syzbot has found a reproducer for the following crash on:

HEAD commit:    333478a7eb21 Merge branch 'for-rc' of git://
git tree:       upstream
console output:
kernel config:
dashboard link:
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:

IMPORTANT: if you fix the bug, please add the following tag to the commit:

IPv6: ADDRCONF(NETDEV_UP): vxcan1: link is not ready
8021q: adding VLAN 0 to HW filter on device batadv0
IPVS: ftp: loaded support on port[0] = 21
chnl_net:caif_netlink_parms(): no params data found
BUG: KASAN: use-after-free in atomic_read include/asm-generic/atomic-instrumented.h:21 [inline] BUG: KASAN: use-after-free in refcount_inc_not_zero_checked+0x91/0x2e0 lib/refcount.c:120
Read of size 4 at addr ffff888089bba440 by task syz-executor5/11381

CPU: 1 PID: 11381 Comm: syz-executor5 Not tainted 5.0.0-rc3+ #40
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+0x1db/0x2d0 lib/dump_stack.c:113
 print_address_description.cold+0x7c/0x20d mm/kasan/report.c:187
 kasan_report.cold+0x1b/0x40 mm/kasan/report.c:317
 check_memory_region_inline mm/kasan/generic.c:185 [inline]
 check_memory_region+0x123/0x190 mm/kasan/generic.c:191
 kasan_check_read+0x11/0x20 mm/kasan/common.c:100
 atomic_read include/asm-generic/atomic-instrumented.h:21 [inline]
 refcount_inc_not_zero_checked+0x91/0x2e0 lib/refcount.c:120
 refcount_inc_checked+0x17/0x70 lib/refcount.c:153
 sock_hold include/net/sock.h:647 [inline]
 nr_release+0x62/0x3c0 net/netrom/af_netrom.c:523
 __sock_release+0xd3/0x250 net/socket.c:579
 sock_close+0x1b/0x30 net/socket.c:1141
 __fput+0x3c5/0xb10 fs/file_table.c:278
 ____fput+0x16/0x20 fs/file_table.c:309
 task_work_run+0x1f4/0x2b0 kernel/task_work.c:113
 exit_task_work include/linux/task_work.h:22 [inline]
 do_exit+0xad7/0x26e0 kernel/exit.c:867
 do_group_exit+0x177/0x430 kernel/exit.c:971
 get_signal+0x8b4/0x19b0 kernel/signal.c:2517
 do_signal+0x91/0x1ea0 arch/x86/kernel/signal.c:816
 exit_to_usermode_loop+0x2f7/0x3b0 arch/x86/entry/common.c:162
 prepare_exit_to_usermode arch/x86/entry/common.c:197 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:268 [inline]
 do_syscall_64+0x696/0x800 arch/x86/entry/common.c:293
RIP: 0033:0x458099
Code: 6d b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 3b b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f7c8a6a7cf8 EFLAGS: 00000246 ORIG_RAX: 00000000000000ca
RAX: fffffffffffffe00 RBX: 000000000073c228 RCX: 0000000000458099
RDX: 0000000000000000 RSI: 0000000000000080 RDI: 000000000073c228
RBP: 000000000073c220 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 000000000073c22c
R13: 00007ffc1cd8e9ef R14: 00007f7c8a6a89c0 R15: 0000000000000005

Allocated by task 11350:
 save_stack+0x45/0xd0 mm/kasan/common.c:73
 set_track mm/kasan/common.c:85 [inline]
 __kasan_kmalloc mm/kasan/common.c:496 [inline]
 __kasan_kmalloc.constprop.0+0xcf/0xe0 mm/kasan/common.c:469
 kasan_kmalloc+0x9/0x10 mm/kasan/common.c:504
 __do_kmalloc mm/slab.c:3711 [inline]
 __kmalloc+0x15c/0x740 mm/slab.c:3720
 kmalloc include/linux/slab.h:550 [inline]
 sk_prot_alloc+0x19c/0x2e0 net/core/sock.c:1477
 sk_alloc+0xd7/0x1690 net/core/sock.c:1531
 nr_create+0xb9/0x5e0 net/netrom/af_netrom.c:436
 __sock_create+0x532/0x930 net/socket.c:1277
 sock_create net/socket.c:1317 [inline]
 __sys_socket+0x106/0x260 net/socket.c:1347
 __do_sys_socket net/socket.c:1356 [inline]
 __se_sys_socket net/socket.c:1354 [inline]
 __x64_sys_socket+0x73/0xb0 net/socket.c:1354
 do_syscall_64+0x1a3/0x800 arch/x86/entry/common.c:290

Freed by task 11170:
 save_stack+0x45/0xd0 mm/kasan/common.c:73
 set_track mm/kasan/common.c:85 [inline]
 __kasan_slab_free+0x102/0x150 mm/kasan/common.c:458
 kasan_slab_free+0xe/0x10 mm/kasan/common.c:466
 __cache_free mm/slab.c:3487 [inline]
 kfree+0xcf/0x230 mm/slab.c:3806
 sk_prot_free net/core/sock.c:1514 [inline]
 __sk_destruct+0x76d/0xa60 net/core/sock.c:1596
 sk_destruct+0x7b/0x90 net/core/sock.c:1604
 __sk_free+0xce/0x300 net/core/sock.c:1615
 sk_free+0x42/0x50 net/core/sock.c:1626
 sock_put include/net/sock.h:1707 [inline]
 nr_heartbeat_expiry+0x489/0x520 net/netrom/nr_timer.c:130
 call_timer_fn+0x254/0x900 kernel/time/timer.c:1325
 expire_timers kernel/time/timer.c:1362 [inline]
 __run_timers+0x6fc/0xd50 kernel/time/timer.c:1681
 run_timer_softirq+0x52/0xb0 kernel/time/timer.c:1694
 __do_softirq+0x30b/0xb11 kernel/softirq.c:292

The buggy address belongs to the object at ffff888089bba3c0
 which belongs to the cache kmalloc-2k of size 2048
The buggy address is located 128 bytes inside of
 2048-byte region [ffff888089bba3c0, ffff888089bbabc0)
The buggy address belongs to the page:
page:ffffea000226ee80 count:1 mapcount:0 mapping:ffff88812c3f0c40 index:0xffff888089bbb4c0 compound_mapcount: 0
flags: 0x1fffc0000010200(slab|head)
raw: 01fffc0000010200 ffffea0002608f08 ffffea0002501688 ffff88812c3f0c40
raw: ffff888089bbb4c0 ffff888089bba3c0 0000000100000001 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff888089bba300: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff888089bba380: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
ffff888089bba400: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff888089bba480: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff888089bba500: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb

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