Re: [syzbot] WARNING in mntput_no_expire (3)

[syzbot <syzbot+5b1e53987f858500ec00@syzkaller.appspotmail.com>]

syzbot has found a reproducer for the following issue on:

HEAD commit:    feb9c5e19e91 Merge tag 'for_linus' of git://git.kernel.org..
git tree:       upstream
console+strace: https://syzkaller.appspot.com/x/log.txt?x=10ea9d8ef00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=79caa0035f59d385
dashboard link: https://syzkaller.appspot.com/bug?extid=5b1e53987f858500ec00
compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=125039fef00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17a27b71f00000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+5b1e53987f858500ec00@syzkaller.appspotmail.com

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WARNING: CPU: 0 PID: 3608 at fs/namespace.c:1236 mntput_no_expire+0xada/0xcd0 fs/namespace.c:1236
Modules linked in:

CPU: 0 PID: 3608 Comm: syz-executor314 Not tainted 5.18.0-rc6-syzkaller-00009-gfeb9c5e19e91 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:mntput_no_expire+0xada/0xcd0 fs/namespace.c:1236
Code: 30 84 c0 0f 84 b9 fe ff ff 3c 03 0f 8f b1 fe ff ff 4c 89 44 24 10 e8 45 50 e9 ff 4c 8b 44 24 10 e9 9d fe ff ff e8 56 bf 9d ff <0f> 0b e9 19 fd ff ff e8 4a bf 9d ff e8 b5 cf 91 07 31 ff 89 c5 89
RSP: 0018:ffffc900030ffcf0 EFLAGS: 00010293

RAX: 0000000000000000 RBX: 1ffff9200061ffa4 RCX: 0000000000000000
RDX: ffff88807c859d80 RSI: ffffffff81db815a RDI: 0000000000000003
RBP: ffff88801bcbca80 R08: 0000000000000000 R09: ffffffff9006d90f
R10: ffffffff81db7e71 R11: 0000000000000001 R12: 0000000000000008
R13: ffffc900030ffd40 R14: 00000000ffffffff R15: 0000000000000002
FS:  0000555556a0e300(0000) GS:ffff8880b9c00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000555556a17628 CR3: 0000000071c9d000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 path_umount+0x7d4/0x1260 fs/namespace.c:1806
 ksys_umount fs/namespace.c:1825 [inline]
 __do_sys_umount fs/namespace.c:1830 [inline]
 __se_sys_umount fs/namespace.c:1828 [inline]
 __x64_sys_umount+0x159/0x180 fs/namespace.c:1828
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x7fcc5b9cc2c7
Code: 00 00 00 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 31 f6 e9 09 00 00 00 66 0f 1f 84 00 00 00 00 00 b8 a6 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 c0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffcb4fdf1a8 EFLAGS: 00000202 ORIG_RAX: 00000000000000a6
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fcc5b9cc2c7
RDX: 00007ffcb4fdf269 RSI: 000000000000000a RDI: 00007ffcb4fdf260
RBP: 00007ffcb4fdf260 R08: 00000000ffffffff R09: 00007ffcb4fdf040
R10: 0000555556a0f693 R11: 0000000000000202 R12: 00007ffcb4fe02e0
R13: 0000555556a0f5f0 R14: 00007ffcb4fdf1d0 R15: 0000000000000002
 </TASK>