I missed that inet6_dump_addr() is calling in6_dump_addrs()
from two points.

First one under RTNL protection, and second one under rcu_read_lock().

Since we want to remove RTNL use from inet6_dump_addr() very soon,
no longer assume in6_dump_addrs() is protected by RTNL (even
if this is still the case).

Use rcu_read_lock() earlier to fix this lockdep splat:

WARNING: suspicious RCU usage
6.8.0-rc5-syzkaller-01618-gf8cbf6bd #0 Not tainted

net/ipv6/addrconf.c:5317 suspicious rcu_dereference_check() usage!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
3 locks held by syz-executor.2/8834:
  #0: ffff88802f554678 (nlk_cb_mutex-ROUTE){+.+.}-{3:3}, at: __netlink_dump_start+0x119/0x780 net/netlink/af_netlink.c:2338
  #1: ffffffff8f377a88 (rtnl_mutex){+.+.}-{3:3}, at: netlink_dump+0x676/0xda0 net/netlink/af_netlink.c:2265
  #2: ffff88807e5f0580 (&ndev->lock){++--}-{2:2}, at: in6_dump_addrs+0xb8/0x1de0 net/ipv6/addrconf.c:5279

stack backtrace:
CPU: 1 PID: 8834 Comm: syz-executor.2 Not tainted 6.8.0-rc5-syzkaller-01618-gf8cbf6bd #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/25/2024
Call Trace:
 <TASK>
  __dump_stack lib/dump_stack.c:88 [inline]
  dump_stack_lvl+0x1e7/0x2e0 lib/dump_stack.c:106
  lockdep_rcu_suspicious+0x220/0x340 kernel/locking/lockdep.c:6712
  in6_dump_addrs+0x1b47/0x1de0 net/ipv6/addrconf.c:5317
  inet6_dump_addr+0x1597/0x1690 net/ipv6/addrconf.c:5428
  netlink_dump+0x6a6/0xda0 net/netlink/af_netlink.c:2266
  __netlink_dump_start+0x59d/0x780 net/netlink/af_netlink.c:2374
  netlink_dump_start include/linux/netlink.h:340 [inline]
  rtnetlink_rcv_msg+0xcf7/0x10d0 net/core/rtnetlink.c:6555
  netlink_rcv_skb+0x1e3/0x430 net/netlink/af_netlink.c:2547
  netlink_unicast_kernel net/netlink/af_netlink.c:1335 [inline]
  netlink_unicast+0x7ea/0x980 net/netlink/af_netlink.c:1361
  netlink_sendmsg+0x8e0/0xcb0 net/netlink/af_netlink.c:1902
  sock_sendmsg_nosec net/socket.c:730 [inline]
  __sock_sendmsg+0x221/0x270 net/socket.c:745
  ____sys_sendmsg+0x525/0x7d0 net/socket.c:2584
  ___sys_sendmsg net/socket.c:2638 [inline]
  __sys_sendmsg+0x2b0/0x3a0 net/socket.c:2667

Fixes: c3718936 ("ipv6: anycast: complete RCU handling of struct ifacaddr6")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20240227222259.4081489-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>