KASAN: null-ptr-deref Write in dst_release

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    b71acb0e3721 Merge branch 'linus' of git://git.kernel.org/..
git tree:       net-next
console output: https://syzkaller.appspot.com/x/log.txt?x=15c947e7400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=b03c5892bb940c76
dashboard link: https://syzkaller.appspot.com/bug?extid=1f4f4025b8564c8da9d4
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+1f4f4025b8564c8da9d4@xxxxxxxxxxxxxxxxxxxxxxxxx

IPVS: ftp: loaded support on port[0] = 21
Enabling of bearer <udp:syz0> rejected, failed to enable media
Enabling of bearer <udp:syz0> rejected, failed to enable media
==================================================================
BUG: KASAN: null-ptr-deref in atomic_sub_return  
include/asm-generic/atomic-instrumented.h:305 [inline]
BUG: KASAN: null-ptr-deref in dst_release net/core/dst.c:186 [inline]
BUG: KASAN: null-ptr-deref in dst_release+0x2a/0xb0 net/core/dst.c:181
Write of size 4 at addr 000000000000013c by task kworker/u4:1/21

CPU: 1 PID: 21 Comm: kworker/u4:1 Not tainted 4.20.0+ #3
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Workqueue: netns cleanup_net
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1db/0x2d0 lib/dump_stack.c:113
 kasan_report_error mm/kasan/report.c:352 [inline]
 kasan_report mm/kasan/report.c:412 [inline]
 kasan_report.cold+0x19f/0x2ba mm/kasan/report.c:396
 check_memory_region_inline mm/kasan/kasan.c:260 [inline]
 check_memory_region+0x123/0x190 mm/kasan/kasan.c:267
 kasan_check_write+0x14/0x20 mm/kasan/kasan.c:278
 atomic_sub_return include/asm-generic/atomic-instrumented.h:305 [inline]
 dst_release net/core/dst.c:186 [inline]
 dst_release+0x2a/0xb0 net/core/dst.c:181
 dst_cache_destroy net/core/dst_cache.c:164 [inline]
 dst_cache_destroy+0xd3/0x1b0 net/core/dst_cache.c:156
 ip_tunnel_dev_free+0x25/0x60 net/ipv4/ip_tunnel.c:970
 netdev_run_todo+0x647/0xae0 net/core/dev.c:8885
 rtnl_unlock+0xe/0x10 net/core/rtnetlink.c:117
 ip_tunnel_delete_nets+0x4df/0x6d0 net/ipv4/ip_tunnel.c:1083
 ipip_exit_batch_net+0x23/0x30 net/ipv4/ipip.c:663
 ops_exit_list.isra.0+0x105/0x160 net/core/net_namespace.c:156
 cleanup_net+0x51d/0xb10 net/core/net_namespace.c:551
 process_one_work+0xd0c/0x1ce0 kernel/workqueue.c:2153
 worker_thread+0x143/0x14a0 kernel/workqueue.c:2296
 kthread+0x357/0x430 kernel/kthread.c:246
 ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:352
==================================================================


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