refcount_t: increment on 0; use-after-free.
------------[ cut here ]------------
WARNING: CPU: 1 PID: 15951 at lib/refcount.c:153 refcount_inc.cold.12+0x18/0x1f lib/refcount.c:153
Kernel panic - not syncing: panic_on_warn set ...

CPU: 1 PID: 15951 Comm: syz-executor4 Not tainted 4.14.90+ #29
Call Trace:
 __dump_stack lib/dump_stack.c:17 [inline]
 dump_stack+0xb9/0x11b lib/dump_stack.c:53
 panic+0x1bf/0x3a4 kernel/panic.c:181
 __warn.cold.7+0x2f/0x35 kernel/panic.c:542
 report_bug+0x1f7/0x26c lib/bug.c:186
 fixup_bug arch/x86/kernel/traps.c:177 [inline]
 do_error_trap+0x1ba/0x2c0 arch/x86/kernel/traps.c:295
 invalid_op+0x18/0x40 arch/x86/entry/entry_64.S:944
RIP: 0010:refcount_inc.cold.12+0x18/0x1f lib/refcount.c:153
RSP: 0018:ffff8881a01f7150 EFLAGS: 00010286
RAX: 000000000000002b RBX: ffff8881c6f29954 RCX: 00000000000087b3
RDX: 0000000000000000 RSI: ffffc9000593b000 RDI: ffffffff979d8560
RBP: 00000000000003e8 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: dffffc0000000000
R13: ffff8881c6f298c0 R14: 0000000000000000 R15: ffff8881c6610062
 igmp_start_timer+0x98/0xc0 net/ipv4/igmp.c:217
 igmp_mod_timer net/ipv4/igmp.c:255 [inline]
 igmp_heard_query net/ipv4/igmp.c:1027 [inline]
 igmp_rcv+0x1346/0x2400 net/ipv4/igmp.c:1062
 ip_local_deliver_finish+0x3f1/0xaa0 net/ipv4/ip_input.c:216
 NF_HOOK include/linux/netfilter.h:250 [inline]
 ip_local_deliver+0x374/0x450 net/ipv4/ip_input.c:257
 dst_input include/net/dst.h:465 [inline]
 ip_rcv_finish+0x5cc/0x1490 net/ipv4/ip_input.c:397
 NF_HOOK include/linux/netfilter.h:250 [inline]
 ip_rcv+0xa19/0x1028 net/ipv4/ip_input.c:493
 __netif_receive_skb_core+0x149d/0x2d10 net/core/dev.c:4477
 __netif_receive_skb+0x58/0x1f0 net/core/dev.c:4515
 netif_receive_skb_internal+0xfc/0x5e0 net/core/dev.c:4588
 tun_rx_batched.isra.12+0x464/0x710 drivers/net/tun.c:1218
 tun_get_user+0x11d1/0x3c90 drivers/net/tun.c:1570
 tun_chr_write_iter+0xcf/0x180 drivers/net/tun.c:1596
 call_write_iter include/linux/fs.h:1784 [inline]
 do_iter_readv_writev+0x3a4/0x560 fs/read_write.c:678
 do_iter_write+0x156/0x530 fs/read_write.c:957
 vfs_writev+0x146/0x2d0 fs/read_write.c:1002
 do_writev+0xcc/0x240 fs/read_write.c:1037
 do_syscall_64+0x19b/0x4b0 arch/x86/entry/common.c:289
 entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x457521
RSP: 002b:00007ff921cc7ba0 EFLAGS: 00000293 ORIG_RAX: 0000000000000014
RAX: ffffffffffffffda RBX: 000000000000002a RCX: 0000000000457521
RDX: 0000000000000001 RSI: 00007ff921cc7bf0 RDI: 00000000000000f0
RBP: 0000000020000640 R08: 00000000000000f0 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000293 R12: 00007ff921cc86d4
R13: 00000000004c53b4 R14: 00000000004d9900 R15: 00000000ffffffff
Kernel Offset: 0x13000000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
Rebooting in 86400 seconds..