vhost_net: handle polling errors when setting backend

Currently, the polling errors were ignored, which can lead following issues:

- vhost remove itself unconditionally from waitqueue when stopping the poll,
  this may crash the kernel since the previous attempt of starting may fail to
  add itself to the waitqueue
- userspace may think the backend were successfully set even when the polling
  failed.

Solve this by:

- check poll->wqh before trying to remove from waitqueue
- report polling errors in vhost_poll_start(), tx_poll_start(), the return value
  will be checked and returned when userspace want to set the backend

After this fix, there still could be a polling failure after backend is set, it
will addressed by the next patch.
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/vhost/net.c

@@ -165,12 +165,16 @@ static void tx_poll_stop(struct vhost_net *net)
 }
 
 /* Caller must have TX VQ lock */
-static void tx_poll_start(struct vhost_net *net, struct socket *sock)
+static int tx_poll_start(struct vhost_net *net, struct socket *sock)
 {
+	int ret;
+
 	if (unlikely(net->tx_poll_state != VHOST_NET_POLL_STOPPED))
-		return;
-	vhost_poll_start(net->poll + VHOST_NET_VQ_TX, sock->file);
+		return 0;
+	ret = vhost_poll_start(net->poll + VHOST_NET_VQ_TX, sock->file);
+	if (!ret)
 		net->tx_poll_state = VHOST_NET_POLL_STARTED;
+	return ret;
 }
 
 /* In case of DMA done not in order in lower device driver for some reason.
@@ -642,20 +646,23 @@ static void vhost_net_disable_vq(struct vhost_net *n,
 		vhost_poll_stop(n->poll + VHOST_NET_VQ_RX);
 }
 
-static void vhost_net_enable_vq(struct vhost_net *n,
+static int vhost_net_enable_vq(struct vhost_net *n,
 				struct vhost_virtqueue *vq)
 {
 	struct socket *sock;
+	int ret;
 
 	sock = rcu_dereference_protected(vq->private_data,
 					 lockdep_is_held(&vq->mutex));
 	if (!sock)
-		return;
+		return 0;
 	if (vq == n->vqs + VHOST_NET_VQ_TX) {
 		n->tx_poll_state = VHOST_NET_POLL_STOPPED;
-		tx_poll_start(n, sock);
+		ret = tx_poll_start(n, sock);
 	} else
-		vhost_poll_start(n->poll + VHOST_NET_VQ_RX, sock->file);
+		ret = vhost_poll_start(n->poll + VHOST_NET_VQ_RX, sock->file);
+
+	return ret;
 }
 
 static struct socket *vhost_net_stop_vq(struct vhost_net *n,
@@ -833,7 +840,9 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
 		r = vhost_init_used(vq);
 		if (r)
 			goto err_used;
-		vhost_net_enable_vq(n, vq);
+		r = vhost_net_enable_vq(n, vq);
+		if (r)
+			goto err_used;
 
 		oldubufs = vq->ubufs;
 		vq->ubufs = ubufs;

drivers/vhost/vhost.c

@@ -77,26 +77,38 @@ void vhost_poll_init(struct vhost_poll *poll, vhost_work_fn_t fn,
 	init_poll_funcptr(&poll->table, vhost_poll_func);
 	poll->mask = mask;
 	poll->dev = dev;
+	poll->wqh = NULL;
 
 	vhost_work_init(&poll->work, fn);
 }
 
 /* Start polling a file. We add ourselves to file's wait queue. The caller must
  * keep a reference to a file until after vhost_poll_stop is called. */
-void vhost_poll_start(struct vhost_poll *poll, struct file *file)
+int vhost_poll_start(struct vhost_poll *poll, struct file *file)
 {
 	unsigned long mask;
+	int ret = 0;
 
 	mask = file->f_op->poll(file, &poll->table);
 	if (mask)
 		vhost_poll_wakeup(&poll->wait, 0, 0, (void *)mask);
+	if (mask & POLLERR) {
+		if (poll->wqh)
+			remove_wait_queue(poll->wqh, &poll->wait);
+		ret = -EINVAL;
+	}
+
+	return ret;
 }
 
 /* Stop polling a file. After this function returns, it becomes safe to drop the
  * file reference. You must also flush afterwards. */
 void vhost_poll_stop(struct vhost_poll *poll)
 {
+	if (poll->wqh) {
 		remove_wait_queue(poll->wqh, &poll->wait);
+		poll->wqh = NULL;
+	}
 }
 
 static bool vhost_work_seq_done(struct vhost_dev *dev, struct vhost_work *work,
@@ -792,7 +804,7 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
 		fput(filep);
 
 	if (pollstart && vq->handle_kick)
-		vhost_poll_start(&vq->poll, vq->kick);
+		r = vhost_poll_start(&vq->poll, vq->kick);
 
 	mutex_unlock(&vq->mutex);
 

drivers/vhost/vhost.h

@@ -42,7 +42,7 @@ void vhost_work_queue(struct vhost_dev *dev, struct vhost_work *work);
 
 void vhost_poll_init(struct vhost_poll *poll, vhost_work_fn_t fn,
 		     unsigned long mask, struct vhost_dev *dev);
-void vhost_poll_start(struct vhost_poll *poll, struct file *file);
+int vhost_poll_start(struct vhost_poll *poll, struct file *file);
 void vhost_poll_stop(struct vhost_poll *poll);
 void vhost_poll_flush(struct vhost_poll *poll);
 void vhost_poll_queue(struct vhost_poll *poll);