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

[Dan Williams]

On Fri, Jan 5, 2018 at 6:52 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> 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.

Oh, bother, yes, good catch. It's been a long week.  I'll take a look
at moving this to uaccess_begin().