linux-kernel - Re: [PATCH v6 6/9] kfence, kasan: make KFENCE compatible with KASAN

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Date:   Fri, 30 Oct 2020 14:46:40 +0100
From:   Marco Elver <elver@...gle.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Potapenko <glider@...gle.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christoph Lameter <cl@...ux.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hillf Danton <hdanton@...a.com>,
        Ingo Molnar <mingo@...hat.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Jonathan Corbet <corbet@....net>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Jörn Engel <joern@...estorage.com>,
        Kees Cook <keescook@...omium.org>,
        Mark Rutland <mark.rutland@....com>,
        Pekka Enberg <penberg@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        SeongJae Park <sjpark@...zon.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vlastimil Babka <vbabka@...e.cz>,
        Will Deacon <will@...nel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH v6 6/9] kfence, kasan: make KFENCE compatible with KASAN

On Fri, 30 Oct 2020 at 03:50, Jann Horn <jannh@...gle.com> wrote:
>
> On Thu, Oct 29, 2020 at 2:17 PM Marco Elver <elver@...gle.com> wrote:
> > We make KFENCE compatible with KASAN for testing KFENCE itself. In
> > particular, KASAN helps to catch any potential corruptions to KFENCE
> > state, or other corruptions that may be a result of freepointer
> > corruptions in the main allocators.
> >
> > To indicate that the combination of the two is generally discouraged,
> > CONFIG_EXPERT=y should be set. It also gives us the nice property that
> > KFENCE will be build-tested by allyesconfig builds.
> >
> > Reviewed-by: Dmitry Vyukov <dvyukov@...gle.com>
> > Co-developed-by: Marco Elver <elver@...gle.com>
> > Signed-off-by: Marco Elver <elver@...gle.com>
> > Signed-off-by: Alexander Potapenko <glider@...gle.com>
>
> Reviewed-by: Jann Horn <jannh@...gle.com>

Thanks!

> with one nit:
>
> [...]
> > diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> [...]
> > @@ -141,6 +142,14 @@ void kasan_unpoison_shadow(const void *address, size_t size)
> >          */
> >         address = reset_tag(address);
> >
> > +       /*
> > +        * We may be called from SL*B internals, such as ksize(): with a size
> > +        * not a multiple of machine-word size, avoid poisoning the invalid
> > +        * portion of the word for KFENCE memory.
> > +        */
> > +       if (is_kfence_address(address))
> > +               return;
>
> It might be helpful if you could add a comment that explains that
> kasan_poison_object_data() does not need a similar guard because
> kasan_poison_object_data() is always paired with
> kasan_unpoison_object_data() - that threw me off a bit at first.

Well, KFENCE objects should never be poisoned/unpoisoned because the
kasan_alloc and free hooks have a kfence guard, and none of the code
in sl*b.c that does kasan_{poison,unpoison}_object_data() should be
executed for KFENCE objects.

But I just noticed that kernel/scs.c seems to kasan_poison and
unpoison objects, and keeps them poisoned for most of the object
lifetime. I think we better add a kfence guard to
kasan_poison_shadow() as well.

Thanks,
-- Marco