Subject: Re: [PULL] vhost: cleanups and fixes

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 2 Nov 2018 12:59:40 -0400

On Fri, Nov 02, 2018 at 09:14:51AM -0700, Linus Torvalds wrote:
> On Fri, Nov 2, 2018 at 6:04 AM Michael S. Tsirkin <[email-masked]> wrote:
> >
> > I've tried making access_ok mask the parameter it gets.
> 
> PLEASE don't do this.

Okay.

> Just use "copy_to/from_user()".

Just for completeness I'd like to point out for vhost the copies are
done from the kernel thread.  So yes we can switch to copy_to/from_user
but for e.g. 32-bit userspace running on top of a 64 bit kernel it is
IIUC not sufficient - we must *also* do access_ok checks on control path
when addresses are passed to the kernel and when current points to the
correct task struct.

> We have had lots of bugs because code bitrots.

Yes, I wish we did not need these access_ok checks and could just rely
on copy_to/from_user.

> And no, the access_ok() checks aren't expensive, not even in a loop.
> They *used* to be somewhat expensive compared to the access, but that
> simply isn't true any more. The real expense in copy_to_user and
> friends are in the user access bit setting (STAC and CLAC on x86),
> which easily an order of magnitude more expensive than access_ok().
> 
> So just get rid of the double-underscore version. It's basically
> always a mis-optimization due to entirely historical reasons. I can
> pretty much guarantee that it's not visible in profiles.
> 
>                   Linus

OK. So maybe we should focus on switching to user_access_begin/end +
unsafe_get_user/unsafe_put_user in a loop which does seem to be
measureable.  That moves the barrier out of the loop, which seems to be
consistent with what you would expect.

-- 
MST