linux-kernel - Re: [PATCH v6 07/18] khwasan: add tag related helper functions

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Date:   Tue, 18 Sep 2018 17:45:11 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     Andrey Ryabinin <aryabinin@...tuozzo.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>,
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        Mark Brand <markbrand@...gle.com>,
        Chintan Pandya <cpandya@...eaurora.org>,
        Vishwath Mohan <vishwath@...gle.com>
Subject: Re: [PATCH v6 07/18] khwasan: add tag related helper functions

On Mon, Sep 17, 2018 at 8:59 PM, Andrey Konovalov <andreyknvl@...gle.com> wrote:
> On Wed, Sep 12, 2018 at 6:21 PM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
>> On Wed, Aug 29, 2018 at 1:35 PM, Andrey Konovalov <andreyknvl@...gle.com> wrote:
>
>>> +void *khwasan_preset_slub_tag(struct kmem_cache *cache, const void *addr)
>>
>> Can't we do this in the existing kasan_init_slab_obj() hook? It looks
>> like it should do exactly this -- allow any one-time initialization
>> for objects. We could extend it to accept index and return a new
>> pointer.
>> If that does not work for some reason, I would try to at least unify
>> the hook for slab/slub, e.g. pass idx=-1 from slub and then use
>> random_tag().
>> It also seems that we do preset tag for slab multiple times (from
>> slab_get_obj()). Using kasan_init_slab_obj() should resolve this too
>> (hopefully we don't call it multiple times).
>
> The issue is that SLAB stores freelist as an array of indexes instead
> of using an actual linked list like SLUB. So you can't store the tag
> in the pointer while the object is in the freelist, since there's no
> pointer. And, technically, we don't preset tags for SLAB, we just use
> the id as the tag every time a pointer is used, so perhaps we should
> rename the callback. As to unifying the callbacks, sure, we can do
> that.

As per offline discussion: potentially we can use
kasan_init_slab_obj() if we add tag in kmalloc hook by using
obj_to_idx().