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

[Marco Elver]

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

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