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

[Linus Torvalds]

On Fri, Jan 5, 2018 at 5:10 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
>
> When access_ok fails we should always stop speculating.
> Add the required barriers to the x86 access_ok macro.

Honestly, this seems completely bogus.

The description is pure garbage afaik.

The fact is, we have to stop speculating when access_ok() does *not*
fail - because that's when we'll actually do the access. And it's that
access that needs to be non-speculative.

That actually seems to be what the code does (it stops speculation
when __range_not_ok() returns false, but access_ok() is
!__range_not_ok()). But the explanation is crap, and dangerous.

             Linus