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From: Kees Cook <keescook@chromium.org>
Date: Wed, 14 Feb 2018 11:48:38 -0800
Message-ID: <CAGXu5j+zkFx+1Dn908iqaTV-yP7Wk_rMXZRvXN32h+i_oAcy6w@mail.gmail.com>
Subject: Re: arm64 physmap (was Re: [kernel-hardening] [PATCH 4/6] Protectable Memory)
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To: Laura Abbott <labbott@redhat.com>
Cc: Jann Horn <jannh@google.com>, Igor Stoppa <igor.stoppa@huawei.com>, Boris Lukashev <blukashev@sempervictus.com>, Christopher Lameter <cl@linux.com>, Matthew Wilcox <willy@infradead.org>, Jerome Glisse <jglisse@redhat.com>, Michal Hocko <mhocko@kernel.org>, Christoph Hellwig <hch@infradead.org>, linux-security-module <linux-security-module@vger.kernel.org>, Linux-MM <linux-mm@kvack.org>, kernel list <linux-kernel@vger.kernel.org>, Kernel Hardening <kernel-hardening@lists.openwall.com>, linux-arm-kernel <linux-arm-kernel@lists.infradead.org>

On Wed, Feb 14, 2018 at 11:06 AM, Laura Abbott <labbott@redhat.com> wrote:
> fixed. Modules yes are not fully protected. The conclusion from past
> experience has been that we cannot safely break down larger page sizes
> at runtime like x86 does. We could theoretically
> add support for fixing up the alias if PAGE_POISONING is enabled but
> I don't know who would actually use that in production. Performance
> is very poor at that point.

XPFO forces 4K pages on the physmap[1] for similar reasons. I have no
doubt about performance changes, but I'd be curious to see real
numbers. Did anyone do benchmarks on just the huge/4K change? (Without
also the XPFO overhead?)

If this, XPFO, and PAGE_POISONING all need it, I think we have to
start a closer investigation. :)

-Kees

[1] http://www.openwall.com/lists/kernel-hardening/2017/09/07/13

-- 
Kees Cook
Pixel Security

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