Re: [PATCH] kasan: skip quarantine if object is still accessible under RCU

[Jann Horn <jannh@google.com>]

On Sun, Jul 27, 2025 at 12:06 AM Andrey Konovalov <andreyknvl@gmail.com> wrote:
> On Wed, Jul 23, 2025 at 4:59 PM Jann Horn <jannh@google.com> wrote:
> >
> > Currently, enabling KASAN masks bugs where a lockless lookup path gets a
> > pointer to a SLAB_TYPESAFE_BY_RCU object that might concurrently be
> > recycled and is insufficiently careful about handling recycled objects:
> > KASAN puts freed objects in SLAB_TYPESAFE_BY_RCU slabs onto its quarantine
> > queues, even when it can't actually detect UAF in these objects, and the
> > quarantine prevents fast recycling.
> >
> > When I introduced CONFIG_SLUB_RCU_DEBUG, my intention was that enabling
> > CONFIG_SLUB_RCU_DEBUG should cause KASAN to mark such objects as freed
> > after an RCU grace period and put them on the quarantine, while disabling
> > CONFIG_SLUB_RCU_DEBUG should allow such objects to be reused immediately;
> > but that hasn't actually been working.
> >
> > I discovered such a UAF bug involving SLAB_TYPESAFE_BY_RCU yesterday; I
> > could only trigger this bug in a KASAN build by disabling
> > CONFIG_SLUB_RCU_DEBUG and applying this patch.
> >
> > Signed-off-by: Jann Horn <jannh@google.com>
> > ---
> >  mm/kasan/common.c | 25 ++++++++++++++++++-------
> >  1 file changed, 18 insertions(+), 7 deletions(-)
> >
> > diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> > index ed4873e18c75..9142964ab9c9 100644
> > --- a/mm/kasan/common.c
> > +++ b/mm/kasan/common.c
> > @@ -230,16 +230,12 @@ static bool check_slab_allocation(struct kmem_cache *cache, void *object,
> >  }
> >
> >  static inline void poison_slab_object(struct kmem_cache *cache, void *object,
> > -                                     bool init, bool still_accessible)
> > +                                     bool init)
> >  {
> >         void *tagged_object = object;
> >
> >         object = kasan_reset_tag(object);
> >
> > -       /* RCU slabs could be legally used after free within the RCU period. */
> > -       if (unlikely(still_accessible))
> > -               return;
> > -
> >         kasan_poison(object, round_up(cache->object_size, KASAN_GRANULE_SIZE),
> >                         KASAN_SLAB_FREE, init);
> >
> > @@ -261,7 +257,22 @@ bool __kasan_slab_free(struct kmem_cache *cache, void *object, bool init,
> >         if (!kasan_arch_is_ready() || is_kfence_address(object))
> >                 return false;
> >
> > -       poison_slab_object(cache, object, init, still_accessible);
> > +       /*
> > +        * If this point is reached with an object that must still be
> > +        * accessible under RCU, we can't poison it; in that case, also skip the
> > +        * quarantine. This should mostly only happen when CONFIG_SLUB_RCU_DEBUG
> > +        * has been disabled manually.
> > +        *
> > +        * Putting the object on the quarantine wouldn't help catch UAFs (since
> > +        * we can't poison it here), and it would mask bugs caused by
> > +        * SLAB_TYPESAFE_BY_RCU users not being careful enough about object
> > +        * reuse; so overall, putting the object into the quarantine here would
> > +        * be counterproductive.
> > +        */
> > +       if (still_accessible)
> > +               return false;
> > +
> > +       poison_slab_object(cache, object, init);
> >
> >         /*
> >          * If the object is put into quarantine, do not let slab put the object
> > @@ -519,7 +530,7 @@ bool __kasan_mempool_poison_object(void *ptr, unsigned long ip)
> >         if (check_slab_allocation(slab->slab_cache, ptr, ip))
> >                 return false;
> >
> > -       poison_slab_object(slab->slab_cache, ptr, false, false);
> > +       poison_slab_object(slab->slab_cache, ptr, false);
> >         return true;
> >  }
> >
> >
> > ---
> > base-commit: 89be9a83ccf1f88522317ce02f854f30d6115c41
> > change-id: 20250723-kasan-tsbrcu-noquarantine-e207bb990e24
> >
> > --
> > Jann Horn <jannh@google.com>
> >
>
> Acked-by: Andrey Konovalov <andreyknvl@gmail.com>

Thanks!

> Would it be hard to add KUnit test to check that KASAN detects such issues?

Sent a separate patch with a kunit test.