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Message-ID: <20190704092044.23gd5o2rhqarisgg@steredhat.homenet.telecomitalia.it>
Date: Thu, 4 Jul 2019 11:20:44 +0200
From: Stefano Garzarella <sgarzare@...hat.com>
To: Jason Wang <jasowang@...hat.com>
Cc: Stefan Hajnoczi <stefanha@...hat.com>, netdev@...r.kernel.org,
kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
"Michael S. Tsirkin" <mst@...hat.com>,
"David S. Miller" <davem@...emloft.net>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] vsock/virtio: use RCU to avoid use-after-free on
the_virtio_vsock
On Thu, Jul 04, 2019 at 11:58:00AM +0800, Jason Wang wrote:
>
> On 2019/7/3 下午6:41, Stefano Garzarella wrote:
> > 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@...hat.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()?
>
>
> I mean instead of:
>
> /* Only one virtio-vsock device per guest is supported */
> if (the_virtio_vsock) {
> ret = -EBUSY;
> goto out;
> }
>
> you should use:
>
> if (rcu_dereference_protected(the_virtio_vosck,
> lock_dep_is_held(&the_virtio_vsock_mutex))
>
> ...
Okay, thanks for confirming! I'll send a v3 to fix this!
>
>
> >
> > > > +
> > > > 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.
>
>
> Yes, but any dereference that is not done in vq_callbacks will be very
> dangerous in the future.
Right.
Do you think make sense to continue with this series in order to fix the
hot-unplug issue, then I'll work to refactor the driver code to use the refcnt
(as you suggested in patch 2) and singleton for the_virtio_vsock?
Thanks,
Stefano
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