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

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Date:   Tue, 31 Jul 2018 17:50:00 +0300
From:   Andrey Ryabinin <aryabinin@...tuozzo.com>
To:     Andrey Konovalov <andreyknvl@...gle.com>, vincenzo.frascino@....com
Cc:     Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...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 04:05 PM, Andrey Konovalov wrote:
> On Wed, Jul 25, 2018 at 3:44 PM, Vincenzo Frascino@...s
> <vincenzo.frascino@....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.

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.