Re: [PATCH RFC] uaccess: user_access_begin_after_access_ok()

Michael S. Tsirkin
2020-06-03T05:17:55Z

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