linux-kernel - Re: [PATCH v6 4/9] mm, kfence: insert KFENCE hooks for SLAB

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Message-ID: <CANpmjNOPmgeVLb5COyE734F-1NNSU4vfok-8AQuDoAcLnQ=PbQ@mail.gmail.com>
Date:   Fri, 30 Oct 2020 16:41:56 +0100
From:   Marco Elver <elver@...gle.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Potapenko <glider@...gle.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christoph Lameter <cl@...ux.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Hillf Danton <hdanton@...a.com>,
        Ingo Molnar <mingo@...hat.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Jonathan Corbet <corbet@....net>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Jörn Engel <joern@...estorage.com>,
        Kees Cook <keescook@...omium.org>,
        Mark Rutland <mark.rutland@....com>,
        Pekka Enberg <penberg@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        SeongJae Park <sjpark@...zon.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vlastimil Babka <vbabka@...e.cz>,
        Will Deacon <will@...nel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH v6 4/9] mm, kfence: insert KFENCE hooks for SLAB

On Fri, 30 Oct 2020 at 03:49, Jann Horn <jannh@...gle.com> wrote:
> On Thu, Oct 29, 2020 at 2:17 PM Marco Elver <elver@...gle.com> wrote:
> > Inserts KFENCE hooks into the SLAB allocator.
> [...]
> > diff --git a/mm/slab.c b/mm/slab.c
> [...]
> > @@ -3416,6 +3427,11 @@ static void cache_flusharray(struct kmem_cache *cachep, struct array_cache *ac)
> >  static __always_inline void __cache_free(struct kmem_cache *cachep, void *objp,
> >                                          unsigned long caller)
> >  {
> > +       if (kfence_free(objp)) {
> > +               kmemleak_free_recursive(objp, cachep->flags);
> > +               return;
> > +       }
>
> This looks dodgy. Normally kmemleak is told that an object is being
> freed *before* the object is actually released. I think that if this
> races really badly, we'll make kmemleak stumble over this bit in
> create_object():
>
> kmemleak_stop("Cannot insert 0x%lx into the object search tree
> (overlaps existing)\n",
>       ptr);

Good catch. Although extremely unlikely, let's just avoid it by moving
the freeing after.

>
> > +
> >         /* Put the object into the quarantine, don't touch it for now. */
> >         if (kasan_slab_free(cachep, objp, _RET_IP_))
> >                 return;