linux-kernel - Re: [PATCH v4 01/11] mm: add Kernel Electric-Fence infrastructure

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Date:   Mon, 12 Oct 2020 16:20:59 +0200
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>,
        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>, SeongJae Park <sjpark@...zon.de>
Subject: Re: [PATCH v4 01/11] mm: add Kernel Electric-Fence infrastructure

[ Sorry for delay, just noticed this one doesn't have a reply yet. ]

On Sat, 3 Oct 2020 at 00:27, Jann Horn <jannh@...gle.com> wrote:
> On Fri, Oct 2, 2020 at 11:28 PM Marco Elver <elver@...gle.com> wrote:
> > On Fri, 2 Oct 2020 at 21:32, Jann Horn <jannh@...gle.com> wrote:
> > > > That's another check; we don't want to make this more expensive.
> > >
> > > Ah, right, I missed that this is the one piece of KFENCE that is
> > > actually really hot code until Dmitry pointed that out.
> > >
> > > But actually, can't you reduce how hot this is for SLUB by moving
> > > is_kfence_address() down into the freeing slowpath? At the moment you
> > > use it in slab_free_freelist_hook(), which is in the super-hot
> > > fastpath, but you should be able to at least move it down into
> > > __slab_free()...
> > >
> > > Actually, you already have hooked into __slab_free(), so can't you
> > > just get rid of the check in the slab_free_freelist_hook()?
> >
> > I missed this bit: the loop that follows wants the free pointer, so I
> > currently see how this might work. :-/
>
> reverse call graph:
> __slab_free
>   do_slab_free
>     slab_free
>       kmem_cache_free (frees a single non-kmalloc allocation)
>       kmem_cache_free_bulk (frees multiple)
>       kfree (frees a single kmalloc allocation)
>     ___cache_free (frees a single allocation for KASAN)
>
> So the only path for which we can actually loop in __slab_free() is
> kmem_cache_free_bulk(); and you've already changed
> build_detached_freelist() (which is used by kmem_cache_free_bulk() to
> group objects from the same page) to consume KFENCE allocations before
> they can ever reach __slab_free(). So we know that if we've reached
> __slab_free(), then we are being called with either a single object
> (which may be a KFENCE object) or with a list of objects that all
> belong to the same page and don't contain any KFENCE allocations.

Yes, while that is true, we still cannot execute the code in
slab_free_freelist_hook(). There are several problems:

- getting the freepointer which accesses object + s->offset, may
result in KFENCE OOB errors.

- similarly for setting the freepointer.

- slab_want_init_on_free zeroing object according to memcache
object_size, because it'll corrupt KFENCE's redzone if memcache
object_size > actual allocation size.

Leaving this here is fine, since we have determined that recent
optimizations make the check in slab_free_freelist_hook() negligible.

Thanks,
-- Marco