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From:   Kees Cook <keescook@chromium.org>
Date:   Wed, 14 Feb 2018 14:27:47 -0800
Message-ID: <CAGXu5jK1taW5JccCTdi6EzfD7RNnFrRyoJQMtLLi5CwT-8eJkQ@mail.gmail.com>
Subject: Re: arm64 physmap (was Re: [kernel-hardening] [PATCH 4/6] Protectable Memory)
To:     Tycho Andersen <tycho@tycho.ws>
Cc:     Laura Abbott <labbott@redhat.com>, 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>,
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On Wed, Feb 14, 2018 at 2:13 PM, Tycho Andersen <tycho@tycho.ws> wrote:
> On Wed, Feb 14, 2018 at 11:48:38AM -0800, Kees Cook wrote:
>> 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. :)
>
> I haven't but it shouldn't be too hard. What benchmarks are you
> thinking?

Unless I'm looking at some specific micro benchmark, I tend to default
to looking at kernel build benchmarks but that gets pretty noisy.
Laura regularly uses hackbench, IIRC. I'm not finding the pastebin I
had for that, though.

I wonder if we need a benchmark subdirectory in tools/testing/, so we
could collect some of these common tools? All benchmarks are terrible,
but at least we'd have the same terrible benchmarks. :)

-Kees

-- 
Kees Cook
Pixel Security