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From: Dan Williams <dan.j.williams@intel.com>
Date: Sat, 6 Jan 2018 11:36:04 -0800
Message-ID: <CAPcyv4jLGwCTVQAxHH5gob_6CSeXkbNDdWCV3t-KHKGXVPjBCQ@mail.gmail.com>
Subject: Re: [PATCH 06/18] x86, barrier: stop speculation for failed access_ok
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arch@vger.kernel.org, Andi Kleen <ak@linux.intel.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Netdev <netdev@vger.kernel.org>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>
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Sender: linux-kernel-owner@vger.kernel.org

On Sat, Jan 6, 2018 at 11:25 AM, Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
> On Sat, Jan 06, 2018 at 10:54:27AM -0800, Dan Williams wrote:
>> On Sat, Jan 6, 2018 at 10:39 AM, Alexei Starovoitov
>> <alexei.starovoitov@gmail.com> wrote:
>> [..]
>> >> retpoline is variant-2, this patch series is about variant-1.
>> >
>> > that's exactly the point. Don't slow down the kernel with lfences
>> > to solve variant 1. retpoline for 2 is ok from long term kernel
>> > viability perspective.
>> >
>>
>> Setting aside that we still need to measure the impact of these
>> changes the end result will still be nospec_array_ptr() sprinkled in
>> various locations. So can we save the debate about what's inside that
>> macro on various architectures and at least proceed with annotating
>> the problematic locations? Perhaps we can go a step further and have a
>> config option to switch between the clever array_access() approach
>> from Linus that might be fine depending on the compiler, and the
>> cpu-vendor-recommended not to speculate implementation of
>> nospec_array_ptr().
>
> recommended by panicing vendors who had no better ideas?
> Ohh, speculation is exploitable, let's stop speculation.
> Instead of fighting it we can safely steer it where it doesn't leak
> kernel data. AND approach is doing exactly that.
> It probably can be made independent of compiler choice to use setbe-like insn.

Right, when that 'probably' is 'certainly' for the architecture you
care about just update the nospec_array_ptr() definition at that
point.