general protection fault in sit_exit_batch_net

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    c01ebd6c r8152: Use guard clause and fix comment typos
git tree:       net-next
console output: https://syzkaller.appspot.com/x/log.txt?x=10329447600000
kernel config:  https://syzkaller.appspot.com/x/.config?x=6ffbfa7e4a36190f
dashboard link: https://syzkaller.appspot.com/bug?extid=1695af5ca559927e2db8
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1156242b600000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17253b19600000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+1695af5ca559927e2db8@xxxxxxxxxxxxxxxxxxxxxxxxx

kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] PREEMPT SMP KASAN
CPU: 0 PID: 21 Comm: kworker/u4:1 Not tainted 5.3.0+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Workqueue: netns cleanup_net
RIP: 0010:dev_net include/linux/netdevice.h:2186 [inline]
RIP: 0010:sit_destroy_tunnels net/ipv6/sit.c:1831 [inline]
RIP: 0010:sit_exit_batch_net+0x3b1/0x750 net/ipv6/sit.c:1894
Code: fb 49 8d 7c 24 18 48 89 f8 48 c1 e8 03 80 3c 18 00 0f 85 27 02 00 00  
4d 8b 44 24 18 49 8d b8 80 05 00 00 48 89 f8 48 c1 e8 03 <80> 3c 18 00 0f  
85 f2 01 00 00 4d 3b a8 80 05 00 00 4c 89 85 58 ff
RSP: 0018:ffff8880a9a07b50 EFLAGS: 00010202
RAX: 00000000000000b0 RBX: dffffc0000000000 RCX: ffffffff865ad59f
RDX: 0000000000000000 RSI: ffffffff865ad488 RDI: 0000000000000580
RBP: ffff8880a9a07c08 R08: 0000000000000000 R09: fffffbfff1332f89
R10: fffffbfff1332f88 R11: ffffffff89997c47 R12: ffff88808b711140
R13: ffff88808f206300 R14: 0000000000000006 R15: ffff88808a9e8e50
FS:  0000000000000000(0000) GS:ffff8880ae800000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000f8e000 CR3: 00000000980db000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 ops_exit_list.isra.0+0xfc/0x150 net/core/net_namespace.c:175
 cleanup_net+0x4e2/0xa60 net/core/net_namespace.c:594
 process_one_work+0x9af/0x1740 kernel/workqueue.c:2269
 worker_thread+0x98/0xe40 kernel/workqueue.c:2415
 kthread+0x361/0x430 kernel/kthread.c:255
 ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352
Modules linked in:
---[ end trace 279fb1eb05d15843 ]---
RIP: 0010:dev_net include/linux/netdevice.h:2186 [inline]
RIP: 0010:sit_destroy_tunnels net/ipv6/sit.c:1831 [inline]
RIP: 0010:sit_exit_batch_net+0x3b1/0x750 net/ipv6/sit.c:1894
Code: fb 49 8d 7c 24 18 48 89 f8 48 c1 e8 03 80 3c 18 00 0f 85 27 02 00 00  
4d 8b 44 24 18 49 8d b8 80 05 00 00 48 89 f8 48 c1 e8 03 <80> 3c 18 00 0f  
85 f2 01 00 00 4d 3b a8 80 05 00 00 4c 89 85 58 ff
RSP: 0018:ffff8880a9a07b50 EFLAGS: 00010202
RAX: 00000000000000b0 RBX: dffffc0000000000 RCX: ffffffff865ad59f
RDX: 0000000000000000 RSI: ffffffff865ad488 RDI: 0000000000000580
RBP: ffff8880a9a07c08 R08: 0000000000000000 R09: fffffbfff1332f89
R10: fffffbfff1332f88 R11: ffffffff89997c47 R12: ffff88808b711140
R13: ffff88808f206300 R14: 0000000000000006 R15: ffff88808a9e8e50
FS:  0000000000000000(0000) GS:ffff8880ae800000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000f8e000 CR3: 00000000980db000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400


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