Re: [PATCH v2 1/3] vsock/virtio: use RCU to avoid use-after-free on the_virtio_vsock

[Jason Wang]

On 2019/6/28 äå8:36, Stefano Garzarella wrote:
> Some callbacks used by the upper layers can run while we are in the
> .remove(). A potential use-after-free can happen, because we free
> the_virtio_vsock without knowing if the callbacks are over or not.
>
> To solve this issue we move the assignment of the_virtio_vsock at the
> end of .probe(), when we finished all the initialization, and at the
> beginning of .remove(), before to release resources.
> For the same reason, we do the same also for the vdev->priv.
>
> We use RCU to be sure that all callbacks that use the_virtio_vsock
> ended before freeing it. This is not required for callbacks that
> use vdev->priv, because after the vdev->config->del_vqs() we are sure
> that they are ended and will no longer be invoked.
>
> We also take the mutex during the .remove() to avoid that .probe() can
> run while we are resetting the device.
>
> Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
> ---
>   net/vmw_vsock/virtio_transport.c | 67 +++++++++++++++++++++-----------
>   1 file changed, 44 insertions(+), 23 deletions(-)
>
> diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
> index 9c287e3e393c..7ad510ec12e0 100644
> --- a/net/vmw_vsock/virtio_transport.c
> +++ b/net/vmw_vsock/virtio_transport.c
> @@ -65,19 +65,22 @@ struct virtio_vsock {
>   	u32 guest_cid;
>   };
>   
> -static struct virtio_vsock *virtio_vsock_get(void)
> -{
> -	return the_virtio_vsock;
> -}
> -
>   static u32 virtio_transport_get_local_cid(void)
>   {
> -	struct virtio_vsock *vsock = virtio_vsock_get();
> +	struct virtio_vsock *vsock;
> +	u32 ret;
>   
> -	if (!vsock)
> -		return VMADDR_CID_ANY;
> +	rcu_read_lock();
> +	vsock = rcu_dereference(the_virtio_vsock);
> +	if (!vsock) {
> +		ret = VMADDR_CID_ANY;
> +		goto out_rcu;
> +	}
>   
> -	return vsock->guest_cid;
> +	ret = vsock->guest_cid;
> +out_rcu:
> +	rcu_read_unlock();
> +	return ret;
>   }
>   
>   static void virtio_transport_loopback_work(struct work_struct *work)
> @@ -197,14 +200,18 @@ virtio_transport_send_pkt(struct virtio_vsock_pkt *pkt)
>   	struct virtio_vsock *vsock;
>   	int len = pkt->len;
>   
> -	vsock = virtio_vsock_get();
> +	rcu_read_lock();
> +	vsock = rcu_dereference(the_virtio_vsock);
>   	if (!vsock) {
>   		virtio_transport_free_pkt(pkt);
> -		return -ENODEV;
> +		len = -ENODEV;
> +		goto out_rcu;
>   	}
>   
> -	if (le64_to_cpu(pkt->hdr.dst_cid) == vsock->guest_cid)
> -		return virtio_transport_send_pkt_loopback(vsock, pkt);
> +	if (le64_to_cpu(pkt->hdr.dst_cid) == vsock->guest_cid) {
> +		len = virtio_transport_send_pkt_loopback(vsock, pkt);
> +		goto out_rcu;
> +	}
>   
>   	if (pkt->reply)
>   		atomic_inc(&vsock->queued_replies);
> @@ -214,6 +221,9 @@ virtio_transport_send_pkt(struct virtio_vsock_pkt *pkt)
>   	spin_unlock_bh(&vsock->send_pkt_list_lock);
>   
>   	queue_work(virtio_vsock_workqueue, &vsock->send_pkt_work);
> +
> +out_rcu:
> +	rcu_read_unlock();
>   	return len;
>   }
>   
> @@ -222,12 +232,14 @@ virtio_transport_cancel_pkt(struct vsock_sock *vsk)
>   {
>   	struct virtio_vsock *vsock;
>   	struct virtio_vsock_pkt *pkt, *n;
> -	int cnt = 0;
> +	int cnt = 0, ret;
>   	LIST_HEAD(freeme);
>   
> -	vsock = virtio_vsock_get();
> +	rcu_read_lock();
> +	vsock = rcu_dereference(the_virtio_vsock);
>   	if (!vsock) {
> -		return -ENODEV;
> +		ret = -ENODEV;
> +		goto out_rcu;
>   	}
>   
>   	spin_lock_bh(&vsock->send_pkt_list_lock);
> @@ -255,7 +267,11 @@ virtio_transport_cancel_pkt(struct vsock_sock *vsk)
>   			queue_work(virtio_vsock_workqueue, &vsock->rx_work);
>   	}
>   
> -	return 0;
> +	ret = 0;
> +
> +out_rcu:
> +	rcu_read_unlock();
> +	return ret;
>   }
>   
>   static void virtio_vsock_rx_fill(struct virtio_vsock *vsock)
> @@ -590,8 +606,6 @@ static int virtio_vsock_probe(struct virtio_device *vdev)
>   	vsock->rx_buf_max_nr = 0;
>   	atomic_set(&vsock->queued_replies, 0);
>   
> -	vdev->priv = vsock;
> -	the_virtio_vsock = vsock;
>   	mutex_init(&vsock->tx_lock);
>   	mutex_init(&vsock->rx_lock);
>   	mutex_init(&vsock->event_lock);
> @@ -613,6 +627,9 @@ static int virtio_vsock_probe(struct virtio_device *vdev)
>   	virtio_vsock_event_fill(vsock);
>   	mutex_unlock(&vsock->event_lock);
>   
> +	vdev->priv = vsock;
> +	rcu_assign_pointer(the_virtio_vsock, vsock);


You probably need to use rcu_dereference_protected() to access 
the_virtio_vsock in the function in order to survive from sparse.


> +
>   	mutex_unlock(&the_virtio_vsock_mutex);
>   	return 0;
>   
> @@ -627,6 +644,12 @@ static void virtio_vsock_remove(struct virtio_device *vdev)
>   	struct virtio_vsock *vsock = vdev->priv;
>   	struct virtio_vsock_pkt *pkt;
>   
> +	mutex_lock(&the_virtio_vsock_mutex);
> +
> +	vdev->priv = NULL;
> +	rcu_assign_pointer(the_virtio_vsock, NULL);


This is still suspicious, can we access the_virtio_vsock through 
vdev->priv? If yes, we may still get use-after-free since it was not 
protected by RCU.

Another more interesting question, I believe we will do singleton for 
virtio_vsock structure. Then what's the point of using vdev->priv to 
access the_virtio_vsock? It looks to me we can it brings extra troubles 
for doing synchronization.

Thanks


> +	synchronize_rcu();
> +
>   	flush_work(&vsock->loopback_work);
>   	flush_work(&vsock->rx_work);
>   	flush_work(&vsock->tx_work);
> @@ -666,12 +689,10 @@ static void virtio_vsock_remove(struct virtio_device *vdev)
>   	}
>   	spin_unlock_bh(&vsock->loopback_list_lock);
>   
> -	mutex_lock(&the_virtio_vsock_mutex);
> -	the_virtio_vsock = NULL;
> -	mutex_unlock(&the_virtio_vsock_mutex);
> -
>   	vdev->config->del_vqs(vdev);
>   
> +	mutex_unlock(&the_virtio_vsock_mutex);
> +
>   	kfree(vsock);
>   }
>