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

[Jann Horn]

On Fri, Oct 30, 2020 at 2:46 PM Marco Elver <elver at google.com> wrote:
> On Fri, 30 Oct 2020 at 03:50, Jann Horn <jannh at google.com> wrote:
> > On Thu, Oct 29, 2020 at 2:17 PM Marco Elver <elver at 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 at google.com>
> > > Co-developed-by: Marco Elver <elver at google.com>
> > > Signed-off-by: Marco Elver <elver at google.com>
> > > Signed-off-by: Alexander Potapenko <glider at google.com>
> >
> > Reviewed-by: Jann Horn <jannh at 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.