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

Dan Williams
2018-01-06T19:36:06Z

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.