net: qualcomm: rmnet: Fix incorrect assignment of real_dev

A null dereference was observed when a sysctl was being set
from userspace and rmnet was stuck trying to complete some actions
in the NETDEV_REGISTER callback. This is because the real_dev is set
only after the device registration handler completes.

sysctl call stack -

<6> Unable to handle kernel NULL pointer dereference at
    virtual address 00000108
<2> pc : rmnet_vnd_get_iflink+0x1c/0x28
<2> lr : dev_get_iflink+0x2c/0x40
<2>  rmnet_vnd_get_iflink+0x1c/0x28
<2>  inet6_fill_ifinfo+0x15c/0x234
<2>  inet6_ifinfo_notify+0x68/0xd4
<2>  ndisc_ifinfo_sysctl_change+0x1b8/0x234
<2>  proc_sys_call_handler+0xac/0x100
<2>  proc_sys_write+0x3c/0x4c
<2>  __vfs_write+0x54/0x14c
<2>  vfs_write+0xcc/0x188
<2>  SyS_write+0x60/0xc0
<2>  el0_svc_naked+0x34/0x38

device register call stack -

<2>  notifier_call_chain+0x84/0xbc
<2>  raw_notifier_call_chain+0x38/0x48
<2>  call_netdevice_notifiers_info+0x40/0x70
<2>  call_netdevice_notifiers+0x38/0x60
<2>  register_netdevice+0x29c/0x3d8
<2>  rmnet_vnd_newlink+0x68/0xe8
<2>  rmnet_newlink+0xa0/0x160
<2>  rtnl_newlink+0x57c/0x6c8
<2>  rtnetlink_rcv_msg+0x1dc/0x328
<2>  netlink_rcv_skb+0xac/0x118
<2>  rtnetlink_rcv+0x24/0x30
<2>  netlink_unicast+0x158/0x1f0
<2>  netlink_sendmsg+0x32c/0x338
<2>  sock_sendmsg+0x44/0x60
<2>  SyS_sendto+0x150/0x1ac
<2>  el0_svc_naked+0x34/0x38

Fixes: b752eff5be24 ("net: qualcomm: rmnet: Implement ndo_get_iflink")
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/qualcomm/rmnet/rmnet_vnd.c

@@ -234,7 +234,7 @@ int rmnet_vnd_newlink(u8 id, struct net_device *rmnet_dev,
 		      struct net_device *real_dev,
 		      struct rmnet_endpoint *ep)
 {
-	struct rmnet_priv *priv;
+	struct rmnet_priv *priv = netdev_priv(rmnet_dev);
 	int rc;
 
 	if (ep->egress_dev)
@@ -247,6 +247,8 @@ int rmnet_vnd_newlink(u8 id, struct net_device *rmnet_dev,
 	rmnet_dev->hw_features |= NETIF_F_IP_CSUM | NETIF_F_IPV6_CSUM;
 	rmnet_dev->hw_features |= NETIF_F_SG;
 
+	priv->real_dev = real_dev;
+
 	rc = register_netdevice(rmnet_dev);
 	if (!rc) {
 		ep->egress_dev = rmnet_dev;
@@ -255,9 +257,7 @@ int rmnet_vnd_newlink(u8 id, struct net_device *rmnet_dev,
 
 		rmnet_dev->rtnl_link_ops = &rmnet_link_ops;
 
-		priv = netdev_priv(rmnet_dev);
 		priv->mux_id = id;
-		priv->real_dev = real_dev;
 
 		netdev_dbg(rmnet_dev, "rmnet dev created\n");
 	}