[v3] nbd: fix potential NULL pointer fault in nbd_genl_disconnect

[Sun Ke <sunke32@huawei.com>]

Open /dev/nbdX first, the config_refs will be 1 and
the pointers in nbd_device are still null. Disconnect
/dev/nbdX, then reference a null recv_workq. The
protection by config_refs in nbd_genl_disconnect is useless.

To fix it, just add a check for a non null task_recv in
nbd_genl_disconnect.

Signed-off-by: Sun Ke <sunke32@huawei.com>
---
v1 -> v2:
Add an omitted mutex_unlock.

v2 -> v3:
Add nbd->config_lock, suggested by Josef.
---
 drivers/block/nbd.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index b4607dd96185..870b3fd0c101 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -2008,12 +2008,20 @@ static int nbd_genl_disconnect(struct sk_buff *skb, struct genl_info *info)
 		       index);
 		return -EINVAL;
 	}
+	mutex_lock(&nbd->config_lock);
 	if (!refcount_inc_not_zero(&nbd->refs)) {
+		mutex_unlock(&nbd->config_lock);
 		mutex_unlock(&nbd_index_mutex);
 		printk(KERN_ERR "nbd: device at index %d is going down\n",
 		       index);
 		return -EINVAL;
 	}
+	if (!nbd->recv_workq) {
+		mutex_unlock(&nbd->config_lock);
+		mutex_unlock(&nbd_index_mutex);
+		return -EINVAL;
+	}
+	mutex_unlock(&nbd->config_lock);
 	mutex_unlock(&nbd_index_mutex);
 	if (!refcount_inc_not_zero(&nbd->config_refs)) {
 		nbd_put(nbd);
-- 
2.17.2