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>

1 files changed, 15 insertions, 3 deletions

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 34389f75fe65..9759249e6d90 100644
--- a/drivers/vhost/vhost.c
+++ b/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)
 {
-        remove_wait_queue(poll->wqh, &poll->wait);
+        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);