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

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

On Fri, Jul 18, 2025 at 02:03:55PM +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.
> 
> We also need to reinit completion in vhost_net_flush(), because
> refcnt == 0 does not mean freeing in that case.
> 
> Cc: stable@vger.kernel.org
> Fixes: 0ad8b480d6ee9 ("vhost: fix ref cnt checking deadlock")
> Reported-by: Andrey Ryabinin <arbn@yandex-team.com>
> Suggested-by: Andrey Smetanin <asmetanin@yandex-team.ru>
> Suggested-by: Hillf Danton <hdanton@sina.com>
> Tested-by: Lei Yang <leiyang@redhat.com> (v1)
> Signed-off-by: Nikolay Kuratov <kniv@yandex-team.ru>


Nikolay should I expect v3?

> ---
> v2:
> * move reinit_completion() into vhost_net_flush(), thanks
>   to Hillf Danton
> * add Tested-by: Lei Yang
> * check that usages of put_and_wait() are consistent across
>   LTS kernels
> 
>  drivers/vhost/net.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 7cbfc7d718b3..69e1bfb9627e 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -94,7 +94,7 @@ struct vhost_net_ubuf_ref {
>  	 * >1: outstanding ubufs
>  	 */
>  	atomic_t refcount;
> -	wait_queue_head_t wait;
> +	struct completion wait;
>  	struct vhost_virtqueue *vq;
>  };
>  
> @@ -240,7 +240,7 @@ vhost_net_ubuf_alloc(struct vhost_virtqueue *vq, bool zcopy)
>  	if (!ubufs)
>  		return ERR_PTR(-ENOMEM);
>  	atomic_set(&ubufs->refcount, 1);
> -	init_waitqueue_head(&ubufs->wait);
> +	init_completion(&ubufs->wait);
>  	ubufs->vq = vq;
>  	return ubufs;
>  }
> @@ -249,14 +249,14 @@ static int vhost_net_ubuf_put(struct vhost_net_ubuf_ref *ubufs)
>  {
>  	int r = atomic_sub_return(1, &ubufs->refcount);
>  	if (unlikely(!r))
> -		wake_up(&ubufs->wait);
> +		complete_all(&ubufs->wait);
>  	return r;
>  }
>  
>  static void vhost_net_ubuf_put_and_wait(struct vhost_net_ubuf_ref *ubufs)
>  {
>  	vhost_net_ubuf_put(ubufs);
> -	wait_event(ubufs->wait, !atomic_read(&ubufs->refcount));
> +	wait_for_completion(&ubufs->wait);
>  }
>  
>  static void vhost_net_ubuf_put_wait_and_free(struct vhost_net_ubuf_ref *ubufs)
> @@ -1381,6 +1381,7 @@ static void vhost_net_flush(struct vhost_net *n)
>  		mutex_lock(&n->vqs[VHOST_NET_VQ_TX].vq.mutex);
>  		n->tx_flush = false;
>  		atomic_set(&n->vqs[VHOST_NET_VQ_TX].ubufs->refcount, 1);
> +		reinit_completion(&n->vqs[VHOST_NET_VQ_TX].ubufs->wait);
>  		mutex_unlock(&n->vqs[VHOST_NET_VQ_TX].vq.mutex);
>  	}
>  }
> -- 
> 2.34.1