linux-kernel - Re: [PATCH v6 6/9] kfence, kasan: make KFENCE compatible with KASAN

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date:   Fri, 30 Oct 2020 16:19:34 +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 6/9] kfence, kasan: make KFENCE compatible with KASAN

On Fri, 30 Oct 2020 at 16:09, Jann Horn <jannh@...gle.com> wrote:
>
> On Fri, Oct 30, 2020 at 2:46 PM Marco Elver <elver@...gle.com> wrote:
> > On Fri, 30 Oct 2020 at 03:50, Jann Horn <jannh@...gle.com> wrote:
> > > On Thu, Oct 29, 2020 at 2:17 PM Marco Elver <elver@...gle.com> wrote:
> > > > We make KFENCE compatible with KASAN for testing KFENCE itself. In
> > > > particular, KASAN helps to catch any potential corruptions to KFENCE
> > > > state, or other corruptions that may be a result of freepointer
> > > > corruptions in the main allocators.
> > > >
> > > > To indicate that the combination of the two is generally discouraged,
> > > > CONFIG_EXPERT=y should be set. It also gives us the nice property that
> > > > KFENCE will be build-tested by allyesconfig builds.
> > > >
> > > > Reviewed-by: Dmitry Vyukov <dvyukov@...gle.com>
> > > > Co-developed-by: Marco Elver <elver@...gle.com>
> > > > Signed-off-by: Marco Elver <elver@...gle.com>
> > > > Signed-off-by: Alexander Potapenko <glider@...gle.com>
> > >
> > > Reviewed-by: Jann Horn <jannh@...gle.com>
> >
> > Thanks!
> >
> > > with one nit:
> > >
> > > [...]
> > > > diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> > > [...]
> > > > @@ -141,6 +142,14 @@ void kasan_unpoison_shadow(const void *address, size_t size)
> > > >          */
> > > >         address = reset_tag(address);
> > > >
> > > > +       /*
> > > > +        * We may be called from SL*B internals, such as ksize(): with a size
> > > > +        * not a multiple of machine-word size, avoid poisoning the invalid
> > > > +        * portion of the word for KFENCE memory.
> > > > +        */
> > > > +       if (is_kfence_address(address))
> > > > +               return;
> > >
> > > It might be helpful if you could add a comment that explains that
> > > kasan_poison_object_data() does not need a similar guard because
> > > kasan_poison_object_data() is always paired with
> > > kasan_unpoison_object_data() - that threw me off a bit at first.
> >
> > Well, KFENCE objects should never be poisoned/unpoisoned because the
> > kasan_alloc and free hooks have a kfence guard, and none of the code
> > in sl*b.c that does kasan_{poison,unpoison}_object_data() should be
> > executed for KFENCE objects.
> >
> > But I just noticed that kernel/scs.c seems to kasan_poison and
> > unpoison objects, and keeps them poisoned for most of the object
> > lifetime.
>
> FWIW, I wouldn't be surprised if other parts of the kernel also ended
> up wanting to have in-object redzones eventually - e.g. inside skb
> buffers, which have a struct skb_shared_info at the end. AFAIU at the
> moment, KASAN can't catch small OOB accesses from these buffers
> because of the following structure.

Sure, and it might also become more interesting with MTE-based KASAN.

But, currently we recommend not to enable generic KASAN+KFENCE,
because it'd be redundant if the instrumentation price for generic (or
SW-tag) KASAN is already paid. The changes here are also mostly for
testing KFENCE itself.

That may change with MTE-based KASAN, however, which may have modes
where stack traces aren't collected and having KFENCE to get
actionable debug-info across a fleet of machines may still be wanted.
But that story is still evolving. The code here is only for the
generic and SW-tag based KASAN modes, and MTE will have its own
kasan_{un,}poison_shadow (afaik it's being renamed to
kasan_{un,}poison_memory) which works just fine with KFENCE AFAIK.

> > I think we better add a kfence guard to
> > kasan_poison_shadow() as well.
>
> Sounds good.