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

["Michael S. Tsirkin" <mst@redhat.com>]

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