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

[Kees Cook]

On Wed, Feb 14, 2018 at 2:13 PM, Tycho Andersen <tycho@xxxxxxxx> 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@xxxxxxxxxx> 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