Subject: Re: [PATCH RFC] uaccess: user_access_begin_after_access_ok()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 3 Jun 2020 01:17:45 -0400

On Tue, Jun 02, 2020 at 11:10:57PM +0100, Al Viro wrote:
> On Tue, Jun 02, 2020 at 04:42:03PM -0400, Michael S. Tsirkin wrote:
> > On Tue, Jun 02, 2020 at 05:30:48PM +0100, Al Viro wrote:
> > > On Tue, Jun 02, 2020 at 04:45:05AM -0400, Michael S. Tsirkin wrote:
> > > > So vhost needs to poke at userspace *a lot* in a quick succession.  It
> > > > is thus benefitial to enable userspace access, do our thing, then
> > > > disable. Except access_ok has already been pre-validated with all the
> > > > relevant nospec checks, so we don't need that.  Add an API to allow
> > > > userspace access after access_ok and barrier_nospec are done.
> > > 
> > > This is the wrong way to do it, and this API is certain to be abused
> > > elsewhere.  NAK - we need to sort out vhost-related problems, but
> > > this is not an acceptable solution.  Sorry.
> > 
> > OK so summarizing what you and Linus both said, we need at
> > least a way to make sure access_ok (and preferably the barrier too)
> > is not missed.
> > 
> > Another comment is about actually checking that performance impact
> > is significant and worth the complexity and risk.
> > 
> > Is that a fair summary?
> > 
> > I'm actually thinking it's doable with a new __unsafe_user type of
> > pointer, sparse will then catch errors for us.
> 
> Er... how would sparse keep track of the range?

Using types. So you start with a user pointer:

struct foo __user *up;

Now you validate it, including a speculation barrier:

struct foo __valdated_user *p = user_access_validate(up, sizeof *up);

and you can save it and use it with something like unsafe_get_user and unsafe_put_user
that gets __valdated_user pointers:

user_access_begin_validated(p, sizeof *p)
valiated_get_user(bar, foo->bar, err_fault)
valiated_put_user(baz, foo->baz, err_fault)
user_access_end()

-- 
MST