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 21:41:33 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Nadav Amit <nadav.amit@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        Juerg Haefliger <juergh@...il.com>,
        Tycho Andersen <tycho@...ho.ws>, jsteckli@...zon.de,
        Kees Cook <keescook@...gle.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Juerg Haefliger <juerg.haefliger@...onical.com>,
        deepa.srinivasan@...cle.com, chris hyser <chris.hyser@...cle.com>,
        Tyler Hicks <tyhicks@...onical.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Jon Masters <jcm@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        iommu <iommu@...ts.linux-foundation.org>,
        X86 ML <x86@...nel.org>,
        "linux-alpha@...r.kernel.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>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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 Wed, Apr 17, 2019 at 5:00 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Wed, Apr 17, 2019 at 4:42 PM Thomas Gleixner <tglx@...utronix.de> wrote:
> >
> > On Wed, 17 Apr 2019, Linus Torvalds wrote:
> >
> > > With SMEP, user space pages are always NX.
> >
> > We talk past each other. The user space page in the ring3 valid virtual
> > address space (non negative) is of course protected by SMEP.
> >
> > The attack utilizes the kernel linear mapping of the physical
> > memory. I.e. user space address 0x43210 has a kernel equivalent at
> > 0xfxxxxxxxxxx. So if the attack manages to trick the kernel to that valid
> > kernel address and that is mapped X --> game over. SMEP does not help
> > there.
>
> Oh, agreed.
>
> But that would simply be a kernel bug. We should only map kernel pages
> executable when we have kernel code in them, and we should certainly
> not allow those pages to be mapped writably in user space.
>
> That kind of "executable in kernel, writable in user" would be a
> horrendous and major bug.
>
> So i think it's a non-issue.
>
> > From the top of my head I'd say this is a non issue as those kernel address
> > space mappings _should_ be NX, but we got bitten by _should_ in the past:)
>
> I do agree that bugs can happen, obviously, and we might have missed something.
>
> But in the context of XPFO, I would argue (*very* strongly) that the
> likelihood of the above kind of bug is absolutely *miniscule* compared
> to the likelihood that we'd have something wrong in the software
> implementation of XPFO.
>
> So if the argument is "we might have bugs in software", then I think
> that's an argument _against_ XPFO rather than for it.
>

I don't think this type of NX goof was ever the argument for XPFO.
The main argument I've heard is that a malicious user program writes a
ROP payload into user memory (regular anonymous user memory) and then
gets the kernel to erroneously set RSP (*not* RIP) to point there.

I find this argument fairly weak for a couple reasons.  First, if
we're worried about this, let's do in-kernel CFI, not XPFO, to
mitigate it.  Second, I don't see why the exact same attack can't be
done using, say, page cache, and unless I'm missing something, XPFO
doesn't protect page cache.  Or network buffers, or pipe buffers, etc.