Date: Sat, 6 Jan 2018 20:41:34 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
cc: Dan Williams <dan.j.williams@intel.com>,
        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>
Subject: Re: [PATCH 06/18] x86, barrier: stop speculation for failed
 access_ok
In-Reply-To: <20180106192517.ykvlcq4564cqy4u6@ast-mbp>
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On Sat, 6 Jan 2018, Alexei Starovoitov 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.

For one particular architecture and that's not a solution for generic
code.

Aside of that I fundamentally disagree with your purely performance
optimized argumentation. We need to make sure that we have a solution which
kills the problem safely and then take it from there. Correctness first,
optimization later is the rule for this. Better safe than sorry.

Thanks,

	tglx