linux-kernel - Re: [syzbot] WARNING in mntput_no

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Date:   Thu, 1 Apr 2021 10:09:18 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Christian Brauner <christian.brauner@...ntu.com>,
        syzbot <syzbot+c88a7030da47945a3cc3@...kaller.appspotmail.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzkaller-bugs@...glegroups.com, viro@...iv.linux.org.uk,
        io-uring@...r.kernel.org
Subject: Re: [syzbot] WARNING in mntput_no_expire (2)

On 4/1/21 9:45 AM, Christian Brauner wrote:
> On Thu, Apr 01, 2021 at 02:09:20AM -0700, syzbot wrote:
>> Hello,
>>
>> syzbot found the following issue on:
>>
>> HEAD commit:    d19cc4bf Merge tag 'trace-v5.12-rc5' of git://git.kernel.o..
>> git tree:       upstream
>> console output: https://syzkaller.appspot.com/x/log.txt?x=1018f281d00000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=d1a3d65a48dbd1bc
>> dashboard link: https://syzkaller.appspot.com/bug?extid=c88a7030da47945a3cc3
>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12f50d11d00000
>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=137694a1d00000
>>
>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>> Reported-by: syzbot+c88a7030da47945a3cc3@...kaller.appspotmail.com
>>
>> ------------[ cut here ]------------
>> WARNING: CPU: 1 PID: 8409 at fs/namespace.c:1186 mntput_no_expire+0xaca/0xcb0 fs/namespace.c:1186
>> Modules linked in:
>> CPU: 1 PID: 8409 Comm: syz-executor035 Not tainted 5.12.0-rc5-syzkaller #0
>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
>> RIP: 0010:mntput_no_expire+0xaca/0xcb0 fs/namespace.c:1186
>> Code: ff 48 c7 c2 e0 cb 78 89 be c2 02 00 00 48 c7 c7 a0 cb 78 89 c6 05 e5 6d e5 0b 01 e8 ff e1 f6 06 e9 3f fd ff ff e8 c6 a5 a8 ff <0f> 0b e9 fc fc ff ff e8 ba a5 a8 ff e8 55 dc 94 ff 31 ff 89 c5 89
>> RSP: 0018:ffffc9000165fc78 EFLAGS: 00010293
>> RAX: 0000000000000000 RBX: 1ffff920002cbf95 RCX: 0000000000000000
>> RDX: ffff88802072d4c0 RSI: ffffffff81cb4b8a RDI: 0000000000000003
>> RBP: ffff888011656900 R08: 0000000000000000 R09: ffffffff8fa978af
>> R10: ffffffff81cb4884 R11: 0000000000000000 R12: 0000000000000008
>> R13: ffffc9000165fcc8 R14: dffffc0000000000 R15: 00000000ffffffff
>> FS:  0000000000000000(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> CR2: 000055a722053160 CR3: 000000000bc8e000 CR4: 00000000001506e0
>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>> Call Trace:
>>  mntput fs/namespace.c:1232 [inline]
>>  cleanup_mnt+0x523/0x530 fs/namespace.c:1132
>>  task_work_run+0xdd/0x1a0 kernel/task_work.c:140
>>  exit_task_work include/linux/task_work.h:30 [inline]
>>  do_exit+0xbfc/0x2a60 kernel/exit.c:825
>>  do_group_exit+0x125/0x310 kernel/exit.c:922
>>  __do_sys_exit_group kernel/exit.c:933 [inline]
>>  __se_sys_exit_group kernel/exit.c:931 [inline]
>>  __x64_sys_exit_group+0x3a/0x50 kernel/exit.c:931
>>  do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
>>  entry_SYSCALL_64_after_hwframe+0x44/0xae
>> RIP: 0033:0x446af9
>> Code: Unable to access opcode bytes at RIP 0x446acf.
>> RSP: 002b:00000000005dfe48 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
>> RAX: ffffffffffffffda RBX: 00000000004ce450 RCX: 0000000000446af9
>> RDX: 000000000000003c RSI: 00000000000000e7 RDI: 0000000000000001
>> RBP: 0000000000000001 R08: ffffffffffffffbc R09: 0000000000000000
>> R10: 0000000000000000 R11: 0000000000000246 R12: 00000000004ce450
>> R13: 0000000000000001 R14: 0000000000000000 R15: 0000000000000001
> 
> [+Cc Jens + io_uring]
> 
> Hm, this reproducer uses io_uring and it's the io_uring_enter() that
> triggers this reliably. With this reproducer I've managed to reproduce
> the issue on v5.12-rc4, and v5.12-rc3, v5.12-rc2 and v5.12-rc1.
> It's not reproducible at
> 9820b4dca0f9c6b7ab8b4307286cdace171b724d
> which is the commit immediately before the first v5.12 io_uring merge.
> It's first reproducible with the first io_uring merge for v5.12, i.e.
> 5bbb336ba75d95611a7b9456355b48705016bdb1

Thanks, that's good info. I'll take a look at it and see if I can
reproduce.

-- 
Jens Axboe