linux-kernel - Re: [PATCH v3 03/10] arm64, kfence: enable KFENCE for ARM64

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Date:   Tue, 22 Sep 2020 11:56:26 +0200
From:   Marco Elver <elver@...gle.com>
To:     Will Deacon <will@...nel.org>
Cc:     Alexander Potapenko <glider@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "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>,
        Dmitriy 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>, Jann Horn <jannh@...gle.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>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Memory Management List <linux-mm@...ck.org>
Subject: Re: [PATCH v3 03/10] arm64, kfence: enable KFENCE for ARM64

On Mon, 21 Sep 2020 at 19:44, Will Deacon <will@...nel.org> wrote:
[...]
> > > > > For ARM64, we would like to solicit feedback on what the best option is
> > > > > to obtain a constant address for __kfence_pool. One option is to declare
> > > > > a memory range in the memory layout to be dedicated to KFENCE (like is
> > > > > done for KASAN), however, it is unclear if this is the best available
> > > > > option. We would like to avoid touching the memory layout.
> > > >
> > > > Sorry for the delay on this.
> > >
> > > NP, thanks for looking!
> > >
> > > > Given that the pool is relatively small (i.e. when compared with our virtual
> > > > address space), dedicating an area of virtual space sounds like it makes
> > > > the most sense here. How early do you need it to be available?
> > >
> > > Yes, having a dedicated address sounds good.
> > > We're inserting kfence_init() into start_kernel() after timekeeping_init().
> > > So way after mm_init(), if that matters.
> >
> > The question is though, how big should that dedicated area be?
> > Right now KFENCE_NUM_OBJECTS can be up to 16383 (which makes the pool
> > size 64MB), but this number actually comes from the limitation on
> > static objects, so we might want to increase that number on arm64.
>
> What happens on x86 and why would we do something different?

On x86 we just do `char __kfence_pool[KFENCE_POOL_SIZE] ...;` to
statically allocate the pool. On arm64 this doesn't seem to work
because static memory doesn't have struct pages?

Thanks,
-- Marco