linux-kernel - Re: [PATCH 00/18] prevent bounds-check bypass via speculative execution

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Date:   Sat, 6 Jan 2018 11:37:03 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc:     Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alan Cox <alan.cox@...el.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Will Deacon <will.deacon@....com>,
        Solomon Peachy <pizza@...ftnet.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Christian Lamparter <chunkeey@...glemail.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        linux-arch@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        linux-scsi <linux-scsi@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>, X86 ML <x86@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Zhang Rui <rui.zhang@...el.com>,
        "Linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>, Jan Kara <jack@...e.com>,
        Eduardo Valentin <edubezval@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>, qla2xxx-upstream@...gic.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        Alan Cox <alan@...ux.intel.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        linux-wireless@...r.kernel.org,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Netdev <netdev@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "David S. Miller" <davem@...emloft.net>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>
Subject: Re: [PATCH 00/18] prevent bounds-check bypass via speculative execution

On Fri, Jan 5, 2018 at 5:09 PM, Dan Williams <dan.j.williams@...el.com> wrote:
> Quoting Mark's original RFC:
>
> "Recently, Google Project Zero discovered several classes of attack
> against speculative execution. One of these, known as variant-1, allows
> explicit bounds checks to be bypassed under speculation, providing an
> arbitrary read gadget. Further details can be found on the GPZ blog [1]
> and the Documentation patch in this series."
>
> This series incorporates Mark Rutland's latest api and adds the x86
> specific implementation of nospec_barrier. The
> nospec_{array_ptr,ptr,barrier} helpers are then combined with a kernel
> wide analysis performed by Elena Reshetova to address static analysis
> reports where speculative execution on a userspace controlled value
> could bypass a bounds check. The patches address a precondition for the
> attack discussed in the Spectre paper [2].
>
> A consideration worth noting for reviewing these patches is to weigh the
> dramatic cost of being wrong about whether a given report is exploitable
> vs the overhead nospec_{array_ptr,ptr} may introduce. In other words,
> lets make the bar for applying these patches be "can you prove that the
> bounds check bypass is *not* exploitable". Consider that the Spectre
> paper reports one example of a speculation window being ~180 cycles.
>
> Note that there is also a proposal from Linus, array_access [3], that
> attempts to quash speculative execution past a bounds check without
> introducing an lfence instruction. That may be a future optimization
> possibility that is compatible with this api, but it would appear to
> need guarantees from the compiler that it is not clear the kernel can
> rely on at this point. It is also not clear that it would be a
> significant performance win vs lfence.
>
> These patches also will also be available via the 'nospec' git branch
> here:
>
>     git://git.kernel.org/pub/scm/linux/kernel/git/djbw/linux nospec

It appears that git.kernel.org has not mirrored out the new branch. In
the meantime here's an alternative location:

    https://github.com/djbw/linux.git nospec

If there are updates to these patches they will appear in nospec-v2,
nospec-v3, etc... branches.