Virtualization - Jul 2019 - [PATCH v2 1/3] vsock/virtio: use RCU to avoid use-after-free on the_virtio_vsock

During the review of "[PATCH] vsock/virtio: Initialize core virtio vsock
before registering the driver", Stefan pointed out some possible issues
in the .probe() and .remove() callbacks of the virtio-vsock driver.

This series tries to solve these issues:
- Patch 1 adds RCU critical sections to avoid use-after-free of
  'the_virtio_vsock' pointer.
- Patch 2 stops workers before to call vdev->config->reset(vdev) to
  be sure that no one is accessing the device.
- Patch 3 moves the works flush at the end of the .remove() to avoid
  use-after-free of 'vsock' object.

v2:
- Patch 1: use RCU to protect 'the_virtio_vsock' pointer
- Patch 2: no changes
- Patch 3: flush works only at the end of .remove()
- Removed patch 4 because virtqueue_detach_unused_buf() returns all the buffers
  allocated.

v1: https://patchwork.kernel.org/cover/10964733/

Stefano Garzarella (3):
  vsock/virtio: use RCU to avoid use-after-free on the_virtio_vsock
  vsock/virtio: stop workers during the .remove()
  vsock/virtio: fix flush of works during the .remove()

 net/vmw_vsock/virtio_transport.c | 131 ++++++++++++++++++++++++-------
 1 file changed, 102 insertions(+), 29 deletions(-)

-- 
2.20.1

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 at 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);
+
 	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);
+	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);
 }
 
-- 
2.20.1

Before to call vdev->config->reset(vdev) we need to be sure that
no one is accessing the device, for this reason, we add new variables
in the struct virtio_vsock to stop the workers during the .remove().

This patch also add few comments before vdev->config->reset(vdev)
and vdev->config->del_vqs(vdev).

Suggested-by: Stefan Hajnoczi <stefanha at redhat.com>
Suggested-by: Michael S. Tsirkin <mst at redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare at redhat.com>
---
 net/vmw_vsock/virtio_transport.c | 51 +++++++++++++++++++++++++++++++-
 1 file changed, 50 insertions(+), 1 deletion(-)

diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
index 7ad510ec12e0..1b44ec6f3f6c 100644
--- a/net/vmw_vsock/virtio_transport.c
+++ b/net/vmw_vsock/virtio_transport.c
@@ -38,6 +38,7 @@ struct virtio_vsock {
 	 * must be accessed with tx_lock held.
 	 */
 	struct mutex tx_lock;
+	bool tx_run;
 
 	struct work_struct send_pkt_work;
 	spinlock_t send_pkt_list_lock;
@@ -53,6 +54,7 @@ struct virtio_vsock {
 	 * must be accessed with rx_lock held.
 	 */
 	struct mutex rx_lock;
+	bool rx_run;
 	int rx_buf_nr;
 	int rx_buf_max_nr;
 
@@ -60,6 +62,7 @@ struct virtio_vsock {
 	 * vqs[VSOCK_VQ_EVENT] must be accessed with event_lock held.
 	 */
 	struct mutex event_lock;
+	bool event_run;
 	struct virtio_vsock_event event_list[8];
 
 	u32 guest_cid;
@@ -94,6 +97,10 @@ static void virtio_transport_loopback_work(struct work_struct
*work)
 	spin_unlock_bh(&vsock->loopback_list_lock);
 
 	mutex_lock(&vsock->rx_lock);
+
+	if (!vsock->rx_run)
+		goto out;
+
 	while (!list_empty(&pkts)) {
 		struct virtio_vsock_pkt *pkt;
 
@@ -102,6 +109,7 @@ static void virtio_transport_loopback_work(struct
work_struct *work)
 
 		virtio_transport_recv_pkt(pkt);
 	}
+out:
 	mutex_unlock(&vsock->rx_lock);
 }
 
@@ -130,6 +138,9 @@ virtio_transport_send_pkt_work(struct work_struct *work)
 
 	mutex_lock(&vsock->tx_lock);
 
+	if (!vsock->tx_run)
+		goto out;
+
 	vq = vsock->vqs[VSOCK_VQ_TX];
 
 	for (;;) {
@@ -188,6 +199,7 @@ virtio_transport_send_pkt_work(struct work_struct *work)
 	if (added)
 		virtqueue_kick(vq);
 
+out:
 	mutex_unlock(&vsock->tx_lock);
 
 	if (restart_rx)
@@ -323,6 +335,10 @@ static void virtio_transport_tx_work(struct work_struct
*work)
 
 	vq = vsock->vqs[VSOCK_VQ_TX];
 	mutex_lock(&vsock->tx_lock);
+
+	if (!vsock->tx_run)
+		goto out;
+
 	do {
 		struct virtio_vsock_pkt *pkt;
 		unsigned int len;
@@ -333,6 +349,8 @@ static void virtio_transport_tx_work(struct work_struct
*work)
 			added = true;
 		}
 	} while (!virtqueue_enable_cb(vq));
+
+out:
 	mutex_unlock(&vsock->tx_lock);
 
 	if (added)
@@ -361,6 +379,9 @@ static void virtio_transport_rx_work(struct work_struct
*work)
 
 	mutex_lock(&vsock->rx_lock);
 
+	if (!vsock->rx_run)
+		goto out;
+
 	do {
 		virtqueue_disable_cb(vq);
 		for (;;) {
@@ -470,6 +491,9 @@ static void virtio_transport_event_work(struct work_struct
*work)
 
 	mutex_lock(&vsock->event_lock);
 
+	if (!vsock->event_run)
+		goto out;
+
 	do {
 		struct virtio_vsock_event *event;
 		unsigned int len;
@@ -484,7 +508,7 @@ static void virtio_transport_event_work(struct work_struct
*work)
 	} while (!virtqueue_enable_cb(vq));
 
 	virtqueue_kick(vsock->vqs[VSOCK_VQ_EVENT]);
-
+out:
 	mutex_unlock(&vsock->event_lock);
 }
 
@@ -619,12 +643,18 @@ static int virtio_vsock_probe(struct virtio_device *vdev)
 	INIT_WORK(&vsock->send_pkt_work, virtio_transport_send_pkt_work);
 	INIT_WORK(&vsock->loopback_work, virtio_transport_loopback_work);
 
+	mutex_lock(&vsock->tx_lock);
+	vsock->tx_run = true;
+	mutex_unlock(&vsock->tx_lock);
+
 	mutex_lock(&vsock->rx_lock);
 	virtio_vsock_rx_fill(vsock);
+	vsock->rx_run = true;
 	mutex_unlock(&vsock->rx_lock);
 
 	mutex_lock(&vsock->event_lock);
 	virtio_vsock_event_fill(vsock);
+	vsock->event_run = true;
 	mutex_unlock(&vsock->event_lock);
 
 	vdev->priv = vsock;
@@ -659,6 +689,24 @@ static void virtio_vsock_remove(struct virtio_device *vdev)
 	/* Reset all connected sockets when the device disappear */
 	vsock_for_each_connected_socket(virtio_vsock_reset_sock);
 
+	/* Stop all work handlers to make sure no one is accessing the device,
+	 * so we can safely call vdev->config->reset().
+	 */
+	mutex_lock(&vsock->rx_lock);
+	vsock->rx_run = false;
+	mutex_unlock(&vsock->rx_lock);
+
+	mutex_lock(&vsock->tx_lock);
+	vsock->tx_run = false;
+	mutex_unlock(&vsock->tx_lock);
+
+	mutex_lock(&vsock->event_lock);
+	vsock->event_run = false;
+	mutex_unlock(&vsock->event_lock);
+
+	/* Flush all device writes and interrupts, device will not use any
+	 * more buffers.
+	 */
 	vdev->config->reset(vdev);
 
 	mutex_lock(&vsock->rx_lock);
@@ -689,6 +737,7 @@ static void virtio_vsock_remove(struct virtio_device *vdev)
 	}
 	spin_unlock_bh(&vsock->loopback_list_lock);
 
+	/* Delete virtqueues and flush outstanding callbacks if any */
 	vdev->config->del_vqs(vdev);
 
 	mutex_unlock(&the_virtio_vsock_mutex);
-- 
2.20.1

This patch moves the flush of works after vdev->config->del_vqs(vdev),
because we need to be sure that no workers run before to free the
'vsock' object.

Since we stopped the workers using the [tx|rx|event]_run flags,
we are sure no one is accessing the device while we are calling
vdev->config->reset(vdev), so we can safely move the workers' flush.

Before the vdev->config->del_vqs(vdev), workers can be scheduled
by VQ callbacks, so we must flush them after del_vqs(), to avoid
use-after-free of 'vsock' object.

Suggested-by: Michael S. Tsirkin <mst at redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare at redhat.com>
---
 net/vmw_vsock/virtio_transport.c | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/net/vmw_vsock/virtio_transport.c b/net/vmw_vsock/virtio_transport.c
index 1b44ec6f3f6c..96dafa978268 100644
--- a/net/vmw_vsock/virtio_transport.c
+++ b/net/vmw_vsock/virtio_transport.c
@@ -680,12 +680,6 @@ static void virtio_vsock_remove(struct virtio_device *vdev)
 	rcu_assign_pointer(the_virtio_vsock, NULL);
 	synchronize_rcu();
 
-	flush_work(&vsock->loopback_work);
-	flush_work(&vsock->rx_work);
-	flush_work(&vsock->tx_work);
-	flush_work(&vsock->event_work);
-	flush_work(&vsock->send_pkt_work);
-
 	/* Reset all connected sockets when the device disappear */
 	vsock_for_each_connected_socket(virtio_vsock_reset_sock);
 
@@ -740,6 +734,15 @@ static void virtio_vsock_remove(struct virtio_device *vdev)
 	/* Delete virtqueues and flush outstanding callbacks if any */
 	vdev->config->del_vqs(vdev);
 
+	/* Other works can be queued before 'config->del_vqs()', so we
flush
+	 * all works before to free the vsock object to avoid use after free.
+	 */
+	flush_work(&vsock->loopback_work);
+	flush_work(&vsock->rx_work);
+	flush_work(&vsock->tx_work);
+	flush_work(&vsock->event_work);
+	flush_work(&vsock->send_pkt_work);
+
 	mutex_unlock(&the_virtio_vsock_mutex);
 
 	kfree(vsock);
-- 
2.20.1

On Fri, Jun 28, 2019 at 02:36:57PM +0200, 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.
->del_vqs() is only called at the very end, did you forget to move it
earlier?

In particular, the virtqueue handler callbacks schedule a workqueue.
The work functions use container_of() to get vsock.  We need to be sure
that the work item isn't freed along with vsock while the work item is
still pending.

How do we know that the virtqueue handler is never called in such a way
that it sees vsock != NULL (there is no explicit memory barrier on the
read side) and then schedules a work item after flush_work() has run?

Stefan
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On Fri, Jun 28, 2019 at 02:36:59PM +0200, Stefano Garzarella
wrote:
> This patch moves the flush of works after
vdev->config->del_vqs(vdev),
> because we need to be sure that no workers run before to free the
> 'vsock' object.
> 
> Since we stopped the workers using the [tx|rx|event]_run flags,
> we are sure no one is accessing the device while we are calling
> vdev->config->reset(vdev), so we can safely move the workers'
flush.
What about send_pkt and loopback work?  How were they stopped safely?

For example, if send_pkt work executes then we're in trouble since it
accesses the tx virtqueue which is deleted by ->del_vqs().
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On Fri, Jun 28, 2019 at 02:36:59PM +0200, Stefano Garzarella
wrote:
> This patch moves the flush of works after
vdev->config->del_vqs(vdev),
> because we need to be sure that no workers run before to free the
> 'vsock' object.
> 
> Since we stopped the workers using the [tx|rx|event]_run flags,
> we are sure no one is accessing the device while we are calling
> vdev->config->reset(vdev), so we can safely move the workers'
flush.
> 
> Before the vdev->config->del_vqs(vdev), workers can be scheduled
> by VQ callbacks, so we must flush them after del_vqs(), to avoid
> use-after-free of 'vsock' object.
Nevermind, I looked back at Patch 2 and saw the send_pkt and loopback
work functions were also updated.  Thanks!

Stefan
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On Fri, Jun 28, 2019 at 02:36:57PM +0200, 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.
My question is answered in Patch 3.
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On Fri, Jun 28, 2019 at 02:36:56PM +0200, Stefano Garzarella
wrote:
> During the review of "[PATCH] vsock/virtio: Initialize core virtio
vsock
> before registering the driver", Stefan pointed out some possible
issues
> in the .probe() and .remove() callbacks of the virtio-vsock driver.
> 
> This series tries to solve these issues:
> - Patch 1 adds RCU critical sections to avoid use-after-free of
>   'the_virtio_vsock' pointer.
> - Patch 2 stops workers before to call vdev->config->reset(vdev) to
>   be sure that no one is accessing the device.
> - Patch 3 moves the works flush at the end of the .remove() to avoid
>   use-after-free of 'vsock' object.
> 
> v2:
> - Patch 1: use RCU to protect 'the_virtio_vsock' pointer
> - Patch 2: no changes
> - Patch 3: flush works only at the end of .remove()
> - Removed patch 4 because virtqueue_detach_unused_buf() returns all the
buffers
>   allocated.
> 
> v1: https://patchwork.kernel.org/cover/10964733/
This looks good to me.

Did you run any stress tests?  For example an SMP guest constantly
connecting and sending packets together with a script that
hotplug/unplugs vhost-vsock-pci from the host side.

Stefan
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On Mon, Jul 01, 2019 at 04:11:13PM +0100, Stefan Hajnoczi
wrote:
> On Fri, Jun 28, 2019 at 02:36:56PM +0200, Stefano Garzarella wrote:
> > During the review of "[PATCH] vsock/virtio: Initialize core
virtio vsock
> > before registering the driver", Stefan pointed out some possible
issues
> > in the .probe() and .remove() callbacks of the virtio-vsock driver.
> > 
> > This series tries to solve these issues:
> > - Patch 1 adds RCU critical sections to avoid use-after-free of
> >   'the_virtio_vsock' pointer.
> > - Patch 2 stops workers before to call vdev->config->reset(vdev)
to
> >   be sure that no one is accessing the device.
> > - Patch 3 moves the works flush at the end of the .remove() to avoid
> >   use-after-free of 'vsock' object.
> > 
> > v2:
> > - Patch 1: use RCU to protect 'the_virtio_vsock' pointer
> > - Patch 2: no changes
> > - Patch 3: flush works only at the end of .remove()
> > - Removed patch 4 because virtqueue_detach_unused_buf() returns all
the buffers
> >   allocated.
> > 
> > v1: https://patchwork.kernel.org/cover/10964733/
> 
> This looks good to me.
Thanks for the review!
> 
> Did you run any stress tests?  For example an SMP guest constantly
> connecting and sending packets together with a script that
> hotplug/unplugs vhost-vsock-pci from the host side.
Yes, I started an SMP guest (-smp 4 -monitor tcp:127.0.0.1:1234,server,nowait)
and I run these scripts to stress the .probe()/.remove() path:

- guest
  while true; do
      cat /dev/urandom | nc-vsock -l 4321 > /dev/null &
      cat /dev/urandom | nc-vsock -l 5321 > /dev/null &
      cat /dev/urandom | nc-vsock -l 6321 > /dev/null &
      cat /dev/urandom | nc-vsock -l 7321 > /dev/null &
      wait
  done

- host
  while true; do
      cat /dev/urandom | nc-vsock 3 4321 > /dev/null &
      cat /dev/urandom | nc-vsock 3 5321 > /dev/null &
      cat /dev/urandom | nc-vsock 3 6321 > /dev/null &
      cat /dev/urandom | nc-vsock 3 7321 > /dev/null &
      sleep 2
      echo "device_del v1" | nc 127.0.0.1 1234
      sleep 1
      echo "device_add vhost-vsock-pci,id=v1,guest-cid=3" | nc
127.0.0.1 1234
      sleep 1
  done

Do you think is enough or is better to have a test more accurate?

Thanks,
Stefano

On 2019/6/28 ??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 at 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.

> +
>   	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);
>   }
>

On 2019/6/28 ??8:36, Stefano Garzarella wrote:
> Before to call vdev->config->reset(vdev) we need to be sure that
> no one is accessing the device, for this reason, we add new variables
> in the struct virtio_vsock to stop the workers during the .remove().
>
> This patch also add few comments before vdev->config->reset(vdev)
> and vdev->config->del_vqs(vdev).
>
> Suggested-by: Stefan Hajnoczi <stefanha at redhat.com>
> Suggested-by: Michael S. Tsirkin <mst at redhat.com>
> Signed-off-by: Stefano Garzarella <sgarzare at redhat.com>
> ---
>   net/vmw_vsock/virtio_transport.c | 51 +++++++++++++++++++++++++++++++-
>   1 file changed, 50 insertions(+), 1 deletion(-)

This should work. But we may consider to convert the_virtio_vosck to 
socket object and use socket refcnt and destructor in the future instead 
of inventing something new by ourselves.

Thanks

>
> diff --git a/net/vmw_vsock/virtio_transport.c
b/net/vmw_vsock/virtio_transport.c
> index 7ad510ec12e0..1b44ec6f3f6c 100644
> --- a/net/vmw_vsock/virtio_transport.c
> +++ b/net/vmw_vsock/virtio_transport.c
> @@ -38,6 +38,7 @@ struct virtio_vsock {
>   	 * must be accessed with tx_lock held.
>   	 */
>   	struct mutex tx_lock;
> +	bool tx_run;
>   
>   	struct work_struct send_pkt_work;
>   	spinlock_t send_pkt_list_lock;
> @@ -53,6 +54,7 @@ struct virtio_vsock {
>   	 * must be accessed with rx_lock held.
>   	 */
>   	struct mutex rx_lock;
> +	bool rx_run;
>   	int rx_buf_nr;
>   	int rx_buf_max_nr;
>   
> @@ -60,6 +62,7 @@ struct virtio_vsock {
>   	 * vqs[VSOCK_VQ_EVENT] must be accessed with event_lock held.
>   	 */
>   	struct mutex event_lock;
> +	bool event_run;
>   	struct virtio_vsock_event event_list[8];
>   
>   	u32 guest_cid;
> @@ -94,6 +97,10 @@ static void virtio_transport_loopback_work(struct
work_struct *work)
>   	spin_unlock_bh(&vsock->loopback_list_lock);
>   
>   	mutex_lock(&vsock->rx_lock);
> +
> +	if (!vsock->rx_run)
> +		goto out;
> +
>   	while (!list_empty(&pkts)) {
>   		struct virtio_vsock_pkt *pkt;
>   
> @@ -102,6 +109,7 @@ static void virtio_transport_loopback_work(struct
work_struct *work)
>   
>   		virtio_transport_recv_pkt(pkt);
>   	}
> +out:
>   	mutex_unlock(&vsock->rx_lock);
>   }
>   
> @@ -130,6 +138,9 @@ virtio_transport_send_pkt_work(struct work_struct
*work)
>   
>   	mutex_lock(&vsock->tx_lock);
>   
> +	if (!vsock->tx_run)
> +		goto out;
> +
>   	vq = vsock->vqs[VSOCK_VQ_TX];
>   
>   	for (;;) {
> @@ -188,6 +199,7 @@ virtio_transport_send_pkt_work(struct work_struct
*work)
>   	if (added)
>   		virtqueue_kick(vq);
>   
> +out:
>   	mutex_unlock(&vsock->tx_lock);
>   
>   	if (restart_rx)
> @@ -323,6 +335,10 @@ static void virtio_transport_tx_work(struct
work_struct *work)
>   
>   	vq = vsock->vqs[VSOCK_VQ_TX];
>   	mutex_lock(&vsock->tx_lock);
> +
> +	if (!vsock->tx_run)
> +		goto out;
> +
>   	do {
>   		struct virtio_vsock_pkt *pkt;
>   		unsigned int len;
> @@ -333,6 +349,8 @@ static void virtio_transport_tx_work(struct work_struct
*work)
>   			added = true;
>   		}
>   	} while (!virtqueue_enable_cb(vq));
> +
> +out:
>   	mutex_unlock(&vsock->tx_lock);
>   
>   	if (added)
> @@ -361,6 +379,9 @@ static void virtio_transport_rx_work(struct work_struct
*work)
>   
>   	mutex_lock(&vsock->rx_lock);
>   
> +	if (!vsock->rx_run)
> +		goto out;
> +
>   	do {
>   		virtqueue_disable_cb(vq);
>   		for (;;) {
> @@ -470,6 +491,9 @@ static void virtio_transport_event_work(struct
work_struct *work)
>   
>   	mutex_lock(&vsock->event_lock);
>   
> +	if (!vsock->event_run)
> +		goto out;
> +
>   	do {
>   		struct virtio_vsock_event *event;
>   		unsigned int len;
> @@ -484,7 +508,7 @@ static void virtio_transport_event_work(struct
work_struct *work)
>   	} while (!virtqueue_enable_cb(vq));
>   
>   	virtqueue_kick(vsock->vqs[VSOCK_VQ_EVENT]);
> -
> +out:
>   	mutex_unlock(&vsock->event_lock);
>   }
>   
> @@ -619,12 +643,18 @@ static int virtio_vsock_probe(struct virtio_device
*vdev)
>   	INIT_WORK(&vsock->send_pkt_work, virtio_transport_send_pkt_work);
>   	INIT_WORK(&vsock->loopback_work, virtio_transport_loopback_work);
>   
> +	mutex_lock(&vsock->tx_lock);
> +	vsock->tx_run = true;
> +	mutex_unlock(&vsock->tx_lock);
> +
>   	mutex_lock(&vsock->rx_lock);
>   	virtio_vsock_rx_fill(vsock);
> +	vsock->rx_run = true;
>   	mutex_unlock(&vsock->rx_lock);
>   
>   	mutex_lock(&vsock->event_lock);
>   	virtio_vsock_event_fill(vsock);
> +	vsock->event_run = true;
>   	mutex_unlock(&vsock->event_lock);
>   
>   	vdev->priv = vsock;
> @@ -659,6 +689,24 @@ static void virtio_vsock_remove(struct virtio_device
*vdev)
>   	/* Reset all connected sockets when the device disappear */
>   	vsock_for_each_connected_socket(virtio_vsock_reset_sock);
>   
> +	/* Stop all work handlers to make sure no one is accessing the device,
> +	 * so we can safely call vdev->config->reset().
> +	 */
> +	mutex_lock(&vsock->rx_lock);
> +	vsock->rx_run = false;
> +	mutex_unlock(&vsock->rx_lock);
> +
> +	mutex_lock(&vsock->tx_lock);
> +	vsock->tx_run = false;
> +	mutex_unlock(&vsock->tx_lock);
> +
> +	mutex_lock(&vsock->event_lock);
> +	vsock->event_run = false;
> +	mutex_unlock(&vsock->event_lock);
> +
> +	/* Flush all device writes and interrupts, device will not use any
> +	 * more buffers.
> +	 */
>   	vdev->config->reset(vdev);
>   
>   	mutex_lock(&vsock->rx_lock);
> @@ -689,6 +737,7 @@ static void virtio_vsock_remove(struct virtio_device
*vdev)
>   	}
>   	spin_unlock_bh(&vsock->loopback_list_lock);
>   
> +	/* Delete virtqueues and flush outstanding callbacks if any */
>   	vdev->config->del_vqs(vdev);
>   
>   	mutex_unlock(&the_virtio_vsock_mutex);