vhost_net: fix ubuf refcount incorrectly when sendmsg fails

Currently the vhost_zerocopy_callback() maybe be called to decrease
the refcount when sendmsg fails in tun. The error handling in vhost
handle_tx_zerocopy() will try to decrease the same refcount again.
This is wrong. To fix this issue, we only call vhost_net_ubuf_put()
when vq->heads[nvq->desc].len == VHOST_DMA_IN_PROGRESS.

Fixes: bab632d6 ("vhost: vhost TX zero-copy support")
Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/1609207308-20544-1-git-send-email-wangyunjian@huawei.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/vhost/net.c

@@ -863,6 +863,7 @@ static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock)
 	size_t len, total_len = 0;
 	int err;
 	struct vhost_net_ubuf_ref *ubufs;
+	struct ubuf_info *ubuf;
 	bool zcopy_used;
 	int sent_pkts = 0;
 
@@ -895,9 +896,7 @@ static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock)
 
 		/* use msg_control to pass vhost zerocopy ubuf info to skb */
 		if (zcopy_used) {
-			struct ubuf_info *ubuf;
 			ubuf = nvq->ubuf_info + nvq->upend_idx;
-
 			vq->heads[nvq->upend_idx].id = cpu_to_vhost32(vq, head);
 			vq->heads[nvq->upend_idx].len = VHOST_DMA_IN_PROGRESS;
 			ubuf->callback = vhost_zerocopy_callback;
@@ -927,7 +926,8 @@ static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock)
 		err = sock->ops->sendmsg(sock, &msg, len);
 		if (unlikely(err < 0)) {
 			if (zcopy_used) {
-				vhost_net_ubuf_put(ubufs);
+				if (vq->heads[ubuf->desc].len == VHOST_DMA_IN_PROGRESS)
+					vhost_net_ubuf_put(ubufs);
 				nvq->upend_idx = ((unsigned)nvq->upend_idx - 1)
 					% UIO_MAXIOV;
 			}