[PATCH 4/4] hyperv: netvsc: improve protection against rescind offer

[Vitaly Kuznetsov <vkuznets@redhat.com>]

The check added in commit c3582a2c4d0b ("hyperv: Add support for vNIC hot
removal") is incomplete as there is no synchronization between
vmbus_onoffer_rescind() and netvsc_send(). In case we get the offer after we
checked out_channel->rescind and before netvsc_send() finishes its job we can
get a crash as we'll be dealing with already freed channel.

Make netvsc_send() take additional reference to the channel with newly
introduced vmbus_get_channel(), this guarantees we won't lose the channel. We
can still get rescind while we're processing but this won't cause a crash.

Reported-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 drivers/net/hyperv/netvsc.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/net/hyperv/netvsc.c b/drivers/net/hyperv/netvsc.c
index 9f49c01..d9b13a1 100644
--- a/drivers/net/hyperv/netvsc.c
+++ b/drivers/net/hyperv/netvsc.c
@@ -763,11 +763,16 @@ int netvsc_send(struct hv_device *device,
 	out_channel = net_device->chn_table[packet->q_idx];
 	if (out_channel == NULL)
 		out_channel = device->channel;
-	packet->channel = out_channel;
+	packet->channel = vmbus_get_channel(out_channel);
 
-	if (out_channel->rescind)
+	if (!packet->channel)
 		return -ENODEV;
 
+	if (out_channel->rescind) {
+		vmbus_put_channel(out_channel);
+		return -ENODEV;
+	}
+
 	if (packet->page_buf_cnt) {
 		ret = vmbus_sendpacket_pagebuffer(out_channel,
 						  packet->page_buf,
@@ -810,6 +815,7 @@ int netvsc_send(struct hv_device *device,
 			   packet, ret);
 	}
 
+	vmbus_put_channel(packet->channel);
 	return ret;
 }
 
-- 
1.9.3