linux-kernel - Re: arm64 physmap (was Re: [kernel-hardening] [PATCH 4/6] Protectable Memory)

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Date:   Wed, 14 Feb 2018 11:48:38 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Laura Abbott <labbott@...hat.com>
Cc:     Jann Horn <jannh@...gle.com>, Igor Stoppa <igor.stoppa@...wei.com>,
        Boris Lukashev <blukashev@...pervictus.com>,
        Christopher Lameter <cl@...ux.com>,
        Matthew Wilcox <willy@...radead.org>,
        Jerome Glisse <jglisse@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: arm64 physmap (was Re: [kernel-hardening] [PATCH 4/6] Protectable Memory)

On Wed, Feb 14, 2018 at 11:06 AM, Laura Abbott <labbott@...hat.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