WARNING: refcount bug in mptcp_close

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: cb957121 Merge tag 'powerpc-5.9-3' of git://git.kernel.org..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=164fa241900000
kernel config: https://syzkaller.appspot.com/x/.config?x=a0437fdd630bee11
dashboard link: https://syzkaller.appspot.com/bug?extid=d66f7ef98d61646e7848
compiler: gcc (GCC) 10.1.0-syz 20200507
CC: [da...@davemloft.net ku...@kernel.org linux-...@vger.kernel.org mathew.j....@linux.intel.com matthie...@tessares.net mp...@lists.01.org net...@vger.kernel.org]

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

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+d66f7e...@syzkaller.appspotmail.com

------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 0 PID: 23362 at lib/refcount.c:28 refcount_warn_saturate+0x1d1/0x1e0 lib/refcount.c:28
Kernel panic - not syncing: panic_on_warn set ...
CPU: 0 PID: 23362 Comm: syz-executor.2 Not tainted 5.9.0-rc1-syzkaller #0
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+0x18f/0x20d lib/dump_stack.c:118
panic+0x2e3/0x75c kernel/panic.c:231
__warn.cold+0x20/0x4a kernel/panic.c:600
report_bug+0x1bd/0x210 lib/bug.c:198
handle_bug+0x38/0x90 arch/x86/kernel/traps.c:234
exc_invalid_op+0x14/0x40 arch/x86/kernel/traps.c:254
asm_exc_invalid_op+0x12/0x20 arch/x86/include/asm/idtentry.h:536
RIP: 0010:refcount_warn_saturate+0x1d1/0x1e0 lib/refcount.c:28
Code: e9 db fe ff ff 48 89 df e8 8c f3 18 fe e9 8a fe ff ff e8 92 da d8 fd 48 c7 c7 00 db 93 88 c6 05 ca e9 11 07 01 e8 81 eb a9 fd <0f> 0b e9 af fe ff ff 0f 1f 84 00 00 00 00 00 41 56 41 55 41 54 55
RSP: 0018:ffffc90007ecfac8 EFLAGS: 00010282
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000040000 RSI: ffffffff815dafc7 RDI: fffff52000fd9f4b
RBP: 0000000000000003 R08: 0000000000000001 R09: ffff8880ae620f8b
R10: 0000000000000000 R11: 0000000000000001 R12: ffff88800014a9b4
R13: ffffed1042560840 R14: ffff888212b04200 R15: ffff88800014a8e0
refcount_sub_and_test include/linux/refcount.h:274 [inline]
refcount_dec_and_test include/linux/refcount.h:294 [inline]
skb_unref include/linux/skbuff.h:1048 [inline]
skb_unref include/linux/skbuff.h:1042 [inline]
kfree_skb+0xec/0x100 net/core/skbuff.c:692
__skb_queue_purge include/linux/skbuff.h:2785 [inline]
mptcp_close+0x730/0xc30 net/mptcp/protocol.c:1646
inet_release+0x12e/0x280 net/ipv4/af_inet.c:431
__sock_release+0xcd/0x280 net/socket.c:596
sock_close+0x18/0x20 net/socket.c:1277
__fput+0x285/0x920 fs/file_table.c:281
task_work_run+0xdd/0x190 kernel/task_work.c:141
get_signal+0xd89/0x1f00 kernel/signal.c:2561
arch_do_signal+0x82/0x2520 arch/x86/kernel/signal.c:811
exit_to_user_mode_loop kernel/entry/common.c:136 [inline]
exit_to_user_mode_prepare+0x15d/0x1c0 kernel/entry/common.c:167
syscall_exit_to_user_mode+0x59/0x2b0 kernel/entry/common.c:242
entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x45d579
Code: 5d b4 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 2b b4 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f9b81ba0c78 EFLAGS: 00000246 ORIG_RAX: 000000000000002c
RAX: 000000000093c880 RBX: 000000000002d040 RCX: 000000000045d579
RDX: 00000000ffffffe7 RSI: 0000000020000100 RDI: 0000000000000004
RBP: 000000000118cf98 R08: 0000000000000000 R09: 0000000000000000
R10: 000000000000c000 R11: 0000000000000246 R12: 000000000118cf4c
R13: 000000000169fb6f R14: 00007f9b81ba19c0 R15: 000000000118cf4c
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.