Re: [PATCH 06/18] x86, barrier: stop speculation for failed access_ok

Thomas Gleixner
2018-01-06T19:42:01Z

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