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From:   Jann Horn <jannh@google.com>
Date:   Fri, 30 Oct 2020 16:08:56 +0100
Message-ID: <CAG48ez30tzadrtJm_ShY8oGjnYpf3GDfcajm7S0xX6UxfTCQZw@mail.gmail.com>
Subject: Re: [PATCH v6 6/9] kfence, kasan: make KFENCE compatible with KASAN
To:     Marco Elver <elver@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Alexander Potapenko <glider@google.com>,
        "H . Peter Anvin" <hpa@zytor.com>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Andrey Konovalov <andreyknvl@google.com>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Andy Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Christoph Lameter <cl@linux.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        David Rientjes <rientjes@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Eric Dumazet <edumazet@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Hillf Danton <hdanton@sina.com>,
        Ingo Molnar <mingo@redhat.com>,
        Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        =?UTF-8?Q?J=C3=B6rn_Engel?= <joern@purestorage.com>,
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        Mark Rutland <mark.rutland@arm.com>,
        Pekka Enberg <penberg@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
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On Fri, Oct 30, 2020 at 2:46 PM Marco Elver <elver@google.com> wrote:
> On Fri, 30 Oct 2020 at 03:50, Jann Horn <jannh@google.com> wrote:
> > On Thu, Oct 29, 2020 at 2:17 PM Marco Elver <elver@google.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@google.com>
> > > Co-developed-by: Marco Elver <elver@google.com>
> > > Signed-off-by: Marco Elver <elver@google.com>
> > > Signed-off-by: Alexander Potapenko <glider@google.com>
> >
> > Reviewed-by: Jann Horn <jannh@google.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.

FWIW, I wouldn't be surprised if other parts of the kernel also ended
up wanting to have in-object redzones eventually - e.g. inside skb
buffers, which have a struct skb_shared_info at the end. AFAIU at the
moment, KASAN can't catch small OOB accesses from these buffers
because of the following structure.

> I think we better add a kfence guard to
> kasan_poison_shadow() as well.

Sounds good.