linux-kernel - Re: [PATCH 06/18] x86, barrier: stop speculation for failed access

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Message-ID: <20180106192517.ykvlcq4564cqy4u6@ast-mbp>
Date:   Sat, 6 Jan 2018 11:25:19 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Netdev <netdev@...r.kernel.org>, Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 06/18] x86, barrier: stop speculation for failed access_ok

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@...il.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.