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

[Hillf Danton <hdanton@sina.com>]

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)