UPSTREAM: ipv6: add complete rcu protection around np->opt

UPSTREAM: 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>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
[sw0312.kim: cherry-pick mainline backported patch to fix CVE-2016-3841
 - from android-3.10.y in https://android.googlesource.com/kernel/common]
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Change-Id: Ia0ad9b75da9c86cfaee15e103784f9a500ae3768