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

[Stefano Garzarella]

On Wed, Jul 03, 2019 at 05:53:58PM +0800, Jason Wang wrote:
> 
> 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.
> 

Ooo, thanks!

Do you mean when we check if the_virtio_vsock is not null at the beginning of
virtio_vsock_probe()?

> 
> > +
> >   	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.

We will free the object only after calling the del_vqs(), so we are sure
that the vq_callbacks ended and will no longer be invoked.
So, IIUC it shouldn't happen.

> 
> 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.

I thought about it when I tried to use RCU to stop the worker and I
think make sense. Maybe can be another series after this will be merged.

@Stefan, what do you think about that?

Thanks,
Stefano