drivers/net/wan/lapbether: Fixed the value of hard_header_len

BugLink: https://bugs.launchpad.net/bugs/1889928

[ Upstream commit 9dc829a1 ]

When this driver transmits data,
  first this driver will remove a pseudo header of 1 byte,
  then the lapb module will prepend the LAPB header of 2 or 3 bytes,
  then this driver will prepend a length field of 2 bytes,
  then the underlying Ethernet device will prepend its own header.

So, the header length required should be:
  -1 + 3 + 2 + "the header length needed by the underlying device".

This patch fixes kernel panic when this driver is used with AF_PACKET
SOCK_DGRAM sockets.
Signed-off-by: Xie He <xie.he.0141@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

drivers/net/wan/lapbether.c

@@ -308,7 +308,6 @@ static void lapbeth_setup(struct net_device *dev)
 	dev->netdev_ops	     = &lapbeth_netdev_ops;
 	dev->destructor	     = free_netdev;
 	dev->type            = ARPHRD_X25;
-	dev->hard_header_len = 3;
 	dev->mtu             = 1000;
 	dev->addr_len        = 0;
 }
@@ -329,6 +328,14 @@ static int lapbeth_new_device(struct net_device *dev)
 	if (!ndev)
 		goto out;
 
+	/* When transmitting data:
+	 * first this driver removes a pseudo header of 1 byte,
+	 * then the lapb module prepends an LAPB header of at most 3 bytes,
+	 * then this driver prepends a length field of 2 bytes,
+	 * then the underlying Ethernet device prepends its own header.
+	 */
+	ndev->hard_header_len = -1 + 3 + 2 + dev->hard_header_len;
+
 	lapbeth = netdev_priv(ndev);
 	lapbeth->axdev = ndev;