Re: [PATCH] kasan, slub: fix handling of kasan_slab_free hook

[Andrey Konovalov <andreyknvl@google.com>]

On Fri, Mar 2, 2018 at 1:10 PM, Andrey Ryabinin <aryabinin@virtuozzo.com> wrote:
> On 02/23/2018 06:53 PM, Andrey Konovalov wrote:
>> The kasan_slab_free hook's return value denotes whether the reuse of a
>> slab object must be delayed (e.g. when the object is put into memory
>> qurantine).
>>
>> The current way SLUB handles this hook is by ignoring its return value
>> and hardcoding checks similar (but not exactly the same) to the ones
>> performed in kasan_slab_free, which is prone to making mistakes.
>>
>
> What are those differences exactly? And what problems do they cause?
> Answers to these questions should be in the changelog.


The difference is that with the old code we end up proceeding with
invalidly freeing an object when an invalid-free (or double-free) is
detected. Will add this in v2.

>
>
>> This patch changes the way SLUB handles this by:
>> 1. taking into account the return value of kasan_slab_free for each of
>>    the objects, that are being freed;
>> 2. reconstructing the freelist of objects to exclude the ones, whose
>>    reuse must be delayed.
>>
>> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
>> ---
>
>
>
>
>>
>> @@ -2965,14 +2974,13 @@ static __always_inline void slab_free(struct kmem_cache *s, struct page *page,
>>                                     void *head, void *tail, int cnt,
>>                                     unsigned long addr)
>>  {
>> -     slab_free_freelist_hook(s, head, tail);
>>       /*
>> -      * slab_free_freelist_hook() could have put the items into quarantine.
>> -      * If so, no need to free them.
>> +      * With KASAN enabled slab_free_freelist_hook modifies the freelist
>> +      * to remove objects, whose reuse must be delayed.
>>        */
>> -     if (s->flags & SLAB_KASAN && !(s->flags & SLAB_TYPESAFE_BY_RCU))
>> -             return;
>> -     do_slab_free(s, page, head, tail, cnt, addr);
>> +     slab_free_freelist_hook(s, &head, &tail);
>> +     if (head != NULL)
>
> That's an additional branch in non-debug fast-path. Find a way to avoid this.

Hm, there supposed to be a branch here. We either have objects that we
need to free, or we don't, and we need to do different things in those
cases. Previously this was done with a hardcoded "if (s->flags &
SLAB_KASAN && ..." statement, not it's a different "if (head !=
NULL)".

I could put this check under #ifdef CONFIG_KASAN if the performance is
critical here, but I'm not sure if that's the best solution. I could
also add an "unlikely()" there.

>
>
>> +             do_slab_free(s, page, head, tail, cnt, addr);
>>  }
>>
>>  #ifdef CONFIG_KASAN
>>

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