Index: head/sys/dev/hyperv/include/vmbus.h
===================================================================
--- head/sys/dev/hyperv/include/vmbus.h
+++ head/sys/dev/hyperv/include/vmbus.h
@@ -129,6 +129,17 @@
 	return device_get_ivars(dev);
 }
 
+/*
+ * vmbus_chan_open_br()
+ * Return values:
+ * 0			Succeeded.
+ * EISCONN		Failed, and the memory passed through 'br' is still
+ *			connected.  Callers must _not_ free the the memory
+ *			passed through 'br', if this error happens.
+ * other values		Failed.  The memory passed through 'br' is no longer
+ *			connected.  Callers are free to do anything with the
+ *			memory passed through 'br'.
+ */
 int		vmbus_chan_open(struct vmbus_channel *chan,
 		    int txbr_size, int rxbr_size, const void *udata, int udlen,
 		    vmbus_chan_callback_t cb, void *cbarg);
Index: head/sys/dev/hyperv/vmbus/vmbus_chan.c
===================================================================
--- head/sys/dev/hyperv/vmbus/vmbus_chan.c
+++ head/sys/dev/hyperv/vmbus/vmbus_chan.c
@@ -324,7 +324,21 @@
 
 	error = vmbus_chan_open_br(chan, &cbr, udata, udlen, cb, cbarg);
 	if (error) {
-		hyperv_dmamem_free(&chan->ch_bufring_dma, chan->ch_bufring);
+		if (error == EISCONN) {
+			/*
+			 * XXX
+			 * The bufring GPADL is still connected; abandon
+			 * this bufring, instead of having mysterious
+			 * crash or trashed data later on.
+			 */
+			vmbus_chan_printf(chan, "chan%u bufring GPADL "
+			    "is still connected upon channel open error; "
+			    "leak %d bytes memory\n", chan->ch_id,
+			    txbr_size + rxbr_size);
+		} else {
+			hyperv_dmamem_free(&chan->ch_bufring_dma,
+			    chan->ch_bufring);
+		}
 		chan->ch_bufring = NULL;
 	}
 	return (error);
@@ -458,7 +472,17 @@
 	sysctl_ctx_free(&chan->ch_sysctl_ctx);
 	vmbus_chan_clear_chmap(chan);
 	if (chan->ch_bufring_gpadl != 0) {
-		vmbus_chan_gpadl_disconnect(chan, chan->ch_bufring_gpadl);
+		int error1;
+
+		error1 = vmbus_chan_gpadl_disconnect(chan,
+		    chan->ch_bufring_gpadl);
+		if (error1) {
+			/*
+			 * Give caller a hint that the bufring GPADL is still
+			 * connected.
+			 */
+			error = EISCONN;
+		}
 		chan->ch_bufring_gpadl = 0;
 	}
 	atomic_clear_int(&chan->ch_stflags, VMBUS_CHAN_ST_OPENED);