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

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Date:   Tue, 31 Jul 2018 19:18:48 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Andrey Konovalov <andreyknvl@...gle.com>,
        vincenzo.frascino@....com, Alexander Potapenko <glider@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Christoph Lameter <cl@...ux.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mark Rutland <mark.rutland@....com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Marc Zyngier <marc.zyngier@....com>,
        Dave Martin <dave.martin@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Ingo Molnar <mingo@...nel.org>,
        Paul Lawrence <paullawrence@...gle.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        linux-doc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-sparse@...r.kernel.org,
        Linux Memory Management List <linux-mm@...ck.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Chintan Pandya <cpandya@...eaurora.org>,
        Jacob Bramley <Jacob.Bramley@....com>,
        Jann Horn <jannh@...gle.com>,
        Ruben Ayrapetyan <Ruben.Ayrapetyan@....com>,
        Lee Smith <Lee.Smith@....com>,
        Kostya Serebryany <kcc@...gle.com>,
        Mark Brand <markbrand@...gle.com>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        Evgeniy Stepanov <eugenis@...gle.com>
Subject: Re: [PATCH v4 13/17] khwasan: add hooks implementation



On 07/31/2018 07:08 PM, Dmitry Vyukov wrote:
> On Tue, Jul 31, 2018 at 6:04 PM, Andrey Ryabinin
> <aryabinin@...tuozzo.com> 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.