WARNING: refcount bug in free_nsproxy (2)

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 6c09d7db Add linux-next specific files for 20200110
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=102b2156e00000
kernel config: https://syzkaller.appspot.com/x/.config?x=7dc7ab9739654fbe
dashboard link: https://syzkaller.appspot.com/bug?extid=a98eee31f5df4261d88c
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=162f16aee00000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=13457571e00000

The bug was bisected to:

commit 14215108a1fd7e002c0a1f9faf8fbaf41fdda50d
Author: Cong Wang <xiyou.w...@gmail.com>
Date: Thu Feb 21 05:37:42 2019 +0000

net_sched: initialize net pointer inside tcf_exts_init()

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=1063da9ee00000
final crash: https://syzkaller.appspot.com/x/report.txt?x=1263da9ee00000
console output: https://syzkaller.appspot.com/x/log.txt?x=1463da9ee00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+a98eee...@syzkaller.appspotmail.com
Fixes: 14215108a1fd ("net_sched: initialize net pointer inside
tcf_exts_init()")

R13: 0000000000000008 R14: 0000000000000000 R15: 0000000000000000
------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 1 PID: 9780 at lib/refcount.c:28
refcount_warn_saturate+0x1dc/0x1f0 lib/refcount.c:28
Kernel panic - not syncing: panic_on_warn set ...
CPU: 1 PID: 9780 Comm: syz-executor174 Not tainted
5.5.0-rc5-next-20200110-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+0x197/0x210 lib/dump_stack.c:118
panic+0x2e3/0x75c kernel/panic.c:221
__warn.cold+0x2f/0x3e kernel/panic.c:582
report_bug+0x289/0x300 lib/bug.c:195
fixup_bug arch/x86/kernel/traps.c:176 [inline]
fixup_bug arch/x86/kernel/traps.c:171 [inline]
do_error_trap+0x11b/0x200 arch/x86/kernel/traps.c:269
do_invalid_op+0x37/0x50 arch/x86/kernel/traps.c:288
invalid_op+0x23/0x30 arch/x86/entry/entry_64.S:1027
RIP: 0010:refcount_warn_saturate+0x1dc/0x1f0 lib/refcount.c:28
Code: e9 d8 fe ff ff 48 89 df e8 b1 a6 13 fe e9 85 fe ff ff e8 47 6e d5 fd
48 c7 c7 80 64 91 88 c6 05 4f 5b 00 07 01 e8 e3 f5 a5 fd <0f> 0b e9 ac fe
ff ff 0f 1f 00 66 2e 0f 1f 84 00 00 00 00 00 55 48
RSP: 0018:ffffc90005fd7cf8 EFLAGS: 00010286
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff815e8546 RDI: fffff52000bfaf91
RBP: ffffc90005fd7d08 R08: ffff8880a65ee500 R09: ffffed1015d26659
R10: ffffed1015d26658 R11: ffff8880ae9332c7 R12: 0000000000000003
R13: ffff88809aff6040 R14: ffff88809aff6044 R15: 00000000000002bc
refcount_sub_and_test include/linux/refcount.h:261 [inline]
refcount_dec_and_test include/linux/refcount.h:281 [inline]
put_net include/net/net_namespace.h:259 [inline]
free_nsproxy+0x2eb/0x330 kernel/nsproxy.c:180
switch_task_namespaces+0xb3/0xd0 kernel/nsproxy.c:225
exit_task_namespaces+0x18/0x20 kernel/nsproxy.c:230
do_exit+0xbc6/0x2f70 kernel/exit.c:800
do_group_exit+0x135/0x360 kernel/exit.c:899
__do_sys_exit_group kernel/exit.c:910 [inline]
__se_sys_exit_group kernel/exit.c:908 [inline]
__x64_sys_exit_group+0x44/0x50 kernel/exit.c:908
do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x441228
Code: 74 20 63 6f 64 65 20 62 61 73 65 2e 00 00 00 00 00 00 72 73 79 73 6c
6f 67 64 3a 20 24 41 62 6f 72 74 4f 6e 55 6e 63 6c 65 61 <6e> 43 6f 6e 66
69 67 20 69 73 20 73 65 74 2c 20 61 6e 64 20 63 6f
RSP: 002b:00007ffedb54aa88 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
RAX: ffffffffffffffda RBX: 0000000000000001 RCX: 0000000000441228
RDX: 0000000000000001 RSI: 000000000000003c RDI: 0000000000000001
RBP: 00000000004c79d0 R08: 00000000000000e7 R09: ffffffffffffffd0
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
R13: 00000000006daa80 R14: 0000000000000000 R15: 0000000000000000
Kernel Offset: disabled
Rebooting in 86400 seconds..


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[syzbot]

Auto-closing this bug as obsolete.
No recent activity, existing reproducers are no longer triggering the issue.