hv_netvsc: fix a race between netvsc_send() and netvsc_init_buf()

Fix in commit 88098834 ("hv_netvsc: set nvdev link after populating
chn_table") turns out to be incomplete. A crash in
netvsc_get_next_send_section() is observed on mtu change when the device
is under load. The race I identified is: if we get to netvsc_send() after
we set net_device_ctx->nvdev link in netvsc_device_add() but before we
finish netvsc_connect_vsp()->netvsc_init_buf() send_section_map is not
allocated and we crash. Unfortunately we can't set net_device_ctx->nvdev
link after the netvsc_init_buf() call as during the negotiation we need
to receive packets and on the receive path we check for it. It would
probably be possible to split nvdev into a pair of nvdev_in and nvdev_out
links and check them accordingly in get_outbound_net_device()/
get_inbound_net_device() but this looks like an overkill.

Check that send_section_map is allocated in netvsc_send().
Signed-off-by: NVitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/hyperv/netvsc.c

@@ -888,6 +888,13 @@ int netvsc_send(struct hv_device *device,
 	if (!net_device)
 		return -ENODEV;
 
+	/* We may race with netvsc_connect_vsp()/netvsc_init_buf() and get
+	 * here before the negotiation with the host is finished and
+	 * send_section_map may not be allocated yet.
+	 */
+	if (!net_device->send_section_map)
+		return -EAGAIN;
+
 	out_channel = net_device->chn_table[q_idx];
 
 	packet->send_buf_index = NETVSC_INVALID_INDEX;