linux-kernel - Re: [PATCH v8 16/20] kasan: add hooks implementation for tag-based mode

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Date:   Fri, 21 Sep 2018 16:28:27 +0200
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Dmitry Vyukov <dvyukov@...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>,
        "open list:DOCUMENTATION" <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-MM <linux-mm@...ck.org>,
        "open list:KERNEL BUILD + fi..." <linux-kbuild@...r.kernel.org>,
        Kostya Serebryany <kcc@...gle.com>,
        Evgeniy Stepanov <eugenis@...gle.com>,
        Lee Smith <Lee.Smith@....com>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        Jacob Bramley <Jacob.Bramley@....com>,
        Ruben Ayrapetyan <Ruben.Ayrapetyan@....com>,
        Jann Horn <jannh@...gle.com>,
        Mark Brand <markbrand@...gle.com>,
        Chintan Pandya <cpandya@...eaurora.org>,
        Vishwath Mohan <vishwath@...gle.com>
Subject: Re: [PATCH v8 16/20] kasan: add hooks implementation for tag-based mode

On Fri, Sep 21, 2018 at 1:37 PM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
> On Wed, Sep 19, 2018 at 8:54 PM, Andrey Konovalov <andreyknvl@...gle.com> wrote:

>> +       /*
>> +        * Since it's desirable to only call object contructors ones during
>
> s/ones/once/

Will fix.

>
>> +        * slab allocation, we preassign tags to all such objects.
>
> While we are here, it can make sense to mention that we can't repaint
> objects with ctors after reallocation (even for
> non-SLAB_TYPESAFE_BY_RCU) because the ctor code can memorize pointer
> to the object somewhere (e.g. in the object itself). Then if we
> repaint it, the old memorized pointer will become invalid.

Will mention.

>> -       kasan_unpoison_shadow(object, size);
>> +       /* See the comment in kasan_init_slab_obj regarding preassigned tags */
>> +       if (IS_ENABLED(CONFIG_KASAN_SW_TAGS) &&
>> +                       (cache->ctor || cache->flags & SLAB_TYPESAFE_BY_RCU)) {
>> +#ifdef CONFIG_SLAB
>> +               struct page *page = virt_to_page(object);
>> +
>> +               tag = (u8)obj_to_index(cache, page, (void *)object);
>> +#else
>> +               tag = get_tag(object);
>> +#endif
>
> This kinda _almost_ matches the chunk of code in kasan_init_slab_obj,
> but not exactly. Wonder if there is some nice way to unify this code?
>
> Maybe something like:
>
> static u8 tag_for_object(struct kmem_cache *cache, const void *object, new bool)
> {
>     if (!IS_ENABLED(CONFIG_KASAN_SW_TAGS) ||
>         !cache->ctor && !(cache->flags & SLAB_TYPESAFE_BY_RCU))
>         return random_tag();
> #ifdef CONFIG_SLAB
>     struct page *page = virt_to_page(object);
>     return (u8)obj_to_index(cache, page, (void *)object);
> #else
>     return new ? random_tag() : get_tag(object);
> #endif
> }
>
> Then we can call this in both places.

Will do, however I think it's better to do the CONFIG_KASAN_SW_TAGS
check outside this helper function.

> As a side effect this will assign tags to pointers during slab
> initialization even if we don't have ctors, but it should be fine (?).

We don't have to assign tag in this case, can just leave 0xff.