Re: Odd-sized kmem_cache_alloc and slub_debug=Z

[Kees Cook <keescook@chromium.org>]

On Mon, Aug 17, 2020 at 08:31:35PM +0200, Marco Elver wrote:
> On Fri, 7 Aug 2020 at 21:06, Pekka Enberg <penberg@gmail.com> wrote:
> ...
> > Yeah, it reproduces with defconfig too, as long as you remember to
> > pass "slub_debug=Z"... :-/
> >
> > The following seems to be the culprit:
> >
> > commit 3202fa62fb43087387c65bfa9c100feffac74aa6
> > Author: Kees Cook <keescook@chromium.org>
> > Date:   Wed Apr 1 21:04:27 2020 -0700
> >
> >     slub: relocate freelist pointer to middle of object
> >
> > Reverting this commit and one of it's follow up fixes from Kees from
> > v5.8 makes the issue go away for me. Btw, please note that caches with
> > size 24 and larger do not trigger this bug, so the issue is that with
> > small enough object size, we're stomping on allocator metadata (I
> > assume part of the freelist).
> 
> Was there a patch to fix this? Checking, just in case I missed it.

Hi! I've finally carved out time to look at this, and I've figured it
out. My prior redzoning fix was actually wrong; I'll send a patch to
fix it harder. In the meantime, I've also discovered that the redzoning
code wasn't safe for sizes smaller than sizeof(void *). This oversight
is, I think, what caused me to incorrectly handle the changed offset the
first time. :P

Anyway, patches incoming...

-- 
Kees Cook