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Date: Tue, 12 Sep 2017 16:11:26 +0800
From: Yisheng Xie <>
To: Juerg Haefliger <>,
        Tycho Andersen
CC: <>, <>,
        Marco Benatto
Subject: Re: [PATCH v6 00/11] Add support for eXclusive Page Frame Ownership

On 2017/9/12 15:40, Juerg Haefliger wrote:
> On 09/12/2017 09:07 AM, Yisheng Xie wrote:
>> Hi Tycho,
>> On 2017/9/11 23:02, Tycho Andersen wrote:
>>> Hi Yisheng,
>>> On Mon, Sep 11, 2017 at 06:34:45PM +0800, Yisheng Xie wrote:
>>>> Hi Tycho ,
>>>> On 2017/9/8 1:35, Tycho Andersen wrote:
>>>>> Hi all,
>>>>> Here is v6 of the XPFO set; see v5 discussion here:
>>>>> Changelogs are in the individual patch notes, but the highlights are:
>>>>> * add primitives for ensuring memory areas are mapped (although these are quite
>>>>>   ugly, using stack allocation; I'm open to better suggestions)
>>>>> * instead of not flushing caches, re-map pages using the above
>>>>> * TLB flushing is much more correct (i.e. we're always flushing everything
>>>>>   everywhere). I suspect we may be able to back this off in some cases, but I'm
>>>>>   still trying to collect performance numbers to prove this is worth doing.
>>>>> I have no TODOs left for this set myself, other than fixing whatever review
>>>>> feedback people have. Thoughts and testing welcome!
>>>> According to the paper of Vasileios P. Kemerlis et al, the mainline kernel
>>>> will not set the Pro. of physmap(direct map area) to RW(X), so do we really
>>>> need XPFO to protect from ret2dir attack?
>>> I guess you're talking about section 4.3? 
>> Yes
>>> They mention that that x86
>>> only gets rw, but that aarch64 is rwx still.
>> IIRC, the in kernel of v4.13 the aarch64 is not rwx, I will check it.
>>> But in either case this still provides access protection, similar to
>>> SMAP. Also, if I understand things correctly the protections are
>>> unmanaged, so a page that had the +x bit set at some point, it could
>>> be used for ret2dir.
>> So you means that the Pro. of direct map area maybe changed to +x, then ret2dir attack can use it?
> XPFO protects against malicious reads from userspace (potentially
> accessing sensitive data). 
This sounds reasonable to me.

> I've also been told by a security expert that
> ROP attacks are still possible even if user space memory is
> non-executable. XPFO is supposed to prevent that but I haven't been able
> to confirm this. It's way out of my comfort zone.
It also quite out of knowledge, and I just try hard to understand it. Thanks so much for
your kind explain.  And hope some security expert can give some more detail explain?

Yisheng Xie

> ...Juerg

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