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

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Message-ID: <CAG48ez0NZYOwafGfXw6pN91zeFH60CSdeQrTLgJffrbu1xPTBA@mail.gmail.com>
Date: Fri, 2 Aug 2024 13:05:58 +0200
From: Jann Horn <jannh@...gle.com>
To: Andrey Konovalov <andreyknvl@...il.com>
Cc: Marco Elver <elver@...gle.com>, Andrey Ryabinin <ryabinin.a.a@...il.com>, 
	Alexander Potapenko <glider@...gle.com>, Dmitry Vyukov <dvyukov@...gle.com>, 
	Vincenzo Frascino <vincenzo.frascino@....com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>, David Rientjes <rientjes@...gle.com>, 
	Joonsoo Kim <iamjoonsoo.kim@....com>, Vlastimil Babka <vbabka@...e.cz>, 
	Roman Gushchin <roman.gushchin@...ux.dev>, Hyeonggon Yoo <42.hyeyoo@...il.com>, 
	kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v5 1/2] kasan: catch invalid free before SLUB
 reinitializes the object

On Thu, Aug 1, 2024 at 2:54 PM Andrey Konovalov <andreyknvl@...il.com> wrote:
> On Thu, Aug 1, 2024 at 6:01 AM Jann Horn <jannh@...gle.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.