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

From: Will Deacon <will@kernel.org>
To: Alexander Potapenko <glider@google.com>
Cc: Marco Elver <elver@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Andrey Konovalov <andreyknvl@google.com>,
	Andrey Ryabinin <aryabinin@virtuozzo.com>,
	Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Christoph Lameter <cl@linux.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	David Rientjes <rientjes@google.com>,
	Dmitriy Vyukov <dvyukov@google.com>,
	Eric Dumazet <edumazet@google.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Hillf Danton <hdanton@sina.com>, Ingo Molnar <mingo@redhat.com>,
	Jann Horn <jannh@google.com>,
	Jonathan.Cameron@huawei.com, Jonathan Corbet <corbet@lwn.net>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	Kees Cook <keescook@chromium.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Pekka Enberg <penberg@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	sjpark@amazon.com, Thomas Gleixner <tglx@linutronix.de>,
	Vlastimil Babka <vbabka@suse.cz>,
	the arch/x86 maintainers <x86@kernel.org>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	kasan-dev <kasan-dev@googlegroups.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Linux Memory Management List <linux-mm@kvack.org>
Subject: Re: [PATCH v3 03/10] arm64, kfence: enable KFENCE for ARM64
Date: Mon, 21 Sep 2020 18:43:59 +0100	[thread overview]
Message-ID: <20200921174357.GB3141@willie-the-truck> (raw)
In-Reply-To: <CAG_fn=XV7JfJDK+t1X6bnV6gRoiogNXsHfww0jvcEtJ2WZpR7Q@mail.gmail.com>

On Mon, Sep 21, 2020 at 05:37:10PM +0200, Alexander Potapenko wrote:
> On Mon, Sep 21, 2020 at 4:58 PM Alexander Potapenko <glider@google.com> wrote:
> >
> > On Mon, Sep 21, 2020 at 4:31 PM Will Deacon <will@kernel.org> wrote:
> > >
> > > On Mon, Sep 21, 2020 at 03:26:04PM +0200, Marco Elver wrote:
> > > > Add architecture specific implementation details for KFENCE and enable
> > > > KFENCE for the arm64 architecture. In particular, this implements the
> > > > required interface in <asm/kfence.h>. Currently, the arm64 version does
> > > > not yet use a statically allocated memory pool, at the cost of a pointer
> > > > load for each is_kfence_address().
> > > >
> > > > Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
> > > > Co-developed-by: Alexander Potapenko <glider@google.com>
> > > > Signed-off-by: Alexander Potapenko <glider@google.com>
> > > > Signed-off-by: Marco Elver <elver@google.com>
> > > > ---
> > > > 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?

Will