ip6mr: fix use-after-free in ip6mr_sk_done()

ip6mr: fix use-after-free in ip6mr_sk_done()

Apparently addrconf_exit_net() is called before igmp6_net_exit()
and ndisc_net_exit() at netns dismantle time:

 net_namespace: call ip6table_mangle_net_exit()
 net_namespace: call ip6_tables_net_exit()
 net_namespace: call ipv6_sysctl_net_exit()
 net_namespace: call ioam6_net_exit()
 net_namespace: call seg6_net_exit()
 net_namespace: call ping_v6_proc_exit_net()
 net_namespace: call tcpv6_net_exit()
 ip6mr_sk_done sk=ffffa354c78a74c0
 net_namespace: call ipv6_frags_exit_net()
 net_namespace: call addrconf_exit_net()
 net_namespace: call ip6addrlbl_net_exit()
 net_namespace: call ip6_flowlabel_net_exit()
 net_namespace: call ip6_route_net_exit_late()
 net_namespace: call fib6_rules_net_exit()
 net_namespace: call xfrm6_net_exit()
 net_namespace: call fib6_net_exit()
 net_namespace: call ip6_route_net_exit()
 net_namespace: call ipv6_inetpeer_exit()
 net_namespace: call if6_proc_net_exit()
 net_namespace: call ipv6_proc_exit_net()
 net_namespace: call udplite6_proc_exit_net()
 net_namespace: call raw6_exit_net()
 net_namespace: call igmp6_net_exit()
 ip6mr_sk_done sk=ffffa35472b2a180
 ip6mr_sk_done sk=ffffa354c78a7980
 net_namespace: call ndisc_net_exit()
 ip6mr_sk_done sk=ffffa35472b2ab00
 net_namespace: call ip6mr_net_exit()
 net_namespace: call inet6_net_exit()

This was fine because ip6mr_sk_done() would not reach the point decreasing
net->ipv6.devconf_all->mc_forwarding until my patch in ip6mr_sk_done().

To fix this without changing struct pernet_operations ordering,
we can clear net->ipv6.devconf_dflt and net->ipv6.devconf_all
when they are freed from addrconf_exit_net()

BUG: KASAN: use-after-free in instrument_atomic_read include/linux/instrumented.h:71 [inline]
BUG: KASAN: use-after-free in atomic_read include/linux/atomic/atomic-instrumented.h:27 [inline]
BUG: KASAN: use-after-free in ip6mr_sk_done+0x11b/0x410 net/ipv6/ip6mr.c:1578
Read of size 4 at addr ffff88801ff08688 by task kworker/u4:4/963

CPU: 0 PID: 963 Comm: kworker/u4:4 Not tainted 5.17.0-rc2-syzkaller-00650-g5a8fb33e5305 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: netns cleanup_net
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
 print_address_description.constprop.0.cold+0x8d/0x336 mm/kasan/report.c:255
 __kasan_report mm/kasan/report.c:442 [inline]
 kasan_report.cold+0x83/0xdf mm/kasan/report.c:459
 check_region_inline mm/kasan/generic.c:183 [inline]
 kasan_check_range+0x13d/0x180 mm/kasan/generic.c:189
 instrument_atomic_read include/linux/instrumented.h:71 [inline]
 atomic_read include/linux/atomic/atomic-instrumented.h:27 [inline]
 ip6mr_sk_done+0x11b/0x410 net/ipv6/ip6mr.c:1578
 rawv6_close+0x58/0x80 net/ipv6/raw.c:1201
 inet_release+0x12e/0x280 net/ipv4/af_inet.c:428
 inet6_release+0x4c/0x70 net/ipv6/af_inet6.c:478
 __sock_release net/socket.c:650 [inline]
 sock_release+0x87/0x1b0 net/socket.c:678
 inet_ctl_sock_destroy include/net/inet_common.h:65 [inline]
 igmp6_net_exit+0x6b/0x170 net/ipv6/mcast.c:3173
 ops_exit_list+0xb0/0x170 net/core/net_namespace.c:168
 cleanup_net+0x4ea/0xb00 net/core/net_namespace.c:600
 process_one_work+0x9ac/0x1650 kernel/workqueue.c:2307
 worker_thread+0x657/0x1110 kernel/workqueue.c:2454
 kthread+0x2e9/0x3a0 kernel/kthread.c:377
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
 </TASK>

Fixes: f2f2325ec799 ("ip6mr: ip6mr_sk_done() can exit early in common cases")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>