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SubjectRe: [PATCH v2 1/3] vsock/virtio: use RCU to avoid use-after-free on the_virtio_vsock
On Wed, Jul 03, 2019 at 05:53:58PM +0800, Jason Wang wrote:
>
> On 2019/6/28 下午8:36, 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@redhat.com>
> > ---
> > 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

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