linux-kernel - Re: [RFC PATCH v9 03/13] mm: Add support for eXclusive Page Frame Ownership (XPFO)

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Date:   Wed, 17 Apr 2019 10:44:56 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Khalid Aziz <khalid.aziz@...cle.com>, juergh@...il.com,
        Tycho Andersen <tycho@...ho.ws>, jsteckli@...zon.de,
        keescook@...gle.com,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Juerg Haefliger <juerg.haefliger@...onical.com>,
        deepa.srinivasan@...cle.com, chris.hyser@...cle.com,
        tyhicks@...onical.com, David Woodhouse <dwmw@...zon.co.uk>,
        Andrew Cooper <andrew.cooper3@...rix.com>, jcm@...hat.com,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        iommu <iommu@...ts.linux-foundation.org>,
        X86 ML <x86@...nel.org>, linux-arm-kernel@...ts.infradead.org,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        Khalid Aziz <khalid@...ehiking.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Dave Hansen <dave@...1.net>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Arjan van de Ven <arjan@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [RFC PATCH v9 03/13] mm: Add support for eXclusive Page Frame
 Ownership (XPFO)

> On Apr 17, 2019, at 10:26 AM, Ingo Molnar <mingo@...nel.org> wrote:
> 
> 
> * Nadav Amit <nadav.amit@...il.com> wrote:
> 
>>> On Apr 17, 2019, at 10:09 AM, Ingo Molnar <mingo@...nel.org> wrote:
>>> 
>>> 
>>> * Khalid Aziz <khalid.aziz@...cle.com> wrote:
>>> 
>>>>> I.e. the original motivation of the XPFO patches was to prevent execution 
>>>>> of direct kernel mappings. Is this motivation still present if those 
>>>>> mappings are non-executable?
>>>>> 
>>>>> (Sorry if this has been asked and answered in previous discussions.)
>>>> 
>>>> Hi Ingo,
>>>> 
>>>> That is a good question. Because of the cost of XPFO, we have to be very
>>>> sure we need this protection. The paper from Vasileios, Michalis and
>>>> Angelos - <http://www.cs.columbia.edu/~vpk/papers/ret2dir.sec14.pdf>,
>>>> does go into how ret2dir attacks can bypass SMAP/SMEP in sections 6.1
>>>> and 6.2.
>>> 
>>> So it would be nice if you could generally summarize external arguments 
>>> when defending a patchset, instead of me having to dig through a PDF 
>>> which not only causes me to spend time that you probably already spent 
>>> reading that PDF, but I might also interpret it incorrectly. ;-)
>>> 
>>> The PDF you cited says this:
>>> 
>>> "Unfortunately, as shown in Table 1, the W^X prop-erty is not enforced 
>>>  in many platforms, including x86-64.  In our example, the content of 
>>>  user address 0xBEEF000 is also accessible through kernel address 
>>>  0xFFFF87FF9F080000 as plain, executable code."
>>> 
>>> Is this actually true of modern x86-64 kernels? We've locked down W^X 
>>> protections in general.
>> 
>> As I was curious, I looked at the paper. Here is a quote from it:
>> 
>> "In x86-64, however, the permissions of physmap are not in sane state.
>> Kernels up to v3.8.13 violate the W^X property by mapping the entire region
>> as “readable, writeable, and executable” (RWX)—only very recent kernels
>> (≥v3.9) use the more conservative RW mapping.”
> 
> But v3.8.13 is a 5+ years old kernel, it doesn't count as a "modern" 
> kernel in any sense of the word. For any proposed patchset with 
> significant complexity and non-trivial costs the benchmark version 
> threshold is the "current upstream kernel".
> 
> So does that quote address my followup questions:
> 
>> Is this actually true of modern x86-64 kernels? We've locked down W^X
>> protections in general.
>> 
>> I.e. this conclusion:
>> 
>>  "Therefore, by simply overwriting kfptr with 0xFFFF87FF9F080000 and
>>   triggering the kernel to dereference it, an attacker can directly
>>   execute shell code with kernel privileges."
>> 
>> ... appears to be predicated on imperfect W^X protections on the x86-64
>> kernel.
>> 
>> Do such holes exist on the latest x86-64 kernel? If yes, is there a
>> reason to believe that these W^X holes cannot be fixed, or that any fix
>> would be more expensive than XPFO?
> 
> ?
> 
> What you are proposing here is a XPFO patch-set against recent kernels 
> with significant runtime overhead, so my questions about the W^X holes 
> are warranted.
> 

Just to clarify - I am an innocent bystander and have no part in this work.
I was just looking (again) at the paper, as I was curious due to the recent
patches that I sent that improve W^X protection.