Re: [PATCH v5 1/2] kasan: catch invalid free before SLUB reinitializes the object

[Jann Horn <jannh@google.com>]

On Thu, Aug 1, 2024 at 2:54 PM Andrey Konovalov <andreyknvl@gmail.com> wrote:
> On Thu, Aug 1, 2024 at 6:01 AM Jann Horn <jannh@google.com> wrote:
> >
> > > > @@ -503,15 +509,22 @@ bool __kasan_mempool_poison_object(void *ptr, unsigned long ip)
> > > >                 kasan_poison(ptr, folio_size(folio), KASAN_PAGE_FREE, false);
> > > >                 return true;
> > > >         }
> > > >
> > > >         if (is_kfence_address(ptr))
> > > >                 return false;
> > > > +       if (!kasan_arch_is_ready())
> > > > +               return true;
> > >
> > > Hm, I think we had a bug here: the function should return true in both
> > > cases. This seems reasonable: if KASAN is not checking the object, the
> > > caller can do whatever they want with it.
> >
> > But if the object is a kfence allocation, we maybe do want the caller
> > to free it quickly so that kfence can catch potential UAF access? So
> > "return false" in that case seems appropriate.
>
> Return false would mean: allocation is buggy, do not use it and do not
> free it (note that the return value meaning here is inverse compared
> to the newly added check_slab_allocation()). And this doesn't seem
> like something we want for KFENCE-managed objects. But regardless of
> the return value here, the callers tend not to free these allocations
> to the slab allocator, that's the point of mempools. So KFENCE won't
> catch a UAF either way.

Oooh, right, I misunderstood the semantics of the function. I'll
change it in v6.