Re: [PATCH v4 13/17] khwasan: add hooks implementation

[Andrey Ryabinin]

On 07/31/2018 07:08 PM, Dmitry Vyukov wrote:
> On Tue, Jul 31, 2018 at 6:04 PM, Andrey Ryabinin
> <aryabinin@xxxxxxxxxxxxx> 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?
>>
>> Yeah, I puzzled by this too. However, I think it's hard but possible to make it work, at least in theory.
>> There must be an initializer, which consists of two parts:
>> a) initilize objects fields
>> b) expose object to the world (add it to list or something like that)
>>
>> (a) part must somehow to be ok to race with another cpu which might already use the object.
>> (b) part must must use e.g. barriers to make sure that racy users will see previously inilized fields.
>> Racy users must have parring barrier of course.
>>
>> But it sound fishy, and very easy to fuck up.
> 
> 
> Agree on both fronts: theoretically possible but easy to fuck up. Even
> if it works, complexity of the code should be brain damaging and there
> are unlikely good reasons to just not be more explicit and use a ctor.
> 
> 
>> I won't be surprised if every single one SLAB_TYPESAFE_BY_RCU user
>> without ->ctor is bogus. It certainly would be better to convert those to use ->ctor.
> 
> I have another hypothesis: they are not bogus, just don't need
> SLAB_TYPESAFE_BY_RCU :)
> 

I'd call this a bug too.