Re: [PATCH v2] vhost/net: Replace wait_queue with completion in ubufs reference

[Lei Yang]

Tested this patch's V2 with the virtio-net regression test, everything
works fine.

Tested-by: Lei Yang <leiyang@xxxxxxxxxx>

On Mon, Jul 21, 2025 at 12:13 AM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> On Sat, Jul 19, 2025 at 07:03:23AM +0800, Hillf Danton wrote:
> > On Fri, 18 Jul 2025 14:03:55 +0300 Nikolay Kuratov wrote:
> > > When operating on struct vhost_net_ubuf_ref, the following execution
> > > sequence is theoretically possible:
> > > CPU0 is finalizing DMA operation                   CPU1 is doing VHOST_NET_SET_BACKEND
> > >                              // &ubufs->refcount == 2
> > > vhost_net_ubuf_put()                               vhost_net_ubuf_put_wait_and_free(oldubufs)
> > >                                                      vhost_net_ubuf_put_and_wait()
> > >                                                        vhost_net_ubuf_put()
> > >                                                          int r = atomic_sub_return(1, &ubufs->refcount);
> > >                                                          // r = 1
> > > int r = atomic_sub_return(1, &ubufs->refcount);
> > > // r = 0
> > >                                                       wait_event(ubufs->wait, !atomic_read(&ubufs->refcount));
> > >                                                       // no wait occurs here because condition is already true
> > >                                                     kfree(ubufs);
> > > if (unlikely(!r))
> > >   wake_up(&ubufs->wait);  // use-after-free
> > >
> > > This leads to use-after-free on ubufs access. This happens because CPU1
> > > skips waiting for wake_up() when refcount is already zero.
> > >
> > > To prevent that use a completion instead of wait_queue as the ubufs
> > > notification mechanism. wait_for_completion() guarantees that there will
> > > be complete() call prior to its return.
> > >
> > Alternatively rcu helps.
> >
> > --- x/drivers/vhost/net.c
> > +++ y/drivers/vhost/net.c
> > @@ -96,6 +96,7 @@ struct vhost_net_ubuf_ref {
> >       atomic_t refcount;
> >       wait_queue_head_t wait;
> >       struct vhost_virtqueue *vq;
> > +     struct rcu_head rcu;
> >  };
> >
> >  #define VHOST_NET_BATCH 64
> > @@ -247,9 +248,13 @@ vhost_net_ubuf_alloc(struct vhost_virtqu
> >
> >  static int vhost_net_ubuf_put(struct vhost_net_ubuf_ref *ubufs)
> >  {
> > -     int r = atomic_sub_return(1, &ubufs->refcount);
> > +     int r;
> > +
> > +     rcu_read_lock();
> > +     r = atomic_sub_return(1, &ubufs->refcount);
> >       if (unlikely(!r))
> >               wake_up(&ubufs->wait);
> > +     rcu_read_unlock();
> >       return r;
> >  }
> >
> > @@ -262,7 +267,7 @@ static void vhost_net_ubuf_put_and_wait(
> >  static void vhost_net_ubuf_put_wait_and_free(struct vhost_net_ubuf_ref *ubufs)
> >  {
> >       vhost_net_ubuf_put_and_wait(ubufs);
> > -     kfree(ubufs);
> > +     kfree_rcu(ubufs, rcu);
> >  }
> >
> >  static void vhost_net_clear_ubuf_info(struct vhost_net *n)
>
> I like that.
>
> --
> MST
>