WARNING: suspicious RCU usage in rt6_check_expired

[syzbot]

Hello,

syzbot hit the following crash on net-next commit
0638eb573cde5888c0886c7f35da604e5db209a6 (Sat Apr 21 20:06:14 2018 +0000)
Merge branch 'ipv6-Another-followup-to-the-fib6_info-change'
syzbot dashboard link:  
https://syzkaller.appspot.com/bug?extid=2422c9e35796659d2273

So far this crash happened 3 times on net-next.
Unfortunately, I don't have any reproducer for this crash yet.
Raw console output:  
https://syzkaller.appspot.com/x/log.txt?id=6081013801287680
Kernel config:  
https://syzkaller.appspot.com/x/.config?id=-8412024688694752032
compiler: gcc (GCC) 8.0.1 20180413 (experimental)

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+2422c9e35796659d2273@xxxxxxxxxxxxxxxxxxxxxxxxx
It will help syzbot understand when the bug is fixed. See footer for  
details.
If you forward the report, please keep this part and the footer.

netlink: 'syz-executor4': attribute type 6 has an invalid length.
netlink: 'syz-executor4': attribute type 1 has an invalid length.
netlink: 'syz-executor4': attribute type 6 has an invalid length.

=============================
WARNING: suspicious RCU usage
4.16.0+ #11 Not tainted
-----------------------------
net/ipv6/route.c:410 suspicious rcu_dereference_check() usage!

other info that might help us debug this:


rcu_scheduler_active = 2, debug_locks = 1
1 lock held by syz-executor7/25958:
 #0: 00000000d1963139 (sk_lock-AF_INET6){+.+.}, at: lock_sock  
include/net/sock.h:1469 [inline]
 #0: 00000000d1963139 (sk_lock-AF_INET6){+.+.}, at:  
sock_setsockopt+0x19c/0x1fe0 net/core/sock.c:717

stack backtrace:
CPU: 1 PID: 25958 Comm: syz-executor7 Not tainted 4.16.0+ #11
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1b9/0x294 lib/dump_stack.c:113
 lockdep_rcu_suspicious+0x14a/0x153 kernel/locking/lockdep.c:4592
 rt6_check_expired+0x38b/0x3e0 net/ipv6/route.c:410
 ip6_negative_advice+0x67/0xc0 net/ipv6/route.c:2204
 dst_negative_advice include/net/sock.h:1786 [inline]
 sock_setsockopt+0x138f/0x1fe0 net/core/sock.c:1051
 __sys_setsockopt+0x2df/0x390 net/socket.c:1899
 SYSC_setsockopt net/socket.c:1914 [inline]
 SyS_setsockopt+0x34/0x50 net/socket.c:1911
 do_syscall_64+0x29e/0x9d0 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x455389
RSP: 002b:00007f7556e30c68 EFLAGS: 00000246 ORIG_RAX: 0000000000000036
RAX: ffffffffffffffda RBX: 00007f7556e316d4 RCX: 0000000000455389
RDX: 0000000000000035 RSI: 0000000000000001 RDI: 0000000000000013
RBP: 000000000072bf58 R08: 0000000000000004 R09: 0000000000000000
R10: 0000000020000000 R11: 0000000000000246 R12: 00000000ffffffff
R13: 00000000000005ff R14: 00000000006fc088 R15: 0000000000000001
netlink: 'syz-executor4': attribute type 4 has an invalid length.
netlink: 'syz-executor4': attribute type 4 has an invalid length.
IPVS: set_ctl: invalid protocol: 59 127.0.0.1:20000 lc
IPVS: set_ctl: invalid protocol: 127 224.0.0.1:20000 rr
IPVS: sync thread started: state = BACKUP, mcast_ifn = ip6tnl0, syncid = 4,  
id = 0
IPVS: set_ctl: invalid protocol: 127 224.0.0.1:20000 rr
netlink: 72 bytes leftover after parsing attributes in process  
`syz-executor2'.
netlink: 72 bytes leftover after parsing attributes in process  
`syz-executor2'.
dccp_xmit_packet: Payload too large (65423) for featneg.
IPVS: set_ctl: invalid protocol: 29 1.0.0.0:2 wlc
IPVS: set_ctl: invalid protocol: 29 1.0.0.0:2 wlc
netlink: 32 bytes leftover after parsing attributes in process  
`syz-executor7'.
netlink: 12 bytes leftover after parsing attributes in process  
`syz-executor1'.
netlink: 12 bytes leftover after parsing attributes in process  
`syz-executor1'.
netlink: 'syz-executor1': attribute type 29 has an invalid length.
netlink: 8 bytes leftover after parsing attributes in process  
`syz-executor1'.
IPVS: set_ctl: invalid protocol: 108 224.0.0.1:20004 lblc
netlink: 'syz-executor1': attribute type 29 has an invalid length.
netlink: 8 bytes leftover after parsing attributes in process  
`syz-executor1'.
IPVS: set_ctl: invalid protocol: 108 224.0.0.1:20004 lblc


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