Re: [PATCH v2 07/19] x86: introduce __uaccess_begin_nospec and ASM_IFENCE

[Josh Poimboeuf]

On Fri, Jan 12, 2018 at 12:01:04PM -0800, Linus Torvalds wrote:
> On Fri, Jan 12, 2018 at 11:26 AM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> >
> > By the time we get to de-reference uptr we know it is not pointing at
> > kernel memory, because access_ok would have failed and the cpu would
> > have waited for that failure result before doing anything else.
> 
> I'm not actually convinced that's right in the original patches,
> exactly because of the issue that Josh pointed out: even if there is a
> comparison inside access_ok() that will be properly serialized, then
> that comparison can (and sometimes does) just cause a truth value to
> be generated, and then there  might be *another* comparison of that
> return value after the lfence. And while the return value is table,
> the conditional branch on that comparison isn't.
> 
> The new model of just doing it together with the STAC should be fine, though.

Aha, that clears it up for me, thanks.  I was still thinking about the
previous version of the patch which had the barrier in access_ok().  I
didn't realize the new version moved the barrier to after the
access_ok() checks.

-- 
Josh