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

Marco Elver
2020-10-30T15:20:23Z

On Fri, 30 Oct 2020 at 16:09, Jann Horn <jannh@google.com> wrote:
>
> 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.

Sure, and it might also become more interesting with MTE-based KASAN.

But, currently we recommend not to enable generic KASAN+KFENCE,
because it'd be redundant if the instrumentation price for generic (or
SW-tag) KASAN is already paid. The changes here are also mostly for
testing KFENCE itself.

That may change with MTE-based KASAN, however, which may have modes
where stack traces aren't collected and having KFENCE to get
actionable debug-info across a fleet of machines may still be wanted.
But that story is still evolving. The code here is only for the
generic and SW-tag based KASAN modes, and MTE will have its own
kasan_{un,}poison_shadow (afaik it's being renamed to
kasan_{un,}poison_memory) which works just fine with KFENCE AFAIK.

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