Re: [RFC PATCH V3 0/5] Hi:

Dan Williams
2019-01-07T21:39:15Z

On Mon, Jan 7, 2019 at 6:11 AM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Sun, Jan 06, 2019 at 11:15:20PM -0800, Dan Williams wrote:
> > On Sun, Jan 6, 2019 at 8:17 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> > >
> > > On Mon, Jan 07, 2019 at 11:53:41AM +0800, Jason Wang wrote:
> > > >
> > > > On 2019/1/7 上午11:28, Michael S. Tsirkin wrote:
> > > > > On Mon, Jan 07, 2019 at 10:19:03AM +0800, Jason Wang wrote:
> > > > > > On 2019/1/3 上午4:47, Michael S. Tsirkin wrote:
> > > > > > > On Sat, Dec 29, 2018 at 08:46:51PM +0800, Jason Wang wrote:
> > > > > > > > This series tries to access virtqueue metadata through kernel virtual
> > > > > > > > address instead of copy_user() friends since they had too much
> > > > > > > > overheads like checks, spec barriers or even hardware feature
> > > > > > > > toggling.
> > > > > > > Will review, thanks!
> > > > > > > One questions that comes to mind is whether it's all about bypassing
> > > > > > > stac/clac. Could you please include a performance comparison with
> > > > > > > nosmap?
> > > > > > >
> > > > > > On machine without SMAP (Sandy Bridge):
> > > > > >
> > > > > > Before: 4.8Mpps
> > > > > >
> > > > > > After: 5.2Mpps
> > > > > OK so would you say it's really unsafe versus safe accesses?
> > > > > Or would you say it's just a better written code?
> > > >
> > > >
> > > > It's the effect of removing speculation barrier.
> > >
> > >
> > > You mean __uaccess_begin_nospec introduced by
> > > commit 304ec1b050310548db33063e567123fae8fd0301
> > > ?
> > >
> > > So fundamentally we do access_ok checks when supplying
> > > the memory table to the kernel thread, and we should
> > > do the spec barrier there.
> > >
> > > Then we can just create and use a variant of uaccess macros that does
> > > not include the barrier?
> > >
> > > Or, how about moving the barrier into access_ok?
> > > This way repeated accesses with a single access_ok get a bit faster.
> > > CC Dan Williams on this idea.
> >
> > It would be interesting to see how expensive re-doing the address
> > limit check is compared to the speculation barrier. I.e. just switch
> > vhost_get_user() to use get_user() rather than __get_user(). That will
> > sanitize the pointer in the speculative path without a barrier.
>
> Hmm it's way cheaper even though IIRC it's measureable.
> Jason, would you like to try?
> Although frankly __get_user being slower than get_user feels very wrong.
> Not yet sure what to do exactly but would you agree?

Agree. __get_user() being faster than get_user() defeats the whole
point of converting code paths to the access_ok() + __get_user()
pattern.

>
>
> > I recall we had a convert access_ok() discussion with this result here:
> >
> > https://lkml.org/lkml/2018/1/17/929
>
> Sorry let me try to clarify. IIUC speculating access_ok once
> is harmless. As Linus said the problem is with "_subsequent_
> accesses that can then be used to perturb the cache".
>
> Thus:
>
> 1. if (!access_ok)
> 2. return
> 3. get_user
> 4. if (!access_ok)
> 5. return
> 6. get_user
>
> Your proposal that Linus nacked was to effectively add a barrier after
> lines 2 and 5 (also using the array_index_nospec trick for speed),
> right? Unfortunately that needs a big API change.
>
> I am asking about adding barrier_nospec within access_ok.
> Thus effectively before lines 1 and 4.
> access_ok will be slower but after all the point of access_ok is
> to then access the same memory multiple times.

If the barrier is before lines 1 and 4 then it offers no real
protection as far I can see. It will start speculating again on the
user controlled pointer value after the barrier resolves.

> So we should be making __get_user faster and access_ok slower ...

I agree, but then the barrier always needs to be after the access_ok()
check unconditionally called in the return path from access_ok(). At
that point it's back to the implementation that Linus nak'd, or I'm
missing some other detail.

...but maybe if it is limited to a new access_ok_nospec() then the
concern is addressed? Then rename current __get_user() to
__get_user_nospec() and make a new __get_user() that is back to being
optimal.