WARNING: refcount bug in pppol2tp_session_sock_hold

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: f6e27dbb Linux 4.14.146
git tree: linux-4.14.y
console output: https://syzkaller.appspot.com/x/log.txt?x=1159622b600000
kernel config: https://syzkaller.appspot.com/x/.config?x=cb75afefe94a0801
dashboard link: https://syzkaller.appspot.com/bug?extid=68295b2c4e1ed14ecf66
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+68295b...@syzkaller.appspotmail.com

refcount_t: increment on 0; use-after-free.
------------[ cut here ]------------
WARNING: CPU: 0 PID: 633 at lib/refcount.c:153 refcount_inc
lib/refcount.c:153 [inline]
WARNING: CPU: 0 PID: 633 at lib/refcount.c:153 refcount_inc.cold+0x18/0x1f
lib/refcount.c:151
Kernel panic - not syncing: panic_on_warn set ...

CPU: 0 PID: 633 Comm: kworker/u4:3 Not tainted 4.14.146 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Workqueue: l2tp l2tp_tunnel_del_work
Call Trace:
__dump_stack lib/dump_stack.c:17 [inline]
dump_stack+0x138/0x197 lib/dump_stack.c:53
panic+0x1f2/0x426 kernel/panic.c:182
__warn.cold+0x2f/0x36 kernel/panic.c:546
report_bug+0x216/0x254 lib/bug.c:186
fixup_bug arch/x86/kernel/traps.c:177 [inline]
fixup_bug arch/x86/kernel/traps.c:172 [inline]
do_error_trap+0x1bb/0x310 arch/x86/kernel/traps.c:295
do_invalid_op+0x1b/0x20 arch/x86/kernel/traps.c:314
invalid_op+0x1b/0x40 arch/x86/entry/entry_64.S:963
RIP: 0010:refcount_inc lib/refcount.c:153 [inline]
RIP: 0010:refcount_inc.cold+0x18/0x1f lib/refcount.c:151
RSP: 0018:ffff8880a8e97c48 EFLAGS: 00010286
RAX: 000000000000002b RBX: ffff888098bb9480 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff866d10e0 RDI: ffffed10151d2f7f
RBP: ffff8880a8e97c50 R08: 000000000000002b R09: ffff8880a8dfea30
R10: 0000000000000000 R11: 0000000000000000 R12: ffff888098bb9500
R13: ffff888093a6b780 R14: ffff8880a52b32e0 R15: ffff8880a52b32a0
sock_hold include/net/sock.h:619 [inline]
pppol2tp_session_sock_hold+0x48/0x60 net/l2tp/l2tp_ppp.c:263
l2tp_tunnel_closeall+0x1ef/0x380 net/l2tp/l2tp_core.c:1332
l2tp_tunnel_del_work+0x64/0x410 net/l2tp/l2tp_core.c:1380
process_one_work+0x863/0x1600 kernel/workqueue.c:2114
worker_thread+0x5d9/0x1050 kernel/workqueue.c:2248
kthread+0x319/0x430 kernel/kthread.c:232
ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:404
Kernel Offset: disabled
Rebooting in 86400 seconds..


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[syzbot]

Auto-closing this bug as obsolete.
Crashes did not happen for a while, no reproducer and no activity.