l2tp: device MTU setup, tunnel socket needs a lock

The MTU overhead calculation in L2TP device set-up
merged via commit b784e7ebfce8cfb16c6f95e14e8532d0768ab7ff
needs to be adjusted to lock the tunnel socket while
referencing the sub-data structures to derive the
socket's IP overhead.

Reported-by: Guillaume Nault <[e-mail]>
Tested-by: Guillaume Nault <[e-mail]>
Signed-off-by: R. Parameswaran <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/net.h

···
 int kernel_sock_ioctl(struct socket *sock, int cmd, unsigned long arg);
 int kernel_sock_shutdown(struct socket *sock, enum sock_shutdown_cmd how);
 
-/* Following routine returns the IP overhead imposed by a socket.  */
+/* Routine returns the IP overhead imposed by a (caller-protected) socket. */
 u32 kernel_sock_ip_overhead(struct sock *sk);
 
 #define MODULE_ALIAS_NETPROTO(proto) \

net/l2tp/l2tp_eth.c

···
 		dev->needed_headroom += session->hdr_len;
 		return;
 	}
+	lock_sock(tunnel->sock);
 	l3_overhead = kernel_sock_ip_overhead(tunnel->sock);
+	release_sock(tunnel->sock);
 	if (l3_overhead == 0) {
 		/* L3 Overhead couldn't be identified, this could be
 		 * because tunnel->sock was NULL or the socket's

net/socket.c

···
 /* This routine returns the IP overhead imposed by a socket i.e.
  * the length of the underlying IP header, depending on whether
  * this is an IPv4 or IPv6 socket and the length from IP options turned
- * on at the socket.
+ * on at the socket. Assumes that the caller has a lock on the socket.
  */
 u32 kernel_sock_ip_overhead(struct sock *sk)
 {