Re: Kernel hardening project suggestion: Normalizing ->ctor slabs and TYPESAFE_BY_RCU slabs

[Dmitry Vyukov <dvyukov@google.com>]

On Tue, Jun 23, 2020 at 10:38 AM Alexander Potapenko <glider@google.com> wrote:
> > > KFENCE also has to ignore both TYPESAFE_BY_RCU and ctors.
> > > For ctors it should be pretty straightforward to fix (and won't
> > > require any changes to SL[AU]B). Not sure if your proposal for RCU
> > > will also work for KFENCE.
> >
> > Does it work for objects freed by call_rcu in normal slabs?
> > If yes, then I would assume it will work for TYPESAFE_BY_RCU after
> > this change, or is there a difference?
>
> If my understanding is correct, TYPESAFE_BY_RCU means that the object
> may be used after it has been freed, that's why we cannot further
> reuse or wipe it before ensuring they aren't used anymore.

Yes, but only within an rcu grace period.
And this proposal will take care of this: from the point of view of
slab, the object is freed after an additional rcu grace period. So
when it reaches slab free, it must not be used anymore.

> Objects allocated from normal slabs cannot be used after they've been
> freed, so I don't see how this change applies to them.
>
> > > Another beneficiary of RCU/ctor normalization would be
> > > init_on_alloc/init_on_free, which also ignore such slabs.
> > >
> > > On Tue, Jun 23, 2020 at 9:18 AM Marco Elver <elver@google.com> wrote:
> > > >
> > > > On Tue, 23 Jun 2020 at 08:45, Dmitry Vyukov <dvyukov@google.com> wrote:
> > > > >
> > > > > On Tue, Jun 23, 2020 at 8:26 AM Jann Horn <jannh@google.com> wrote:
> > > > > >
> > > > > > Hi!
> > > > > >
> > > > > > Here's a project idea for the kernel-hardening folks:
> > > > > >
> > > > > > The slab allocator interface has two features that are problematic for
> > > > > > security testing and/or hardening:
> > > > > >
> > > > > >  - constructor slabs: These things come with an object constructor
> > > > > > that doesn't run when an object is allocated, but instead when the
> > > > > > slab allocator grabs a new page from the page allocator. This is
> > > > > > problematic for use-after-free detection mechanisms such as HWASAN and
> > > > > > Memory Tagging, which can only do their job properly if the address of
> > > > > > an object is allowed to change every time the object is
> > > > > > freed/reallocated. (You can't change the address of an object without
> > > > > > reinitializing the entire object because e.g. an empty list_head
> > > > > > points to itself.)
> > > > > >
> > > > > >  - RCU slabs: These things basically permit use-after-frees by design,
> > > > > > and stuff like ASAN/HWASAN/Memory Tagging essentially doesn't work on
> > > > > > them.
> > > > > >
> > > > > >
> > > > > > It would be nice to have a config flag or so that changes the SLUB
> > > > > > allocator's behavior such that these slabs can be instrumented
> > > > > > properly. Something like:
> > > > > >
> > > > > >  - Let calculate_sizes() reserve space for an rcu_head on each object
> > > > > > in TYPESAFE_BY_RCU slabs, make kmem_cache_free() redirect to
> > > > > > call_rcu() for these slabs, and remove most of the other
> > > > > > special-casing, so that KASAN can instrument these slabs.
> > > > > >  - For all constructor slabs, let slab_post_alloc_hook() call the
> > > > > > ->ctor() function on each allocated object, so that Memory Tagging and
> > > > > > HWASAN will work on them.
> > > > >
> > > > > Hi Jann,
> > > > >
> > > > > Both things sound good to me. I think we considered doing the ctor's
> > > > > change with KASAN, but we did not get anywhere. The only argument
> > > > > against it I remember now was "performance", but it's not that
> > > > > important if this mode is enabled only with KASAN and other debugging
> > > > > tools. Performance is definitely not as important as missing bugs. The
> > > > > additional code complexity for ctors change should be minimal.
> > > > > The rcu change would also be useful, but I would assume it will be larger.
> > > > > Please add them to [1], that's KASAN laundry list.
> > > > >
> > > > > +Alex, Marco, will it be useful for KFENCE [2] as well? Do ctors/rcu
> > > > > affect KFENCE? Will we need any special handling for KFENCE?
> > > > > I assume it will also be useful for KMSAN b/c we can re-mark objects
> > > > > as uninitialized only after they have been reallocated.
> > > >
> > > > Yes, we definitely need to handle TYPESAFE_BY_RCU.
> > > >
> > > > > [1] https://bugzilla.kernel.org/buglist.cgi?bug_status=__open__&component=Sanitizers&list_id=1063981&product=Memory%20Management
> > > > > [2] https://github.com/google/kasan/commits/kfence
> > >
> > >
> > >
> > > --
> > > Alexander Potapenko
> > > Software Engineer
> > >
> > > Google Germany GmbH
> > > Erika-Mann-Straße, 33
> > > 80636 München
> > >
> > > Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
> > > Registergericht und -nummer: Hamburg, HRB 86891
> > > Sitz der Gesellschaft: Hamburg
>
>
>
> --
> Alexander Potapenko
> Software Engineer
>
> Google Germany GmbH
> Erika-Mann-Straße, 33
> 80636 München
>
> Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
> Registergericht und -nummer: Hamburg, HRB 86891
> Sitz der Gesellschaft: Hamburg