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

[Alexander Potapenko <glider@google.com>]

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.

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



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Alexander Potapenko
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