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

[Andrey Ryabinin]

On 07/31/2018 04:05 PM, Andrey Konovalov wrote:
> On Wed, Jul 25, 2018 at 3:44 PM, Vincenzo Frascino@Foss
> <vincenzo.frascino@xxxxxxx> 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.

As for non-SLAB_TYPESAFE_BY_RCU caches with constructors, it's probably ok to reinitialize and retag such objects.
I don't see how could any code rely on the current ->ctor() behavior in non-SLAB_TYPESAFE_BY_RCU case,
unless it does something extremely stupid or weird.
But let's not do it now. If you care, you cand do it later, with a separate patch, so we could just revert
it if anything goes wrong.