------------[ cut here ]------------
(skb->_skb_refdst & ~(1UL)) && !(skb->_skb_refdst & 1UL)
WARNING: ./include/linux/skbuff.h:1170 at skb_dst_check_unset include/linux/skbuff.h:1170 [inline], CPU#1: syz.0.17/5925
WARNING: ./include/linux/skbuff.h:1170 at skb_dst_set_noref include/linux/skbuff.h:1234 [inline], CPU#1: syz.0.17/5925
WARNING: ./include/linux/skbuff.h:1170 at nf_flow_offload_ip_hook+0xf6c/0x2b60 net/netfilter/nf_flow_table_ip.c:864, CPU#1: syz.0.17/5925
Modules linked in:
CPU: 1 UID: 0 PID: 5925 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
RIP: 0010:skb_dst_check_unset include/linux/skbuff.h:1170 [inline]
RIP: 0010:skb_dst_set_noref include/linux/skbuff.h:1234 [inline]
RIP: 0010:nf_flow_offload_ip_hook+0xf6c/0x2b60 net/netfilter/nf_flow_table_ip.c:864
Code: 89 ee e8 87 4b 14 f8 48 83 fd 01 76 20 e8 dc 50 14 f8 83 e5 01 31 ff 48 89 ee e8 6f 4b 14 f8 48 85 ed 75 09 e8 c5 50 14 f8 90 <0f> 0b 90 e8 bc 50 14 f8 e8 e7 73 f7 f7 31 ff 89 c5 89 c6 e8 1c 4b
RSP: 0018:ffffc900006a0798 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffff89f61151
RDX: ffff88802eca4a80 RSI: ffffffff89f6115b RDI: ffff88802eca4a80
RBP: 0000000000000000 R08: 0000000000000007 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff88802ef0bd18
R13: ffff888027da2900 R14: ffff8880327961c0 R15: ffff888027da2900
FS:  000055557d4c8500(0000) GS:ffff8880d5ed8000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 0000000030863000 CR4: 0000000000352ef0
Call Trace:
 <IRQ>
 nf_hook_entry_hookfn include/linux/netfilter.h:158 [inline]
 nf_hook_slow+0xbf/0x220 net/netfilter/core.c:619
 nf_hook_ingress include/linux/netfilter_netdev.h:34 [inline]
 nf_ingress net/core/dev.c:5975 [inline]
 __netif_receive_skb_core.constprop.0+0x1689/0x3530 net/core/dev.c:6071
 __netif_receive_skb_one_core+0xb0/0x1e0 net/core/dev.c:6210
 __netif_receive_skb+0x1f/0x120 net/core/dev.c:6325
 process_backlog+0x37a/0x1580 net/core/dev.c:6676
 __napi_poll.constprop.0+0xaf/0x450 net/core/dev.c:7735
 napi_poll net/core/dev.c:7798 [inline]
 net_rx_action+0xa40/0xf20 net/core/dev.c:7955
 handle_softirqs+0x1ea/0x9b0 kernel/softirq.c:622
 do_softirq kernel/softirq.c:523 [inline]
 do_softirq+0xac/0xe0 kernel/softirq.c:510
 </IRQ>
 <TASK>
 __local_bh_enable_ip+0xf8/0x120 kernel/softirq.c:450
 local_bh_enable include/linux/bottom_half.h:33 [inline]
 tun_rx_batched.isra.0+0x407/0x750 drivers/net/tun.c:1512
 tun_get_user+0x1e31/0x3c20 drivers/net/tun.c:1955
 tun_chr_write_iter+0xdc/0x200 drivers/net/tun.c:2001
 new_sync_write fs/read_write.c:595 [inline]
 vfs_write+0x6ac/0x1050 fs/read_write.c:687
 ksys_write+0x12a/0x250 fs/read_write.c:739
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x115/0x870 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fac0f15e84e
Code: 08 0f 85 a5 a8 ff ff 49 89 fb 48 89 f0 48 89 d7 48 89 ce 4c 89 c2 4d 89 ca 4c 8b 44 24 08 4c 8b 4c 24 10 4c 89 5c 24 08 0f 05 <c3> 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 80 00 00 00 00 48 83 ec 08
RSP: 002b:00007ffcf31b55e8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 000055557d4c8500 RCX: 00007fac0f15e84e
RDX: 000000000000002a RSI: 0000200000000300 RDI: 00000000000000c8
RBP: 00007fac0f23500c R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fac0f425fac R14: 00007fac0f425fa0 R15: 00007fac0f425fa0
 </TASK>