ipv6: add complete rcu protection around np->opt

[ Upstream commit 45f6fad84cc305103b28d73482b344d7f5b76f39 ]

This patch addresses multiple problems :

UDP/RAW sendmsg() need to get a stable struct ipv6_txoptions
while socket is not locked : Other threads can change np->opt
concurrently. Dmitry posted a syzkaller
(http://github.com/google/syzkaller) program desmonstrating
use-after-free.

Starting with TCP/DCCP lockless listeners, tcp_v6_syn_recv_sock()
and dccp_v6_request_recv_sock() also need to use RCU protection
to dereference np->opt once (before calling ipv6_dup_options())

This patch adds full RCU protection to np->opt

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2:
 - Drop changes to l2tp
 - Fix an additional use of np->opt in tcp_v6_send_synack()
 - Fold in commit 43264e0bd963 ("ipv6: remove unnecessary codes in tcp_ipv6.c")
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --cc include/linux/ipv6.h
Simple merge

diff --cc include/net/ipv6.h
Simple merge

diff --cc net/dccp/ipv6.c
Simple merge

diff --cc net/ipv6/af_inet6.c
Simple merge

diff --cc net/ipv6/datagram.c
Simple merge

diff --cc net/ipv6/exthdrs.c
Simple merge

diff --cc net/ipv6/inet6_connection_sock.c
Simple merge

diff --cc net/ipv6/ipv6_sockglue.c
Simple merge

diff --cc net/ipv6/raw.c
Simple merge

diff --cc net/ipv6/syncookies.c
Simple merge

diff --cc net/ipv6/tcp_ipv6.c
Simple merge

diff --cc net/ipv6/udp.c
Simple merge