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 10:49:26 -0600
From:   Khalid Aziz <khalid.aziz@...cle.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     juergh@...il.com, tycho@...ho.ws, jsteckli@...zon.de,
        keescook@...gle.com, konrad.wilk@...cle.com,
        Juerg Haefliger <juerg.haefliger@...onical.com>,
        deepa.srinivasan@...cle.com, chris.hyser@...cle.com,
        tyhicks@...onical.com, dwmw@...zon.co.uk,
        andrew.cooper3@...rix.com, jcm@...hat.com,
        boris.ostrovsky@...cle.com, iommu@...ts.linux-foundation.org,
        x86@...nel.org, linux-arm-kernel@...ts.infradead.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, 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 4/17/19 10:15 AM, Ingo Molnar wrote:
> 
> [ Sorry, had to trim the Cc: list from hell. Tried to keep all the 
>   mailing lists and all x86 developers. ]
> 
> * Khalid Aziz <khalid.aziz@...cle.com> wrote:
> 
>> From: Juerg Haefliger <juerg.haefliger@...onical.com>
>>
>> This patch adds basic support infrastructure for XPFO which protects 
>> against 'ret2dir' kernel attacks. The basic idea is to enforce 
>> exclusive ownership of page frames by either the kernel or userspace, 
>> unless explicitly requested by the kernel. Whenever a page destined for 
>> userspace is allocated, it is unmapped from physmap (the kernel's page 
>> table). When such a page is reclaimed from userspace, it is mapped back 
>> to physmap. Individual architectures can enable full XPFO support using 
>> this infrastructure by supplying architecture specific pieces.
> 
> I have a higher level, meta question:
> 
> Is there any updated analysis outlining why this XPFO overhead would be 
> required on x86-64 kernels running on SMAP/SMEP CPUs which should be all 
> recent Intel and AMD CPUs, and with kernel that mark all direct kernel 
> mappings as non-executable - which should be all reasonably modern 
> kernels later than v4.0 or so?
> 
> 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.

Thanks,
Khalid