>> On Wed, Jul 25, 2018 at 3:44 PM, Vincenzo Frascino@Foss
>> <vincenzo.frascino@arm.com> wrote:
>>> On 06/26/2018 02:15 PM, Andrey Konovalov wrote:
>>>
>>>> @@ -325,18 +341,41 @@ void kasan_init_slab_obj(struct kmem_cache *cache,
>>>> const void *object)
>>>>     void *kasan_slab_alloc(struct kmem_cache *cache, void *object, gfp_t
>>>> flags)
>>>>   {
>>>> -       return kasan_kmalloc(cache, object, cache->object_size, flags);
>>>> +       object = kasan_kmalloc(cache, object, cache->object_size, flags);
>>>> +       if (IS_ENABLED(CONFIG_KASAN_HW) && unlikely(cache->ctor)) {
>>>> +               /*
>>>> +                * Cache constructor might use object's pointer value to
>>>> +                * initialize some of its fields.
>>>> +                */
>>>> +               cache->ctor(object);
>>>>
>>> This seams breaking the kmem_cache_create() contract: "The @ctor is run when
>>> new pages are allocated by the cache."
>>> (https://elixir.bootlin.com/linux/v3.7/source/mm/slab_common.c#L83)
>>>
>>> Since there might be preexisting code relying on it, this could lead to
>>> global side effects. Did you verify that this is not the case?
>>>
>>> Another concern is performance related if we consider this solution suitable
>>> for "near-production", since with the current implementation you call the
>>> ctor (where present) on an object multiple times and this ends up memsetting
>>> and repopulating the memory every time (i.e. inode.c: inode_init_once). Do
>>> you know what is the performance impact?
>>
>> We can assign tags to objects with constructors when a slab is
>> allocated and call constructors once as usual. The downside is that
>> such object would always have the same tag when it is reallocated, so
>> we won't catch use-after-frees.
>
> Actually you should do this for SLAB_TYPESAFE_BY_RCU slabs. Usually they are with ->ctors but there
> are few without constructors.
> We can't reinitialize or even retag them. The latter will definitely cause false-positive use-after-free reports.

Somewhat offtopic, but I can't understand how SLAB_TYPESAFE_BY_RCU
slabs can be useful without ctors or at least memset(0). Objects in
such slabs need to be type-stable, but I can't understand how it's
possible to establish type stability without a ctor... Are these bugs?
Or I am missing something subtle? What would be a canonical usage of
SLAB_TYPESAFE_BY_RCU slab without a ctor?