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

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Message-Id: <56A175F6-E5DA-4BBD-B244-53B786F27B7F@gmail.com>
Date:   Wed, 17 Apr 2019 10:19:54 -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 10:09 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.”